Effects of Daylight - The Ergonomics Center Of North Carolina

Transcription

Principal Investigator: Sharon Joines, PhD
Director of Research
Co-Authors:
Elizabeth Covalla,
Hollis Dickens, and
Jeffrey Hoyle
Partial funding for this project provided by:
Knape & Vogt
© 2003 The Ergonomics Center of North Carolina
All rights reserved. This white paper contains general knowledge and analysis of published guidelines,
standards, and research on ergonomics and musculoskeletal disorders. The Ergonomics Center of North
Carolina cannot assume responsibility for the utilization of the document or for the consequences of its
use.
This document may not be reproduced, stored in a retrieval system, or transmitted in any form or by any
means, electronic or mechanical, including photocopying and recording, without prior written permission of
The Ergonomics Center of North Carolina, Raleigh, North Carolina.
The Ergonomics Center of North Carolina
3701 Neil Street
Raleigh, North Carolina 27607
919.515.2052
www.TheErgonomicsCenter.com
TheErgonomicsCenter
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Advancing the Science of Ergonomics in the Workplace SM
P 919.515.2052
3701 Neil Street, Raleigh, NC 27607
F 919.515.8156
Table of Contents
List of Tables……………………………………………….……………………………….…v
List of Figures……………………………………………………….………………………..iv
1
INTRODUCTION.................................................................................................................. 1
1.1
PRINCIPLE COMPONENTS ................................................................................................. 1
1.2
ERGONOMIC STANDARDS AND GUIDELINES .................................................................... 2
1.2.4
Standards Organizations.......................................................................................... 2
1.2.5
Limitations to Current Standards ............................................................................ 3
1.3
BASIC RESEARCH PRINCIPLES.......................................................................................... 3
1.3.1
Types of Studies........................................................................................................ 4
1.3.2
Independent versus Dependent Variables................................................................ 4
1.3.3
Basic Statistics ......................................................................................................... 4
1.3.4
Common Data Collection Methods.......................................................................... 5
1.3.5
Evaluating the Strength of Research........................................................................ 5
1.4
LITERATURE REVIEW METHOD AND SCOPE .................................................................... 7
2
VISUAL DISPLAY TERMINALS....................................................................................... 9
2.1
RESEARCH ........................................................................................................................ 9
2.1.1
VDT Dose-Response Relationship ........................................................................... 9
2.1.2
VDT Position.......................................................................................................... 10
2.1.3
VDT Design............................................................................................................ 11
2.1.4
VDT Filters ............................................................................................................ 12
2.2
STANDARDS.................................................................................................................... 12
2.3
RECOMMENDATIONS ...................................................................................................... 13
2.4
LIMITATIONS .................................................................................................................. 16
2.5
FUTURE RESEARCH ........................................................................................................ 17
3
INPUT DEVICES ................................................................................................................ 19
3.1
KEYBOARDS ................................................................................................................... 19
3.1.1
Research................................................................................................................. 19
3.1.1.1 Alternative Keyboards ....................................................................................... 20
3.1.1.2 Keyboard Dose – Response Relationship .......................................................... 22
3.1.1.3 Wrist Rests ......................................................................................................... 23
3.1.1.4 Keystroke Makeforce ......................................................................................... 23
3.1.1.5 Keyboard Location............................................................................................. 24
3.1.2
Standards ............................................................................................................... 25
3.1.3
Recommendations .................................................................................................. 27
3.1.4
Limitations ............................................................................................................. 29
3.1.5
Future Research..................................................................................................... 29
3.2
POINTING DEVICES......................................................................................................... 30
3.2.1
Research................................................................................................................. 30
3.2.1.1 Discomfort and Pointing Devices ...................................................................... 30
3.2.1.2 Pointing Device Design ..................................................................................... 31
3.2.1.3 Pointing Devices Position .................................................................................. 32
3.2.2
Standards ............................................................................................................... 32
3.2.3
Recommendations .................................................................................................. 34
iii
3.2.4
3.2.5
4
Limitations ............................................................................................................. 34
Future Research..................................................................................................... 35
SEATING ............................................................................................................................. 37
4.1
RESEARCH ...................................................................................................................... 37
4.1.1
Backrest Inclination ............................................................................................... 38
4.1.2
Seat Pan Inclination............................................................................................... 38
4.1.3
Lumbar Support ..................................................................................................... 39
4.1.4
Arm Support ........................................................................................................... 41
4.1.5
Task Variability...................................................................................................... 41
4.1.6
Summary of Research ............................................................................................ 41
4.2
STANDARDS ................................................................................................................... 41
4.3
4.4
LIMITATIONS .................................................................................................................. 49
4.5
FUTURE RESEARCH ........................................................................................................ 49
5
LIGHTING........................................................................................................................... 51
5.1
RESEARCH ...................................................................................................................... 51
5.1.1
Natural Lighting..................................................................................................... 51
5.1.2
Overhead Lighting ................................................................................................. 52
5.1.3
Task Lighting ......................................................................................................... 53
5.2
STANDARDS ................................................................................................................... 54
5.3
5.4
RESEARCH LIMITATIONS ................................................................................................ 55
5.5
FUTURE RESEARCH ........................................................................................................ 55
6
BREAKS ............................................................................................................................... 57
6.1
RESEARCH ...................................................................................................................... 57
6.1.1
Muscle Activity and Discomfort............................................................................. 57
6.1.2
Productivity and Costs ........................................................................................... 58
6.1.3
Emotional State...................................................................................................... 58
6.1.4
Exercise Breaks...................................................................................................... 59
6.1.5
Break Enforcement................................................................................................. 59
6.2
STANDARDS ................................................................................................................... 60
6.3
6.4
LIMITATIONS .................................................................................................................. 60
6.5
FUTURE RESEARCH ........................................................................................................ 61
7
EXPECTATIONS AND OPPORTUNITIES .................................................................... 63
Glossary ......................................................................................................................................... 65
References...................................................................................................................................... 67
Appendix A – Keywords and Search Results ................................................................................ 75
Appendix B – Input Device Research............................................................................................ 79
Appendix C – Seating Research .................................................................................................... 83
Appendix D – Breaks Research ..................................................................................................... 87
iv
List of Tables
TABLE 1.1: DATA COLLECTION METHODS ....................................................................................................... 5
TABLE 1.2: GUIDELINES FOR EVALUATING RESEARCH STUDIES ...................................................................... 6
TABLE 2.1: VDT GUIDELINES OUTLINED IN ERGONOMIC STANDARDS AND RECOMMENDATIONS .................. 13
TABLE 2.2: VDT: RECOMMENDATIONS BASED ON CURRENT STANDARDS AND RESEARCH ........................... 15
TABLE 2.3: VDT: WORKSTATION DESIGN IMPLICATION FOR RECOMMENDATIONS IN TABLE 2.2.................. 16
TABLE 3.1: METHODS FOR QUANTIFYING DEPENDENT VARIABLES ................................................................ 19
TABLE 3.2: EFFECTS OF KEYBOARD DESIGN ON USER POSTURE ..................................................................... 22
TABLE 3.3: EFFECTS OF KEYBOARD LOCATION ............................................................................................. 25
TABLE 3.4: KEYBOARD GUIDELINES OUTLINED IN ERGONOMIC STANDARDS ................................................. 26
TABLE 3.5: KEYBOARDS: RECOMMENDATIONS BASED ON CURRENT STANDARDS AND RESEARCH ............... 28
TABLE 3.6: KEYBOARDS: WORKSTATION DESIGN IMPLICATION FOR RECOMMENDATIONS IN TABLE 3.3 ...... 29
TABLE 3.7: INPUT DEVICE GUIDELINES OUTLINED IN ERGONOMIC STANDARDS ............................................. 33
TABLE 3.8: POINTING DEVICE: RECOMMENDATIONS BASED ON CURRENT STANDARDS AND RESEARCH ....... 34
TABLE 4.1: METHODS FOR QUANTIFYING DEPENDENT VARIABLES ................................................................ 38
TABLE 4.2: LUMBAR SUPPORT HEIGHT RESEARCH DATA ............................................................................... 40
TABLE 4.3: LUMBAR SUPPORT DEPTH RESEARCH DATA................................................................................. 40
TABLE 4.4: CHAIR GUIDELINES OUTLINED IN ERGONOMIC STANDARDS ........................................................ 42
TABLE 4.5: CHAIRS: RECOMMENDATIONS BASED ON ERGONOMIC STANDARDS AND RESEARCH ................... 47
TABLE 5.1: FLICKER RATES TESTED IN THE LITERATURE ............................................................................... 53
TABLE 5.2: LIGHTING GUIDELINES OUTLINED IN ERGONOMIC STANDARDS ................................................... 54
TABLE 6.1: BREAK GUIDELINES OUTLINED IN ERGONOMIC STANDARDS ........................................................ 60
v
List of Figures
FIGURE 2.1: THE MPS MODEL............... ……………………………………………………………………11
FIGURE 2.2: ILLUSTRATION OF DIMENSIONS IN TABLE 2.2 .............................................................................14
…………………………………………………………20
FIGURE 3.1: ALTERNATIVE KEYBOARDS
FIGURE 4.1: CHAIR ADJUSTABILITY (A) SEAT PAN DEPTH (B) BACKREST DEPTH ............................................45
FIGURE 4.2: CHAIR, ILLUSTRATION OF DIMENSIONS IN TABLE 4.5 .................................................................46
vi
1 Introduction
The purpose of this paper is to review and summarize available ergonomics literature on critical areas
of office ergonomics. The following five areas will be addressed:
1. Video Display Terminals (VDTs)
2. Input Devices: Keyboards and Mice
3. Seating
4. Lighting
5. Breaks
These five target areas constitute the foundation of the physical office environment. The prevalence
of ergonomics research concerning these target areas emphasizes the importance of these office
components. However, simple ergonomic recommendations regarding these components are not
found in any one study. Just as it is difficult to identify an object from a series of detailed
photographs, it is difficult to identify ergonomic solutions from a series of specialized research
studies. The large quantities of ergonomics literature taken individually generate a confusing and
sometimes contradictory view of office ergonomics solutions. When considered together, certain
trends and solutions emerge in the literature. The following paper has been crafted to alleviate
confusion by presenting a unified view of current office ergonomics literature.
The work of researchers is far from complete. The trends and solutions presented below are only
preliminary resolutions of current office ergonomic issues. Future research must be continually
synthesized and the important findings added to the models presented below. An explanation of
scientific terms and methodologies has been included to help readers assess the quality of future
research and interpret future research results.
1.1
Principal Components
The paper is split into six chapters, the Introduction and five Target Area chapters. The Introduction
(Chapter 1) contains background information to assist the reader in understanding the Target Area
chapters. Chapter 1 breaks down into the following sections:
Section 1.2: Ergonomic Standards and Guidelines. Limitations of current ergonomic guidelines that
make it necessary to review the literature rather than accepting the guidelines by themselves.
Section 1.3: Basic Research Principles. Basic research terms and methodologies including a guide to
help the reader rank and compare the quality of research studies.
Section 1.4: Literature Review Method and Scope. Step-by-step details of the article selection
process. This section includes the databases and the keywords used to narrow down the vast quantity
of ergonomics literature available. The reader may use this section as a guide for updating the
information presented here.
Each of the five Target Area chapters (Chapters 2 through 6) contain the following sections where
“X” designates chapter number:
Introduction. Brief background information on the target area including common office usage and
ergonomic concerns associated with the target area.
1
Section X.1: Research. The central questions surrounding the target area and research studies that
have addressed these questions.
Section X.2: Standards. A comparison of current research findings and ergonomic standards.
Section X.3: Recommendations. A concise summary of the best-known ergonomic solutions given
current research and consideration of standards. These recommendations may include, but are not
limited to, ranges of adjustability and positions of individuals relative to their environment and office
equipment.
Section X.4: Limitations. Limitations of current research due to either technical or ethical issues.
Section X.5: Future Research. Topic suggestions for future research concerning the target area.
In addition, a Glossary is included before the Appendices to assist the reader with unfamiliar terms.
1.2
Ergonomic Standards and Guidelines
A standard is a set of rules officially endorsed by a professional or governmental standards
organization. The standards must be followed in order to maintain a professional certification or stay
in compliance. In contrast, a guideline is a set of recommendations compiled by the government or a
professional organization to assist companies in achieving a specific outcome. Currently, no
mandatory office ergonomic standards exist in the United States. However, several voluntary
standards and guidelines are available to the public.
1.2.4
Standards Organizations
International Organization for Standardization (ISO).
ISO is a network of national standards
institutes from 145 countries working with international organizations, governments, industry,
business, and consumer representatives. Once the need for a specific industry standard is
recognized, working groups determine the scope of the standard and the detailed specifications. The
standard is approved by two-thirds of the members that compiled the standard and by 75% of all
voting members. Each standard is reevaluated at least every five years. ISO has released the ISO
9241 standard relevant to the office environment. ISO 9241 titled “Ergonomic requirements for
office work with visual display terminals” was released between 1991 and 2001. This standard is
comprised of seventeen sections on furniture and software requirements associated with the office
environment.
American National Standards Institute (ANSI). ANSI is a private, non-profit group that organizes
U.S. voluntary standards. ANSI has released the BSR/HFES 100 standard relevant to the office
environment. BSR/HFES 100 titled “Human factors engineering and computer workstations” is a
draft standard built upon the 1998 ANSI/HFES 100: “Ergonomic standard for computer
workstations.” The draft covers input devices, VDTs, and office furniture.
Canada Standards Association (CSA). CSA is a non-profit membership-based association serving
Canadian business and government. CSA has released the CSA Z412 relevant to the office
environment. CSA Z412 titled “Guideline on Office Ergonomics” was published in 2000 and strives
2
to provide optimal office system designs. The intended audience is health and safety professionals,
but is also useful to designers associated with office ergonomics.
The Business and Institutional Furniture Manufacturer’s Association (BIFMA). BIFMA is a business
organization that pulls information from ANSI and ISO standards to form guidelines for furniture
manufacturers. BIFMA has collaborated on the BIFMA G1 standard relevant to the office
environment. BIFMA G1 titled “2002 Ergonomics Guideline” covers ergonomics for furniture
manufacturers.
Occupational Safety and Health Administration (OSHA). OSHA is a governmental department that
strives to provide a safe working environment for the American workforce. OSHA has released the
OSHA 29CFR1910 standard relevant to the office environment. OSHA 29CFR1910, OSHA’s
Ergonomics Program Standard became effective in January 2001 only to be removed the following
April. However, the standard contains useful information related to office ergonomics. A checklist
was also included in the standard to help employers and employees identify, analyze and control
musculoskeletal disorder (MSD) hazards in computer workstation tasks.
1.2.5
Limitations to Current Standards
Current office ergonomics standards should not be considered the ultimate solution to office
ergonomic issues. The limitations listed below make it necessary to continually review current
ergonomics literature.
Lack of Citations. Many standards are created by a consensus approach. The recommendations are
based only on the professional opinions of committee members. Standards do not always identify the
research, if any, that support its claims.
Standard Conflicts. The consensus approach to standard creation does not require every committee
member to agree with all the recommendations in the standard. A consensus implies that more than
the majority but not necessarily unanimity is required to “pass” a standard. Occasionally, experts
disagree resulting in conflicting standards.
Lack of Quantification. Ergonomic standards often provide only general recommendations with few
numerical ranges identified.
Out of Date. Standards are expensive to create and publish, thus, standards are not updated
frequently. As modern office environments continue to evolve to incorporate new technologies,
standards may not reflect the current technology or information available.
Limited Scope. Ergonomic standards may not cover every aspect of the office environment discussed
below.
1.3
Basic Research Principles
Scientists use a variety of research methodologies that can be complex and confusing. A basic
understanding of research methods and terminology is needed to interpret published research results.
The following sections concerning study types, variable types, statistics, and data collection methods
are meant to give the reader a basic understanding of how ergonomics research is conducted. In
3
addition, general guidelines on how to evaluate the strength and weakness of research methods are
also included to help readers gauge the value of research studies.
1.3.1
Types of Studies
Experimental research may take place in a laboratory or in the field. Researchers have better control
over variables in a laboratory setting. However, laboratory research may lack the realism achieved
by field research. Realism is lost because subjects may modify normal behavior when conscious of
being watched. Laboratory researchers may also unknowingly fail to control for variables or
introduce new variables not found in real world conditions. In addition, costs limit the length of
laboratory studies. As a result, long-term effects will not be observed in laboratory research.
Researchers must confront another set of problems when conducting field research. In the field,
researchers may not be able to identify all the possible variables that influence the results. The
researcher may never know if their findings are a result of the factors being observed or the results of
some other characteristic of the environment not considered by the researcher.
When researching health problems in the workplace, one type of field study, called an
epidemiological study, is frequently used. Epidemiology is the study of the spread of disease and
other factors relating to health. Epidemiological studies investigate correlations between injuries or
illness and potential influencing variables. For instance, epidemiological studies have looked at
correlations between carpal tunnel syndrome (CTS) and occupation. A combination of laboratory and
field research is required to fully answer any research question.
1.3.2
Independent versus Dependent Variables
Experimental research requires the researcher to make systematic changes to the independent
variables while recording the reactions of the dependent variables. Independent variables influence
dependent variables. If you wanted to know how long you had to boil an egg before it became hardboiled, you might place six eggs in a pot of boiling water and remove an egg every minute. When all
the eggs are removed, you could crack each egg open and compare yolk hardness. In this simple
experiment, cooking time is the independent variable while egg yolk hardness is the dependent
variable. Yolk hardness is dependent upon the length of cooking time.
Experimental research may also be used to establish a dose-response relationship. The doseresponse relationship describes the trade-off between exposure levels and risk. For example,
researchers commonly use dose-response relationships to define a population’s risk of developing
cancer from a chemical exposure. As exposure to the chemical increases, the risk of cancer
occurring in the study population increases. The nature of this increase is described by the doseresponse relationship. Dose-response relationships could also be established for musculoskeletal
disorders (MSDs). Eventually, researchers hope to define “safe” exposure levels for computer use.
1.3.3
Basic Statistics
The goal of most experiments is to demonstrate that changes to the independent variable produces
changes in the dependent variable. However, the independent variable may, in fact, have no
influence or only a small influence on the dependent variable. Researchers use statistics to
determine the strength of the relationship between the independent and dependent variable. A
4
statistic is some quantity used to describe the data, such as an average. The probability value or pvalue is a common statistic used to describe statistical significance. If results are statistically
significant, then it is reasonable to assume that the observed changes in the dependent variable are
caused by changes in the independent variable. The smaller the p-value the stronger the statistical
significance and the more confident researchers are about their results. Researchers typically
consider a p-value of 0.05 the maximum p-value allowable to call results statistically significant.
1.3.4
Common Data Collection Methods
Data collected in ergonomics research falls into two categories: assessing changes within the human
body and assessing the interface between the human body and the environment. For example,
electromyography (EMG) is the sampling of electric signals generated during muscle activity thereby
assessing change within the body. Table 1.1 outlines the most common data collection methods used
in ergonomics research reviewed for this paper. Each of these methods directly or indirectly assesses
change within the body.
Table 1.1: Data collection methods
Methods for
Data Collection
EMG
Force Sensors
Goniometer
Pressure
Mapping
Advantages
Disadvantages
•
•
•
Measures muscle activity and fatigue
Gives quantitative results
Is inexpensive once equipment is
purchased
•
•
•
Requires expensive equipment
Is difficult to analyze and interpret data
Is intrusive, may alter subject’s
behavior
•
•
Measures dynamic or static loads
Is compact and versatile
•
Are limited to normal forces only
•
•
Measures postural angles
In inexpensive and accurate
•
•
Is difficult to analyze and interpret data
behavior
•
•
•
•
Measures pressure distributions
Gives quantitative results
Is unobtrusive
Is easy to interpret
•
•
Requires expensive equipment
Requires a level surface
Measures subject’s discomfort
Is easy and inexpensive
•
Gives subjective interpretation, based
solely on subject’s opinions
Measures body postures
Is easy and inexpensive
•
behavior
May be biased by analyst
Requires expensive software and
equipment for objective analysis
Surveys and •
Questionnaires •
•
•
•
•
Video Analysis
1.3.5
Evaluating the Strength of Research
Just as the p-value indicates the strength of the relationship found between independent and
dependent variables, indications of the strength of the design of a research investigation can also be
assessed. Research designs and methodologies are import factors for evaluating studies. Logical
approaches and consistent data collection methodologies make for stronger, believable results. The
strength of a research investigation can also be assessed through several measures such as: sample
5
size, trial length, and experimental design. Table 1.2 offers general guidelines for evaluating a
research study.
Sample Size. The number of subjects used in the experiment is called the sample size. Larger
sample sizes produce stronger results. With large sample sizes, a broader range of the population
may be accounted for making the research results more universal. Larger sample sizes diminish the
influence of individual differences on the results, allowing the researcher to focus on the influences
of the independent variables. Increasing the sample size increases the researchers ability to observe
significant differences. Investigations of multiple independent variables and multiple levels of those
variables necessitate larger sample sizes to obtain sufficient statistical power for their analysis. That
is why some investigators will break a larger problem into smaller constituent pieces and run smaller
experiments with smaller sample sizes.
Table 1.2: Guidelines for evaluating research studies
Strength of
Research
Increasing
Sample Size
N<12
Increasing sample size increases the
researchers ability to observe significant
differences. Optimal sample size depends
on other factors including number of
variables and the use of pilot work
Trial Length
Brief exposure to trial conditions compared
to expected response rate or real world
conditions
(e.g. <1 hr)
Mimics real world requirements
(e.g.>4 hrs)
Control Group
No control group
Control group used
Experimental
Design
Design lacking structure and difficult to
Well structured design easily repeatable by
repeat by other researchers
other researchers
(e.g. a correlation study that does not
(e.g. establishes causal relationship
establish a causal relationship between
between variables and randomizes
variables and does not randomize subjects)
subjects)
Trial Length. In ergonomics research, longer trial lengths will yield better results. Most of the health
effects associated with ergonomics, such as CTS or chronic back pain, only arise after an extended
period of time. The trial length of a study should be long enough for the studied phenomenon to be
observed.
Control Groups. Control groups are key to coping with extraneous variables. An extraneous variable
is anything that influences the dependent variable that is not the focus of the current study (i.e. the
independent variable). A control group is usually a set of subjects that do not receive a change in the
independent variable. A control group ensures that changes observed in the experimental group are
due to the independent variables and not to some unknown factor.
Experimental Design. A strong experimental design will be well structured and use research
techniques found to strengthen results. For example, determining a causal relationship by
systematically changing the independent variable. Randomization is another research technique
used in simpler experimental designs to ensure that subjects are equally likely to be placed in the
6
control or treatment group. The experimental design must be objective and clear so that future
researches could achieve similar results by repeating the research.
1.4
Literature Review Method and Scope
The research referenced in this document was collected based on an extensive review of current
ergonomics literature. The information in this section and in Appendix A is meant to outline the
literature review and selection process such that it may be repeated in the future.
Step 1: Planning phase. During the preliminary planning phase, a process was outlined in which
keywords would be identified, a primary search would be performed, the findings would be
systematically reduced, articles would be collected and reviewed (a highly iterative process),
research synthesized, white paper written, secondary search performed, final draft written, and final
internal review performed.
Step 2: Keyword Selection. Through brainstorming, a list of keywords for each of the five research
areas was compiled. The compiled list was reduced and combined to create approximately ten
central keywords. In addition, a list of word modifiers and eliminators were created. Modifiers are
words added to the keyword to limit the search. For example, the word “office” could be added to
the keyword “chair” to concentrate the focus of the search. An eliminator was used to cut out
extraneous articles. For example, “wheelchair” and “mass transit” were excluded when the keyword
“chair” was used.
Step 3: Primary Search. The Ergonomics Abstract database was used for the primary search.
Ergonomics Abstract is a service covering sixty-eight thousand ergonomics and human factors
literature abstracts from 1986 to the present. However, the preliminary searches were limited to
abstracts released after 1990. The keywords selected in Step 2 were entered into the database’s
search engine as listed in Appendix A. If the search returned more than two hundred articles, the
modifiers and eliminators were used to exclude extraneous articles. The remaining search results
were downloaded into Microsoft’s Endnote.
Step 4: Search Reduction. In Microsoft’s Endnote, the collected abstracts were reviewed one at a
time. An article was eliminated or accepted based on the relevance of information contained in the
abstract. Twenty to forty central articles were chosen for each of the five topic areas.
Step 5: Collection and Review. The articles were collected from the North Carolina State University
Library or downloaded from electronic journals. Each article was read and summarized.
Step 6: Research Synthesis. The article summaries were complied into comprehensive tables with
breakdowns of all independent variables, dependent variables, trial lengths, subject sizes, research
methodologies, and results. General conclusions were drawn from the summary tables.
Step 7: White Paper Development. Important research questions were pulled from the literature.
The summary tables were used to form a draft of the white paper.
Step 8: Secondary Search. Once the central research questions were decided upon in the white
paper draft, a second literature review was performed in Ergonomics Abstracts to verify that all
pertinent articles are included in the final white paper.
7
Step 9: Internal White Paper Review. The staff of The Ergonomics Center internally reviewed the
contents of the white paper to ensure the accuracy of the information compiled.
8
2 Visual Display Terminals
Postural and visual components of VDTs are thought to influence employee health. Poor positioning
of the VDT within the workstation places users in awkward postures, increasing the risk of
developing some MSDs (Aarås et al., 2000). Screen characteristics of the VDT, including contrast
and illuminance, affect the visual system of users (Aarås et al., 2000). Though a specific doseresponse relationship is unknown, excessive VDT work is known to cause visual discomfort
(Aronsson and Strömberg, 1995; Fogleman and Lewis, 2002). Currently, researchers are
investigating which factors of VDT use result in discomfort and if that discomfort leads to permanent
injury. In addition to assessing the effects of VDT work, researchers are evaluating the ergonomic
impact of new technologies as they appear on the market, such as flat screen monitors. The following
section covers research on VDT usage, position, screen characteristics, screen design, and screen
filters.
2.1
Research
The seventeen articles chosen for review examine VDT use in the office environment. Currently,
VDT research strives to answer one or more of the following questions:
1. What is the dose-response relationship for VDT use?
2. Where should the VDT be positioned on the desk (viewing height and distance) to minimize
musculoskeletal and visual discomfort?
3. How does VDT design influence worker comfort?
4. Does the use of VDT filters reduce visual discomfort?
These four questions encompass the central focus of VDT research. The following sections explore
each of these research questions.
2.1.1
VDT Dose-Response Relationship
VDT research focuses primarily on the effect VDT usage has on musculoskeletal and visual
discomfort. Musculoskeletal discomfort is commonly qualified through discomfort surveys, in which
the subject is asked to subjectively quantify the amount of pain felt in one or more body parts. EMG
measurements are used to indirectly assess discomfort by measuring muscle activity and fatigue.
EMG signals measure muscle activity; various aspects of those signals are correlated with increased
reporting of muscular discomfort. Visual discomfort is also qualified through discomfort surveys,
while eye examinations used to diagnose eye disorders such as cataracts or myopia.
Increased VDT usage increases the risk of muscle and/or visual discomfort (Zhang et al., 1998;
Cornelio et al., 1993; Faucett and Rempel, 1994). However, little is known about how much VDT
work is too much VDT work. As little as 20 hours a week of VDT work has been associated with an
increase in visual and muscle discomfort (Zhang et al., 1998). Broken up daily, more than four hours
of VDT work a day increases visual discomfort (Cornelio et al., 1993). Over a ten-year period VDT
workers have had a higher incidence of cataracts as well as significant increases in the number,
frequency, and type of work-related musculoskeletal symptoms (Zhang et al., 1998).
Factors beyond length of VDT usage may play a part in a dose-response relationship. Task type as
well as individual characteristics may be an important component of a dose-response relationship.
Workers performing more than one VDT task, such as data processing mixed with computer-aided
9
design, complained less about eye discomfort than workers performing only one VDT task (Aronsson
and Strömberg, 1995). Visual fatigue is also more common among users with uncorrected or only
partially corrected refractive errors (Cornelio et al., 1993). However, users with visual acuities lower
than 7 to 9 of 10 seem to be protected from visual fatigue (Cornelio et al., 1993). Even psychosocial
factors may be a factor in a dose-response relationship. Low decision latitude along with hours per
day of VDT use was found to be significant risk factors of musculoskeletal symptoms in one study
(Faucett and Rempel, 1994).
2.1.2
VDT Position
VDT position consists of five components:
• The height of the VDT in relation to the user’s eyes
• The distance of the VDT from the user’s eyes
• The tilt of the VDT in relation to the user’s field of vision
• The viewing arc of the VDT in relation to the user’s eyes
• The left to right horizontal orientation of the VDT in relation to the user’s eyes.
The combination of these five components is thought to reduce or exacerbate visual and muscular
discomfort among VDT workers.
VDT Height. VDT height is commonly described in terms of the viewing angle. The viewing angle is
the angle formed between a horizontal line at the height of the user’s eyes and from there to the
center of the screen. Individual studies recommend viewing angles anywhere from negative 40° to
positive 15°. To understand these seemingly contradictory results, monitor-viewing angle must be
examined in relation to the type of discomfort studied by the researchers. High monitor placement
has been associated with increased visual strain (Berqvist and Knave, 1994; Jaschinski et al., 1998;
Sotoyama et al., 1996). However, low monitor placement has been associated with increased
musculoskeletal strain (de Wall et al., 1992, Sommerich et al., 2001). Based on user preferences the
optimal viewing angle is around negative 10° to negative 18° (Straker, 2000; de Wall et al., 1992;
Turville et al., 1998; Sommerich et al., 2001). The lowest and highest viewing angles were reported
as less preferable by subjects.
The monitor placement strain model (MPS) shown in Figure 2.1 was developed to explain this
relationship (Sommerich et al., 2001). Though both types of strain cannot be minimized by a single
monitor height, a reasonable compromise may be possible. The MPS model was developed in 1999
and is based on research published up to that point. Research on VDT viewing angle published after
1999 also supports the model (Straker and Mekhora, 2000; Svensson and Svensson, 2001). The MPS
model does not identify numerical ranges of viewing angles that mark the boundaries between
increases in musculoskeletal strain and increases in visual strain. The studies do not uniformly
measure viewing angles; therefore, a one to one comparison across articles is difficult.
In addition to considering the viewing angle when assessing the height for monitor placement, the
vertical distance between the keyboard and the monitor should also considered. This distance has
more of an effect on eyestrain than VDT height alone. Larger monitor-keyboard distances induced
larger ocular surface areas (OSA) (Sotoyama et al., 1996), which have been associated with increased
eye irritation and visual fatigue.
10
…… Muscles
____ Vision
source: Sommerich et al. (2001)
Strain
High
Visual Strain
• Accommodation
• Ocular surface area
• Discomfort
Musculoskeletal Strain
• Posture
• Muscle activity
• Discomfort
Low
Low
Monitor Placement
High
Figure 2.1: The MPS Model
VDT Distance. The distance between the user and the front of the VDT may affect the user’s comfort.
Near viewing distances of less than 7 in. (18 cm) increase eyestrain among some users. Users prefer
viewing distances between 24 and 39 in. (60 and 100 cm). Viewing distance effects may be
confounded with viewing angle. Viewing distance becomes more important as the viewing angle
increased from negative 18° to 0°. (Jaschinski et al., 1998) The improper selection of viewing
distances may cause users to adopt awkward postures. Users may leans back or forward in their seats
to better see the screen.
VDT Tilt. Although Ergonomists commonly recommend tilting a monitor to reduce glare, monitor tilt
has not been examined in current research.
Viewing Arc. If the viewing distance remains constant while the monitor is raised or lowered the
monitor will follow a viewing arch. It is generally accepted that a constant viewing arc should be
maintained.
VDT Horizontal Orientation. It is widely accepted that the VDT should be placed directly in front of
and centered about the user. All recent research uses this horizontal orientation while examining
other VDT components.
2.1.3
VDT Design
VDT designs are continually improving with new technologies. As a result, limited research is
available concerning the latest designs. However, a few studies have examined the effects of VDT
designs (including screen size, flat screens, and screen characteristics) on user comfort and
performance.
Monitor Size. The biggest monitor available may not be the best solution for workers’ computing
needs. In addition to higher initial costs, larger monitors take up more work surface. As a result,
monitor size may limit the range of monitor positions. However, research has indicated that the
11
smaller monitor sizes should also be avoided. One researcher found higher muscle activity levels
occurred among subjects using a 14 in. monitor versus a 19 in. monitor (Sommerich et al., 2001).
Flat Screens. Flat panel displays (FPD) appeared on the market in the mid-nineties. An obvious
advantage of an FPD for an employer is the space saved with the smaller design. Workstations can
be between 10 to 20% smaller and still retain the same functional work surface area (Peebly, 1998).
In addition to subjects’ preference over traditional cathode ray tube (CRT) monitors (Shieh and Lin,
2000), visual performance improvements have also been observed with FPDs (Shieh and Lin, 2000).
Screen Characteristics.
Screen characteristics such as color schemes may also affect visual
performance and comfort among users. Blue letters on yellow backgrounds result in better visual
performance and greater user satisfaction than other test color combinations (Shieh and Lin, 2000).
The worst visual performance and lowest user satisfaction occurred with purple letters on red
backgrounds.
2.1.4
VDT Filters
VDT filters are transparent, polarized films placed over the monitor screen to reduce glare. Screen
filters have not been shown to have any long-term effects on musculoskeletal conditions of the upper
body, productivity, sick leave, or user preferences (Fostervold et al., 2001). However, most research
focuses on preventing glare though lighting designs (Chapter 5) opposed to evaluating these and
other control options.
2.2
Standards
CSA, OSHA, BIFMA, ISO, and HFES have all issued standards or guidelines related to VDTs as
seen in Table 2.1. The CSA recommends a monitor size from 11 to 19 in. for traditional CRT
screens. However, no recommendations have been made concerning flat panel displays. Screen
widths of 15° to 20° are recommended though no research was reviewed on this topic. Since the field
of vision depends on viewing distance, screen width is given in degrees. The standards consistently
recommend viewing distances between 16 to 29 in. viewing distances. However, research has shown
that the optimal viewing distance may be dependent upon viewing angles. The standards do not
consistently recommend the same viewing angle though the majority of the standards recommend that
the monitor should be at or slightly below eye level. Viewing angles from negative 60° to positive 20°
have been recommended. No recommendations have been made on VDT tilt or viewing arch. More
research is needed to eliminate the uncertainty surrounding viewing angle recommendations.
12
Table 2.1: VDT guidelines outlined in ergonomic standards and recommendations
VDT
Standard
VDT
Screen Size
CSA-Z412
VDT Width
Recommendations
•
•
BSR/HFES 100 •
Draft
Screen should be visible when eyes positioned within ± 20° from
vertical center
•
center
•
horizontal
•
Greater than 16 in. (40 cm)
CSA-Z412
ISO 9241
BIFMA
BSR/HFES 100
•
Draft
VDT Viewing
Distance
CSA-Z412
ISO 9241
OSHA
16 to 29 in. (40 to 74 cm)
•
At least 16 in. (40 cm). Optimal viewing distance is 24 in. (60 cm),
however individuals prefer between 18 to 29 in. (45 to 75 cm)
•
Should allow employee to read screen without leaning head, neck,
or trunk forward or backward
•
View entire screen between horizontal eye level and 60° below eye
level (preferably between 20° to 50° below eye level when seated,
15° to 45° standing)
BSR/HFES 100 •
Draft
2.3
horizontal
•
•
Top of monitor screen should be slightly below horizontal eye level
View entire screen between horizontal eye level and 30° below eye
level
•
horizontal
OSHA
•
Top line of screen is at or below eye level
OSHA
•
Directly in front of employee
CSA-Z412
ISO 9241
VDT Position
At least 16 in. (40 cm)
•
BIFMA
VDT Height
(Viewing
Angle)
15 in. (38 cm) recommended for editing text, over a limited
duration of time, with a character height of 0.11 in. (0.03 cm)
11.4 to 18.9 in. (29 to 48 cm) acceptable
Recommendations
What is the dose-response relationship for VDT use?
Research concerning the dose-response relationship between VDT usage and discomfort has shown
that musculoskeletal and visual discomfort increase as the user spends more time at the computer.
None of the standards specify safe working limits for VDT usage and little research exists that specify
the exact duration of VDT usage that leads to an increased risk of symptom development. While a
quantifiable dose-response relationship is unknown, working at a VDT for more than four hours a day
has been shown to increase visual discomfort though working time may be increased with frequent
breaks, varying tasks, and correcting all vision problems.
13
Where should the VDT be positioned on the desk (viewing height and distance) to minimize
musculoskeletal and visual discomfort?
A good initial monitor set-up for a 19 in. (48 cm) screen positions the monitor with a viewing angle
between 0° and negative 18° at a distance of more than 16 in. (40 cm) directly in front of the user.
However, optimal monitor placement will depend upon the user. If a user is experiencing visual
discomfort, the monitor may be lowered. If a user experiences musculoskeletal discomfort, the
monitor may be raised.
Viewing distance may be left up to the discretion of the user. Research has shown that only when
viewing distances are less than 7 in. (18 cm) does eyestrain develop among users; the standards
recommend a minimum viewing distance of 16 in. (40 cm). In general, users prefer viewing
distances between 24 and 39 in. (60 and 100 cm). Further recommendations and the workstation
design implications can be found below in Tables 2.2 (supported by Figure 2.2) and Table 2.3.
2
5
1
3, 4
Front View
1
2
3
4
5
Side View
diagonal distance across the viewing area of the monitor
horizontal distance across the viewing area of the monitor
vertical distance between center of viewing area and top of work surface
horizontal distance between screen and user’s eyes
Figure 2.2: Illustration of dimensions in Table 2.2
14
Research
OSHA
ISO
CSA
BIFMA
Topic
BSR/HFES
Table 2.2: VDT: Recommendations based on current standards and research
Recommendations based on
current standards and
Rational of Recommendation
research
To allow large text sizes and
high resolution to reduce visual
fatigue
Screen Size1
19 in. diagonal measurement
VDT Width2
To allow large text sizes and
Visible within plus/minus 15° to
high resolution to reduce visual
20° from vertical center
fatigue
VDT Height
3
while sitting
VDT height should allow a
viewing angle between 0° and
–18°
To keep the neck in an upright
posture
VDT Height
while standing4
VDT height should allow a
viewing angle between 0° and
–18°
posture
Adjust based on user
preferences
To minimize harmful glare
Allow user to maintain a
viewing distance between
16 in. (40 cm) and 39 in. (100
cm)
posture and to reduce visual
fatigue
16 to 39 in. (40 to 100 cm)
To prevent user from leaning
back or forward to see the
screen and to reduce visual
fatigue
Place monitor directly in front
of and centered about the user
To keep the upper extremities
in a neutral position
VDT Tilt
Viewing Arc
VDT
Viewing
5
Distance
VDT Horizontal
Orientation
9- Guideline or standard reasonably agrees with given recommendation
- Guideline or standard significantly disagrees with given recommendation
- Generally accepted recommendations based on well established biomechanical principles
- Preliminary recommendations that still must be confirmed by future research
Illustrated in Figure 2.2
1
diagonal distance across the viewing area of the monitor
2
horizontal distance across the viewing area of the monitor
3
4
5
horizontal distance between screen and user’s eyes
15
Table 2.3: VDT: Workstation design implication for recommendations in Table 2.2
Topic
Recommendations
Screen Size
19 in. diagonal
measurement
VDT Height
While Sitting
(Viewing
Angle)
Implication for Workstation Design
•
VDT height should allow •
a viewing angle
•
between 0° and –18°
Monitor arms should be able to support the weight of a 19
in. flat screen monitor
Top of screen level with sitting eye height
Combined desk and monitor height of 27.4 to 34.1 in. (69.5
to 86.5 cm)
VDT Height
VDT height should allow •
While Standing
a viewing angle
•
(Viewing
between 0° and –18°
Angle)
Top of screen level with sitting eye height
Combined desk and monitor height of 60.9 to 68.7 in.
(154.7 to 174.6 cm)
Allow user to maintain a
viewing distance
•
between
16 and 39 in. (40 and
100 cm)
Monitor arms should follow the visual curve and allow
users to maintain recommended viewing distance
VDT
Viewing Arc
VDT
Viewing
Distance
16 and 39 in. (40 and
100 cm)
•
Combined desk and keyboard tray width of at least 39 in.
(100 cm)
•
VDT Position
Keep vertical center of
monitor inline with
vertical center of body
to keep head and neck
in a neutral posture.
Monitor arms should be mounted so that the monitor is in
line with vertical center of users body
Keyboard trays should be mounted in line with vertical
center of VDT
•
How does VDT design influence worker comfort?
Flat screen monitors use less workspace, are preferred over standard monitor styles (with similar
resolution), and have been shown to improve performance. However, flat screens are a new
technology and not enough research has been performed to determine all the advantages and
disadvantages associated with flat screens.
Does the use of VDT filters reduce visual discomfort?
VDT screen filters have not been shown to help or harm users. Anecdotally, by reducing glare,
screen filters may improve working posture for workers that adopt awkward postures to circumvent
severe screen glare.
2.4
Limitations
Though one study showed a strong link between VDT usage and musculoskeletal and visual
discomfort, not enough research has been performed to establish a dose-response relationship. Since
this study was conducted in the field, it is unclear how factors such as break schedules, workload, or
corporate cultures may have influenced these findings. In addition, this study only examined two
level of VDT usage, more than or less than twenty hours a week. A more systematic research
approach is needed to establish a dose-response relationship.
Selection of an ideal monitor height has been hindered by the lack of consistency among researchers’
documentation and implementation of posture analysis methods. Current research lacks a systematic
16
approach of defining viewing angles and few studies examine more than three viewing angles under
the same circumstances. No central body exists to universally define research parameters.
One of the greatest limitations to current VDT research is that it may soon be out-dated as flat
screens replace CRT monitors. It is unknown if research related to strain and CRT usage will be
transferable to liquid crystal technology.
2.5
Future Research
Future research should concentrate on the newer flat screen technologies currently available. In
addition, to longer trial lengths and larger sample sizes, future research must by systematic and
comprehensive accounting for all of the following variables:
• Monitor size
• Monitor distance
• Monitor tilt
• Subject anthropometry
• Task
• Chair and workstation
• Breaks
The most important unsolved question is whether or not VDT usage will lead to an increased risk of
permanent musculoskeletal or visual disorders. Once a relationship is established, researches must
determine how much time in front of a VDT is safe for a wide range of working conditions.
17
18
3 Input Devices
The term input device refers to all equipment used to enter or manipulate data in a computer. Two of
the most commonly used input devices are the keyboard and the mouse. While these devices are
widely available in the traditional designs, more companies are creating alternative designs to
increase worker comfort and decrease the risk factors associated with prolonged computer use. With
so many styles available, research is needed to determine which products are most beneficial.
3.1
Keyboards
As the most frequently used input device, researchers have explored the link between keyboards and
computer related injuries. The standard flat QWERTY keyboard is the most commonly used
keyboard design in office environments. However, the number of alternative keyboards available has
increased over recent years. These alternative keyboards are generally more expensive than the
standard keyboard, but in some cases have been shown to improve working postures. Currently,
researchers are attempting to determine the effectiveness of alternative keyboards in reducing the
risk of injury. In addition to comparing keyboard design, researchers are also evaluating physical
aspects of keying tasks to determine associated risks and offer suggestions for improvement.
3.1.1
Research
Twenty studies concerning keyboard use in the office environment were reviewed for this paper. All
of the studies focused on jobs involving VDT tasks. One of the studies was performed in the field,
while the remaining nineteen studies were performed under more controlled laboratory settings. A
summary of the articles’ results may be found in Appendix B. Each of these studies used various
methods to qualify results. These methods are listed below in Table 3.1.
Table 3.1: Methods for quantifying dependent variables
Dependent Variables
Methods for Qualifying Variables
Productivity
•
•
Keystrokes per minute
Errors per minute
Posture
•
•
Electrogoniometer
Video analysis
Discomfort
•
•
Discomfort surveys
EMG
Force
•
•
•
•
Force plate
Biomechanical modeling with EMG
Strain gauge
Spring balance
19
Research on keyboard use strives to answer the following questions:
1. What advantages/disadvantages do alternative keyboard designs have over standard designs?
2. What is the relationship between duration of keyboard use and musculoskeletal discomfort?
3. Are wrist rests beneficial during keying tasks?
4. What affect does keystroke force have on the musculoskeletal system?
5. What is the optimal location for the keyboard?
Answering these questions will allow researchers to develop keyboard designs, furniture, and
accessories to place keyboards in optimal locations that will benefit all users. The following sections
explore each of these research questions.
3.1.1.1
Alternative Keyboards
Over recent years the rising number of injuries and illnesses related to computer use have
precipitated an increase in the amount of research and development for new keyboard designs.
These new alternative keyboards are generally designed to decrease the risk factors for cumulative
trauma disorders (CTDs) by reducing the awkward wrist postures typically assumed during typing.
While alternative keyboards can offer postural improvements, the associated advantages are often
overshadowed by the costs. New designs can easily cost as much as two to ten times more than a
standard keyboard.
Alternative designs most often maintain the standard QWERTY key layout while altering the shape of
the actual keyboard. Alternative designs can range in complexity from a simple fixed split keyboard
to more extreme designs with completely altered keyboard shape and key layout.
The three main designs for alternative keyboards discussed in the research section below include:
• Fixed split keyboard – Typically the keyboard is divided
into two sections that are separated and angled to form a
slight v-shape. The Microsoft® Natural® Keyboard Elite is
an example.
• Adjustable split keyboard – The keyboard is divided into
two sections and are typically hinged together so the user
(a)
is able to angle and tent the keyboard to its best fit. The
Goldtouch® PS2 Adjustable Ergonomic Keyboard shown in
Figure 3.1a is an example.
• Contoured split keyboard – These keyboards are designed
to not only split the keys, but to also curve the keys to help
the user maintain a neutral hand posture. The Kinesis®
(b)
Contoured Keyboard shown in Figure 3.1b is an example.
Figure 3.1: Alternative
Keyboards (a) Kinesis®
Contoured (b) Goldtouch®
PS2 Adjustable
Eight of the twenty studies on keyboard design reviewed for this paper examined the advantages and
disadvantages of alternative keyboards. Five of the eight articles indicated that productivity is
20
highest on a standard keyboard (Gerard et al., 1994; Gilad et al., 2000; Smith et al., 1998; Swanson
et al., 1997; Zecevic et al., 2000). One of these studies showed that 72% of normal speed
proficiency had been achieved on the Kinesis® Contoured Keyboard in 115 min. and 97% of normal
accuracy had been achieved in 65 min. (Gerard et al., 1994). Another study showed that it took
subjects an average of two hours on an adjustable split keyboard to reach the same performance
levels as seen on the standard keyboard (Smith et al., 1998). An additional method of evaluating
worker speed and productivity, as discussed by Hoffman et al. (1995), is to study the effects of
keyboard design with respect to inter-key spacing. This study noted that the travel times between
keys are minimized when the inter-key spacing is approximately equal to the finger pad size.
Therefore, alternative keyboards with optimal inter-key spacing will lead to an increased typing
speed and higher productivity.
Three of the eight articles focus on the effects of keyboard design on user posture (Serina et al.,
1999; Smith et al., 1998; Zecevic et al., 2000). See the glossary for definitions of frequently cited
postures. One of these studies showed that even on an ‘ideal’ workstation set-up with a standard
keyboard, the majority of subjects typed with a mean wrist extension angle greater than 15° and more
than one-fourth of subjects typed with a mean wrist ulnar deviation angle greater than 20° (Serina et
al., 1999). Smith et al. (1998) showed that the adjustable split keyboard reduced wrist/hand
pronation. Zecevic et al. (2000) showed that the adjustable split keyboard and contoured keyboard
reduced wrist extension compared to the standard design. Also, the contoured split keyboard
significantly reduced ulnar deviation while the adjustable split keyboard significantly increased
radial deviation. Finally, more time was spent in neutral and moderate ranges of extension/flexion
and radial/ulnar deviation when typing on the contoured split than on the adjustable split or standard
keyboards. A summary of the results from these studies can be found in Table 3.2.
Finally, two of the eight studies researched a possible link between keyboard design and worker
discomfort (Smith et al., 1998; Swanson et al., 1997). However, results from both studies show no
significant relationship between keyboard design and discomfort development.
Current research demonstrates the advantages and disadvantages associated with alternative
keyboards. The main disadvantage of alternative keyboards documented by the research is the loss
of productivity. However, two studies concluded that most of the lost productivity could be recovered
within two hours of work on the new keyboard. The main advantage shown by this research is that
alternative keyboards help the worker to maintain neutral hand and wrist postures, which have been
shown to decrease risk for CTD development.
21
Table 3.2: Effects of keyboard design on user posture
Postures
Contour
Standard
Study
Split-Adjustable
Keyboard
Neutral
Flexion/Extension
1
At an ‘ideal’
workstation,
most subjects
typed with
extension more
than 15°
Serina et
al., 1999
Radial/Ulnar
Deviation
Pronation/
Supination
At ‘ideal’
workstation1,
more than 1/4
subjects typed
with ulnar
deviation more
than 20°
Split keyboard
reduced
wrist/hand
pronation
Smith et
al., 1998
Zecevic et
al., 2000
More time spent
in neutral
posture when
typing on the
contoured split
than on the other
designs
Alternative
keyboards
reduced wrist
extension
compared to
standard
-Adjustable split
keyboard
significantly
increased radial
deviation.
-Contoured split
keyboard
significantly
reduced ulnar
deviation
Alternative
designs reduced
pronation
compared to the
standard design
1
Ideal workstation defined as a computer workstation adjusted to the subject’s body dimensions. The workstation was
configured according to the ANSI/HFES 1988 guideline.
3.1.1.2
Keyboard Dose – Response Relationship
Intensive keying tasks involve frequent, repetitive hand and finger movements, which place typists at
a high risk for developing CTDs. In order to evaluate the link between keyboard use and repetitive
injuries, some researchers are investigating the link between time spent using the keyboard and
musculoskeletal discomfort.
Three studies examined the effects of time spent keying and body discomfort (Hanson et al., 1997;
Smith et al., 1998; Swanson et al., 1997). Hanson et al. (1997) shows that after adjusting for age and
gender, the most significant factor associated with symptoms of upper limb disorders is the length of
time spent at the keyboard during the workweek. Swanson et al. (1997) also shows a link between
typing duration and discomfort by reporting a significant increase in worker discomfort across a
single workday as well as reporting significant increases in discomfort across the three days of the
study. Smith et al. (1998) offer similar results. In a comparison of alternative keyboard designs,
Smith et al. (1998) report that all subjects experienced pain at the end of the trial regardless of the
type of keyboard used. Thus, offering support for the relationship between discomfort and keying
duration rather than discomfort and keyboard style.
22
While these studies do not give a quantifiable dose-response relationship linking a specific duration
to the onset of discomfort, they do show that musculoskeletal discomfort is proportional to the
duration of time spent using the keyboard.
3.1.1.3
Wrist Rests
Wrist rests, alternatively referred to as palm rests, are primarily used to protect wrists from resting on
sharp edges on work surfaces while not actively engaged in typing activities. However, in many
cases workers misuse the wrist rests by using them continuously during keying tasks. The
effectiveness of wrist rests has been debated as wrist rests can be seen as both beneficial and
harmful. In turn, researchers have begun to study the benefits and risks associated with wrist rest
usage.
Four of the studies surveyed examined the effects of using a wrist rest while typing (Smith et al.,
1998; Fernstrom et al., 1994; Parsons, 1991; Keller et al., 1998). Smith et al. (1998) focused on the
effects of a wrist rest on productivity, posture, and discomfort. It was determined that when a wrist
rest was not used, performance on the first and second days of using the keyboard was very similar.
However, when a wrist rest was used, performance on the second day of using the keyboard was
considerably higher. In regards to posture, more left hand extension was found in users who did not
use the wrist rest than those who did. Participants reported feeling more in control of their typing
when using wrist rests than without. Finally, participants who did not use a wrist rest reported more
pain in the front of their shoulders and more pain at the outside of their elbow/forearm than those who
did use a wrist rest. The research conducted by Keller et al. (1998) also promotes wrist rest use after
documenting that muscle activity for wrist rest users was half the amount seen in non-wrist rest users.
In contrast to these results, Fernstrom et al. (1994) concluded that palm rests do not decrease strain
in the forearms or shoulders. Parsons (1991) completed a study evaluating nine different types of
wrist rests. At the end of the study only four of the forty subjects found the wrist rests useful in
decreasing discomfort, the remaining thirty-six subjects did not find them useful, and seven found
that discomfort increased while using the wrist rests. The contradicting results reported by these
research studies show that wrist rests should undergo further evaluation.
3.1.1.4
Keystroke Makeforce
Keystroke makeforce is another factor in keyboard design. Makeforce is the amount of force required
to depress the key on a keyboard to its activation point. Research studies discussed below report the
effects of various levels of makeforce on the finger flexor muscles.
Three of the twenty studies on keyboard design studied the effects of keystroke makeforce
(Armstrong et al., 1994; Rempel et al., 1997; Thompson et al., 1999). Armstrong et al. (1994)
determined that subjects use 2.5 to 3.9 times the required activation force to depress the keys. The
average keying force was lowest for the keyboard with lowest required makeforce. Thompson et al.
(1999) study results support findings by Rempel et al. (1997) that subjects generally apply two to
three times the force required to depress the key completely. The study conducted by Rempel et al.
(1997) shows that no differences in applied fingertip force or finger flexor EMG were observed during
typing on keyboards with makeforce of 0.34 or 0.47 N. Also, applied fingertip force increased by
approximately 40% when the keystroke makeforce was
23
increased from 0.47 to 1.02 N. Finally, EMG activity increased by approximately 20% when the
keystroke makeforce was increased from 0.47 to 1.02 N. Therefore, keyboards with a keystroke
makeforce of less than or equal to 0.47 N are recommended over a keyboard with a makeforce of 1.02
N.
3.1.1.5
Keyboard Location
Five of the studies looked at the effects of keyboard location on posture and musculoskeletal
discomfort (Sauter et al., 1991; Wolstad et al., 1993; Black et al., 1997, Simoneau et al., 2001;
Gilad, 2000). Keyboard location is comprised of three main components:
• Height
• Tilt
• Position in front of user
Two of the studies looked at keyboard height and its effects on discomfort. The research conducted
by Sauter et al. (1991) concluded that arm discomfort increases as the keyboard is raised above
elbow level. Black et al. (1997) studied keyboard location by using EMG. The results of this study
support the conclusions of Sauter et al. (1991). As the keyboard height was raised the muscle
activity of the trapezius and shoulder flexors increased. The biceps and deltoids had the lowest EMG
values at elbow height. In this study it was also concluded that the increase in trapezius muscle
activity correlated with discomfort levels. Therefore, this study also supports locating the keyboard
at or below elbow height. Simoneau et al. (2001) studied the effects of keyboard height on wrist
extension angle. This study found that wrist extension angles decrease as keyboard height increases
above elbow height.
Keyboard location is also affected by keyboard tilt. Simoneau et al. (2001) not only looked at the
effects of keyboard height on the wrist extension angle, as discussed previously, but also looked at
the effects of the slope of the keyboard on wrist posture. In this study, it was shown that wrist
extension angles reached a minimum of 9° at a negative 15° keyboard slope, supporting a negatively
tilted keyboard. Gilad (2000) also studied the effects of a downward tilting keyboard on working
conditions. The results of this study show that the downward tilting keyboard with a negative 10°
slope improved the quality of the work and decreased forearm muscle activity. However, users
preferred the flat keyboard position to the negative tilt position. Wolstad et al. (1993) studied the
relationship between keyboard height and slope and the effects on wrist posture. It was concluded
that the minimum deviations were seen when higher keyboard heights were coupled with positive
slopes, and the least deviation for lower keyboard heights was observed when coupled with negative
slopes.
Even in studies that did not focus directly on keyboard position, an ideal keyboard location was
discussed. When testing the effects of various factors, such as different styles of alternative
keyboards, the researchers positioned the keyboard in a neutral posture as defined by the user’s
anthropometry. This neutral posture is described in most cases as allowing the upper arms to fall
comfortably by the user’s sides while allowing the forearm to remain parallel to the floor. The
keyboard is also positioned directly in front of the user as determined by shoulder breadth. Wrist
angles and hand positions were often affected by the keyboard design, but the main goal in finding
the optimal keyboard location was to find a position where the user could maintain neutral postures
in the entire upper body. A summary of the results from these studies can be found in Table 3.3.
24
Table 3.3: Effects of keyboard location
Sauter et al.,
1991
Black et al.,
1997
Position
Tray Tilt
Study
Height
Keyboard
Placement
Measurement
Discomfort
Muscle Activity
Wrist Posture
Arm discomfort
increases as the
keyboard is raised
above elbow level
Muscle activity of the
trapezius/ shoulder
Increase in trapezius flexors increases as
muscle activity
keyboard height
correlated with
increase. Biceps
discomfort levels
and deltoids had
lowest EMG values
at elbow height.
Simoneau et
al., 2001
Wrist extension
angles decrease as
keyboard height
increases above
elbow height Wrist
extension angles
reached a minimum
of 9° at a negative
15° keyboard slope
Gilad 2000
Downward tilting
keyboard with a
negative 10° slope
decreased forearm
muscle activity
Wolstad et
al., 1993
Minimum deviations
were seen when
higher keyboard
heights were
coupled with positive
slopes, and the least
deviation for lower
keyboard heights
was observed when
coupled with
negative slopes
3.1.2
Work Quality
Downward tilting
keyboard with a
negative 10° slope
improved the quality
of the work
Standards
Table 3.4 summarizes existing standards and guidelines on keyboard design. Both CSA-Z412 and
OSHA offer general recommendations without quantifiable details. However, the ISO 9241 and the
BSR/HFES 100 standards include very detailed information on keyboard design and characteristics.
Certain topics covered by the standards contain technical information beyond the scope of this paper.
In these cases the specific sections of the standards were excluded from the following table and
resulting discussions. Examples of excluded standards include the ISO and HFES standards on the
distances between keys, keytop design, key displacement distances, and keytop legends.
25
Table 3.4: Keyboard guidelines outlined in ergonomic standards
Input Devices Keyboards
Alternative
Designs
Standard
CSA-Z412
BSR/HFES 100
Draft
Recommendations
•
Evaluate with usability testing
• For keyboards with attached palm rests, the rests should be
matched to the width, height, and shape of the front edge of the
keyboard
• Rests are considered optional because they may impede motion
during typing
Wrist Rests
CSA-Z412
ISO 9241
OSHA
BSR/HFES 100
Draft
Keyboard
Height and
Position
CSA-Z412
OSHA
BSR/HFES 100
Draft
Key force
ISO 9241
BSR/HFES 100
Draft
Keying
feedback
ISO 9241
•
Resting heel of hand on wrist supports while keying is not
recommended
• Palm rests attached to the keyboard should have a depth of 2 to
4 in. (5 to 10 cm) in front of the bottom row of characters
• Wrists or hands do not rest on sharp or hard edge
• Keyboard should be placed within the space delimited by the
user’s forearm length and shoulder breadth
• Should allow worker to maintain neutral seated posture defined
by: Elbow angles between 70o and 135o, Shoulder abduction
o
o
less than 20 , Shoulder flexion less than 25 , Wrist flexion less
o
o
than 30 , Wrist extension less than 10 , Torso to thigh angle
o
greater than or equal to 90
• Keyboards should be located at height that allows the worker to
key with the upper arms hanging relaxed from the shoulders, the
elbows at roughly right angles, and to allow the wrists to be fairly
straight
• Forearms, wrists, and hands should be straight and parallel to
the floor, while also keeping hands and wrists straight (not bent
sideways toward the little finger)
• Force to activate keys should be between 0.25 and 1.5 N, but
preferably between 0.5 and 0.6 N
• Initial resistance shall be between 25% and 75% of the force at
the character generation point
• Force at the character generation point should be between 0.5
and 0.8 N
• Actuation of a key shall be accompanied by tactile feedback,
auditory feedback, or both
• Actuation of a key shall be accompanied by feedback. Can be a
combination of kinesthetic, auditory, or visual
• Auditory feedback should occur within 100 ms after key
activation
26
3.1.3
Recommendations
What advantages/disadvantages do alternative keyboard designs have over standard designs?
As shown in the research, some alternative keyboards have been shown to improve wrist posture.
However, keyboards should be selected on an individual basis where the chosen keyboard style
allows the worker to keep a neutral wrist posture while typing. In some individual cases, a standard
keyboard may be sufficient, but others may require one of the alternative keyboard designs.
When alternative keyboards are necessary, users should expect a period of decreased productivity as
they adjust to the new design. As shown in the research, workers using common alternative designs
can regain most of their productivity in approximately two hours.
What is the relationship between duration of keyboard use and musculoskeletal discomfort?
Research focusing on the dose-response relationship between time spent keying and discomfort has
shown that musculoskeletal discomfort increases as the user spends more time typing. However, no
research has been conducted to show the exact point at which users begin to experience discomfort.
While these studies do not give a quantifiable dose-response relationship, they do show that
musculoskeletal discomfort is proportional to the duration of time spent using the keyboard.
Therefore, to decrease discomfort users should spend less time typing or utilize a work rest pattern as
discussed in Chapter 6.
Are wrist rests beneficial during keying tasks?
Research studies have produced contradicting results, as have the current ergonomic standards.
However, some recommendations concerning wrist rests can be made. Wrist rests should be
optional, they should be used to protect the wrists from resting on sharp edges or work surfaces while
not actively engaged in typing activities, and most importantly, the rests should not interfere with
keying tasks or cause the user to assume awkward postures.
What effect does keystroke makeforce have on the musculoskeletal system?
Users commonly exert about two to three times the necessary fingertip force when typing. To reduce
the applied force several steps should be taken. According to current research, keyboards should
require no more than 0.47 N of keystroke makeforce. In addition, keying feedback should include
tactile and/or auditory response. When tactile feedback from the keyboard and a low keystroke
makeforce are combined, users are able to minimize the amount of force they apply.
What is the optimal location for the keyboard?
Current research and ergonomic standards encourage keyboard placement in a location that allows
users to maintain a neutral seated posture while typing. This neutral posture is described in most
cases as allowing the upper arms to fall comfortably by the user’s sides while allowing the forearm to
remain parallel to the floor. The keyboard is also positioned directly in front of the user as
determined by shoulder breadth. The BSR/HFES 100 Draft Standard offers a more detailed
description of a neutral seated posture as seen in Table 3.3. However, recent research has shown
that discomfort decreases when the keyboard is located slightly above elbow level. Additionally,
27
wrist posture improves when the keyboard has a slight negative tilt. In most cases a keyboard tray or
height adjustable workstation will be needed to position the keyboard correctly. Further
recommendations and workstation design implications can be found below in Tables 3.5 and Table
3.6.
Alternative
Keyboards
Keying
Duration
OSHA
Research
ISO
CSA
BIFMA
Topic
BSR/HFES
Table 3.5: Keyboards: Recommendations based on current standards and research
Recommendations given
research
Keyboards should be selected
on an individual basis
Goal of Recommendation
The chosen keyboard style
should allow the worker to keep
a neutral wrist posture while
typing
Discomfort is proportional to the
duration of time spent using the Decrease worker discomfort
that results from time intensive
keyboard. Therefore, to
decrease discomfort users
keying tasks
should spend less time typing
Protect wrists from sharp edges
on work surfaces while not
actively engaged in typing
activities
Wrist Rests
Use of wrist rest is optional.
When used, wrist rests should
not interfere with typing
Keystroke
Makeforce
Keystroke makeforce should be To reduce finger flexor muscle
no more than 0.47N
activity during typing tasks
Keyboard
Height
Keyboards should be located at
height that allows the worker to
key with the upper arms
hanging relaxed from the
shoulders and the elbows at
roughly right angles, to allow
the wrists to be fairly straight
Should be combined with
keyboard distance to allow
worker to maintain neutral
postures while typing
Keyboard
Distance
Keyboard should be placed
within the space delimited by
the user’s forearm length
Should be combined with
keyboard height to allow worker
to maintain neutral postures
while typing
Keyboard
Position
Keyboard should be placed
within the space delimited by
the user’s shoulder breadth.
To decrease ulnar and radial
deviations of the wrists
Keyboard Tray
Tilt
Keyboard tray should have a
slight negative tilt.
To maintain neutral postures.
- Guideline or standard reasonably agrees with given recommendation
28
Table 3.6: Keyboards: Workstation design implication for recommendations in Table 3.3
Topic
Recommendations
Implication for Workstation Design
Keyboards should be
located at height that
allows the worker to key
with the upper arms
Keyboard tray should be located at a height between 6.5 in.
Keyboard Height hanging relaxed from the (16.5 cm) (95th% male) and 10.5 in. (26.7 cm) (5th% female)
1
shoulders and the
from the top of the user’s chair seat pan
elbows at roughly right
angles, to allow the
wrists to be fairly straight
Keyboard
Distance
Keyboard should be
placed within the space
delimited by the user’s
forearm length and
shoulder breadth
Keyboard tray should be located between 10.9 in. (27.7 cm)
(5th% female) and 15.7 in. (39.9 cm) (95th% male) from the
back of the user’s elbow when forearms are positioned parallel
1
to the floor
Keyboard Tray
Tilt
Keyboard tray should
have a slight negative tilt
to keep wrists in a
neutral posture
Keyboard tray should have a slight negative tilt
Use of wrist rest is
optional. When used,
wrist rests should not
interfere with typing
If wrist rests are used, they should fit the width and height of
the keyboard, so as not to impede motion during typing or
cause the user to assume awkward postures
Wrist Rests
1
values based on anthropometric data from US military personnel in the 1970’s
3.1.4
Limitations
Researchers are dependent upon subject opinion when quantifying musculoskeletal discomfort. As a
result, research on keyboard use may be biased by personal preferences. Though three of the studies
show a strong link between keyboard usage and muscular discomfort, insufficient research has been
performed to establish a dose-response relationship. A more systematic approach is needed in order
to establish this relationship.
Productivity as well as subjective rating of discomfort may be influenced by feelings of boredom or
stress among subjects. Most of the research comparing alternative keyboards to standard keyboards
takes place in the laboratory over a short period of time. In general, the subjects involved in this
type of research are not familiar with the alternative keyboards used in the test. Therefore, they
spend the majority of the testing period adjusting to the new arrangement. Since workers are not
working at full productivity with the alternative keyboards, it is difficult to compare aspects of
keyboard use, such as discomfort, between different types.
3.1.5
Future Research
Time spent using the keyboard has been linked to increased complaints of musculoskeletal
discomfort. However, current research has been unable to pin point the exact link between duration
of keying tasks and body discomfort. Therefore, a more systematic research should be conducted to
determine how long workers are able to type before experiencing discomfort and perhaps the effects
of breaks on the amount of discomfort experienced.
29
Research has also been conducted to compare alternative designs to standard keyboards. These
comparisons would be more beneficial if the workers could perform proficiently on all keyboard types
before beginning the experiment. The trials should include a significant period of time for the
subjects to become accustomed to the new designs before beginning the actual comparisons.
The effectiveness of wrist rests has been debated as wrist rests can be seen as both beneficial and
harmful. Research studies have produced contradicting results, as have the current ergonomic
standards. Further research should be completed to determine a definitive conclusion on the
effectiveness of wrist rests.
New technologies should also be investigated in future research. Some of the new technologies
include passive motion keyboard trays and recent modifications to alternative keyboard designs.
3.2
Pointing Devices
Consumers are no longer limited to the small two-button mouse that comes standard with most
computers. Flat, curved, and upright, manufacturers have unleashed many “ergonomic” mice on the
market. In addition to “ergonomic” mice, consumers may chose from a variety of trackballs, touch
pads, and graphics tablets and even specialty input devices controlled by the feet or eyes. With so
many choices on the market, research is needed to determine which products best meet the needs of
computer users.
3.2.1
Research
Eighteen articles concerning pointing devices in office environments were reviewed. Pointing device
research strives to answer one or more of the following questions:
1. Does using a pointing device increase your risk of developing an MSD?
2. What is the optimal pointing device design to reduce upper extremity discomfort?
3. Where should a pointing device be located to reduce upper extremity discomfort?
3.2.1.1 Discomfort and Pointing Devices
Researchers are trying to establish if pointing device use increases a worker’s risk of developing
musculoskeletal discomfort and injuries. MSD development is typically associated with work-related
risk factors such as high force, awkward posture, high repetition, and static loading. Possible risk
factors that occur during pointing device usage are sustained awkward postures and repetitive
finger/wrist movements (Armstrong et al., 1995). Though an exact dose-response relationship is yet
unknown, links between pointing device use and discomfort have been identified by several
researchers (Cook et al., 2000; Hagberg, 1995; Haward, 1998; Jensen et al., 2002). Physiological
evidence supports this link. Carpal tunnel pressures greater than pressures known to alter nerve
function and structure (28.8-33.1 mmHg) have been recorded among subjects performing intensive
mousing tasks (Keir et al., 1999). Increased carpal tunnel pressures have been associated with
increased risk of developing CTS. Other physiological stresses have been associated with pointing
device use, for example, muscle loads of the shoulder (anterior deltoid) increase with pointing device
use (Cooper and Straker, 1998).
Further evidence of ties between pointing device use and increases in health risks come from posture
research. It is commonly accepted that working in awkward postures increases an individual’s
30
likelihood of developing an MSD. In one comparison of office employees or data entry personnel,
pointing device users spent 30% of working time with ulnar deviation of the wrist greater than 30°
and 64% of working time with at least 15° of ulnar deviation. On the other hand, non-pointing device
users spent 96% of working time in neutral postures with only slight radial deviation (Karlqvist et al.,
1994). Additionally, the same study found that pointing device users spent 81% working time with
the shoulder of their mousing arm rotated outward more than 30° while non-pointing device users
spent 100% of working time with shoulder in neutral position towards inward rotation (Karlqvist et
al., 1994).
However, field researchers have not been able to associate increased musculoskeletal symptoms to
pointing device use. There has been no observable correlation between the percent of time spent
using a pointing device and symptoms (Cook et al., 2000; Haward, 1998). Frequent computer users
who use a pointing device only experience non-significant increases in upper extremity disorders
over frequent computer users who do not use a pointing device (Blatter and Bongers, 2002). This
research suggests that the link between pointing device use and MSDs previously observed by some
studies may be due to non-pointing device specific risk factors such as stress or screen height.
3.2.1.2 Pointing Device Design
Trackballs, alternative mice, and track points are only three types of pointing device designs
available to today’s computer user. Other pointing devices, such as touch screens and writing
tablets, generally only used by specialty industries are not discussed here.
Trackballs.
Research has shown that while trackballs have some physiological and postural
advantages, their use has not been shown to reduce musculoskeletal symptoms or muscle fatigue
compared to mice (Haward, 1998). Trackball use has been observed to lower overall muscle tension
levels in the shoulder (Harvey and Peper, 1997; Karlqvist et al., 1999). For example, trackballs
have also been shown to reduce the amount of ulnar deviation, wrist flexion, and shoulder elevation
among computer users (Burgess-Limerick et al., 1999; Haward, 1998; Karlqvist et al., 1999;
Wardell and Mrozowki, 2001). However, the benefit achieved from these reductions may be offset by
increased exposures to extreme wrist extension also observed among trackball users (BurgessLimerick and Green, 2000; Karlqvist et al., 1999).
Alternative Mice. In general, mouse use has been associated with smaller wrist extension compared
to trackballs (Burgess-Limerick et al., 1999). Alternative designs vary in their ability to improve
posture and reduce discomfort. Contoured mice, designed to fit the user’s hand, have not been shown
to have significant effects on carpal tunnel pressure or wrist posture (Keir et al., 1999). The only
exceptions to these findings are the whale and joystick mouse, which produced more neutral postures
when compared to a standard mouse (Fernström and Ericson, 1997; Aarås and Ro, 1997). In
addition, the joystick mouse has been shown to reduce forearm muscle loads (Aarås et al., 2001).
Overall, mouse position (see section 3.2.1.3) seems to have more of an effect on posture and
discomfort than does mouse style (Wardell and Mrozowki, 2001).
Trackpoints. Trackpoints are touch pads imbedded in the keyboard itself. Though trackpoints may
decrease shoulder muscle load, they have also been shown to increase forearm load as well as finger
flexor activity when compared to standard mice (Fernström and Ericson, 1997). The position of
trackpoints has also been shown to significantly affect user posture, neck and shoulder discomfort,
and perceptions of performance (Kelaher et al., 2001). For subjects who use their right hand with
31
peripheral input devices, trackpoints located in the bottom right-hand corner or on the right side of a
notebook computer were better than the traditional bottom center location of most trackpoints on
notebook computers. Research has shown that position is an important factor for other pointing
devices as well and will be discussed in the next section.
3.2.1.3 Pointing Devices Position
Researchers have used posture analysis and EMG measures to quantify the effects of mouse position
on working posture and muscle activity. Research has shown that even the four inches gained by
using a keyboard without a numeric keypad significantly improves working posture and reduces
muscle activity among right handed mouse users (Cook and Kothiyal, 1997). Though not significant,
muscle activity in the trapezius has also been reduced by moving the mouse closer to the body (Cook
and Kothiyal, 1997). However, lower EMG values occurred with a trackball in front of the user over a
mouse to the right of the user (Harvey and Peper, 1997). Though this improvement may have been
due to the pointing device design instead of position.
Forearm support may be one solution for reducing muscle load while using a pointing device.
Supporting the forearm while using a pointing device has been shown to reduce the muscle load in
the neck and shoulder (Aarås and Ro, 1997; Karlqvist et al., 1999).
3.2.2
Standards
ISO, OSHA, CSA, and BSR/HFES have all released recommendations concerning pointing devices as
seen in Table 3.7. The majority of the recommendations concern button force and displacement,
which was not covered by the studies examined in this paper. However, the research does support
the conclusions of all four organizations on input device location.
32
Table 3.7: Input device guidelines outlined in ergonomic standards
Type
Standard
Recommendations
•
BSR/HFES 100
Draft
Input device
location
CSA-Z412
ISO 9241
OSHA
Mouse Fit
•
•
Place at same height as keyboard and as close to the keyboard
as possible
•
Design of an input device should allow it to be located and be
accessible within the user’s reach envelope
•
•
Located close to keyboard for operation without reaching
Wrists and hands do not rest on sharp or hard edge
BSR/HFES 100
•
Draft
OSHA
CSA-Z412
ISO 9241
Shape fits hand of employee
•
The diameter of a desktop mounted trackball should be between
2 and 6 in. (5 cm and 15 cm)
o
The exposed surface should be between 100° and 140
Resistance of trackball should be less than 1.0 N and preferably
less than 0.3 N
•
•
•
•
Exposed arc of 100° to 140°
Starting resistance of 0.2-0.4 N
Rolling force of 0.2-1.5 N
Exposed area of trackball at least 1 in. (2.5 cm)
•
Chord length of the exposed area should be at least 1 in. (2.5
o
o
o
cm). Exposed arc should be between 100 - 140 (120
recommended)
Rolling force should be between 0.2-1.5 N
Starting resistance between 0.2-0.4 N
•
•
BSR/HFES 100
•
Draft
Button Force
Mouse shape and size shall allow single handed operation
•
BSR/HFES 100
•
Draft
•
Trackballs
Input device should be placed within the space delimited by the
user’s forearm length and shoulder breadth
Should allow worker to maintain neutral seated posture defined
by: Elbow angles between 70o and 135o, Shoulder abduction less
o
o
than 20 , Shoulder flexion less than 25 , Wrist flexion less than
o
o
30 , Wrist extension less than 10 , Torso to thigh angle greater
o
than or equal to 90
Button displacement force should fall between 0.25-1.0 N
CSA-Z412
•
Button displacement force should fall between 0.5-1.5 N
ISO 9241
•
Displacement force should be within range of 0.5-1.5 N
OSHA
•
Easy to activate
•
BSR/HFES 100
Draft
•
Button
Displacement
•
CSA-Z412
•
•
Displacement should be between 0.6 and 0.2 in. (0.15 and 0.2
cm)
Activating buttons shall not cause inadvertent movements of the
input device
Displacement should be between 0.02 and 0.2 in. (0.05 and 0.6
cm)
Button should be designed so it can be activated without causing
the fingers to deviate excessively from neutral posture
Pressing the buttons should not reduce control of the device
33
3.2.3
Recommendations
In general, neutral postures decrease the risk for MSD development. During mousing, better posture
can be achieved when the mouse is located as close to the body as possible. This requires a
workstation designed to fit the worker. An example of a workstation designed around the user
regarding the pointing devices, is selecting a keyboard with no attached numeric keypad for an
individual whose work requires an equal combination of typing and mousing. Breaking up intensive
pointing device use with frequent breaks may also help reduce upper extremity discomfort by
providing time for recovery. In addition, pointing device users may increase available upper
extremity recovery time by alternating pointing device use between the left and right hands. Research
has not identified one particular type of pointing device that consistently helps workers maintain
proper posture or reduces discomfort. Therefore, workers should be evaluated on an individual basis
to determine which type of pointing device best meets their needs. Recommendations pertaining to
pointing device use and placement based on standards and research are highlighted in Table 3.8.
Research
OSHA
ISO
CSA
BSR/HFES
Topic
BIFMA
Table 3.8: Pointing Device: Recommendations based on current standards and research
Recommendations given
research
Pointing Device
Design
Pointing device design should
fit the user’s needs and
anthropometry
Pointing Device
Use
Break up pointing device use
To give the body recovery
between the left and right
hands or by taking short breaks time and reduce fatigue
Pointing Device
Placement
Place at same height as
keyboard and as close to the
keyboard as possible
To minimize awkward hand
grips
To maintain upper body in a
neutral posture
3.2.4
Limitations
Current pointing device studies have been limited by small sample sizes. In order to take into
account the wide range of individual differences among computer users it is necessary to study a
large group of individuals. However, none of the studies reviewed have used more than twenty-four
subjects. In addition, the reviewed research used trial lengths of less than thirty minutes. Such short
trial lengths may not give users enough time to become accustomed to alternative pointing device
designs. As a result, users may not adopt the same working habits they would if they used the mouse
for several days or even several hours.
34
3.2.5
Future Research
Ideally, research should identify a dose-response relation between mouse use and MSDs. This would
necessitate a large scale and expensive research endeavor. Smaller studies could be performed
systematically comparing the alternative pointing devices on the market. Many product evaluations
only examine one or two measures per study. Measurements of discomfort, productivity, posture, and
muscle activity should all be collected on new pointing devices to fully ascertain the advantages and
disadvantages of the new design. Product evaluations should be performed on a wide range of
subjects over trial periods long enough for subjects to familiarize themselves with the new design.
One way to allow subjects enough time to grow accustomed to a new design while minimizing costly
laboratory time would be to select computer operators as subjects. For example, the subjects would
be given the new design to use in the workplace for a workweek after which the user would perform
controlled pointing tasks in a laboratory environment.
Future studies could also examine the relationship between pointing devices and potential
confounding factors including: task type, task variety, organizational factors, psychosocial factors,
and individual factors. For instance, are different designs better for different tasks? How does the
success of a pointing devices design vary by gender, age, or profession? Eventually researchers
must answer all of these questions to obtain a greater understanding of the characteristics and impact
of pointing devices in the workplace.
35
36
4 Seating
Seating is a primary focus of every office environment. It is the first workstation element that should
be adjusted to fit the user in order to provide maximum blood circulation throughout the body (while
seated). In an ideal world, once the chair adjustments are made, the rest of the workstation can be
adjusted to the employee in the chair. Therefore, correct seating is a critical component in every
ergonomic office setting and as a result has been the source of significant ergonomic research.
In an effort to better understand the effects of chair characteristics, researchers are focusing on the
relationship between chair designs and musculoskeletal discomfort, energy consumption, contact
pressure between the body and the chair, and awkward postures. Based on the premise that
minimizing these stressors leads to increased productivity and reduced work-related injuries, the
result of such changes is expected to result in cost savings. The majority of reported discomfort while
seated occurs in the back (Bendix et al., 1988; Bendix et al., 1996; Coleman et al., 1998; Dieen et
al., 1997). However, inadequate seating can also lead to discomfort in other areas of the body
including the shoulders, arms, and legs. The following section covers research on several seating
features as they relate to discomfort as well as the influence of task variability on seat design.
4.1
Research
Twenty-five studies on the effects of office seating were included in this review. Of those, seven were
field studies while the remaining eighteen were performed under more controlled laboratory settings.
The studies focused on general office tasks, ranging from reading and writing to VDT work. A
summary of the articles results as well as an overall rating of each article based on the criteria given
in section 4.2 may be found in Appendix C.
Investigations in office seating research have addressed one or more of the following questions:
1. How does backrest inclination affect worker discomfort?
2. How does seat pan inclination affect worker discomfort?
3. How does lumbar support affect worker discomfort?
4. How does arm support affect worker discomfort?
5. Why should task variability be considered in office seating?
These issues were quantified using a variety of methods that are summarized in Table 4.1. For a
detailed itemization of the methods used to quantify different chair features, refer to
Appendix C.
37
Table 4.1: Methods for quantifying dependent variables
Dependent Variable
Methods of Quantification
Discomfort
• Discomfort surveys
Muscle Activity
• EMG
• Strain gauges
• Pressure mapping
Posture
• Statometric measures
• Infrared emitting diodes - Optoelectronic
movement recording system
Emotional State
• Questionnaires
• Heart rate
Among the four dependent variables listed above, the primary focus of recent chair research has been
the effect on worker discomfort. Discomfort is commonly qualified through discomfort surveys, in
which the subject is asked to subjectively quantify the amount of pain felt in one or more body parts.
EMG measurements, strain gauges, and pressure mapping are methods used to assess measuring
muscle activity, fatigue, and pressure points. Typically, the more muscle activity required or the
faster a muscle fatigues, the more discomfort that muscle will experience (Soderberg, 1986; Dieen,
1997). It has been shown through EMG ratings and discomfort surveys that areas with highest levels
of muscle activity correspond to areas of greatest discomfort. Also, areas of high-pressure
concentrations have restricted blood flow, which can result in focal points for discomfort. The
principal chair features associated with discomfort in office seating research were backrest
inclination, seat pan inclination, lumbar support, and arm support. Task variability was also found to
have an effect on office seating.
4.1.1
Backrest Inclination
Backrest inclination is a common feature among most ergonomic office chairs and is a factor in
worker comfort. Backrest inclination adjustability has been associated with decreased discomfort,
and was preferred by the majority of users over fixed backrest seating (Bendix et al., 1988; BurgessLimerick et al., 2000; Treaster and Marras, 1987; Vergara and Page, 2000; Vergara and Page,
2002). The traditional notion that the trunk and thighs should be perpendicular (90°) while seated
was also challenged by research (Burgess-Limerick et al., 2000). Three of the five articles
concluded that as the angle between the torso and thighs (trunk angle) increased, the amount of
musculoskeletal discomfort decreased, especially on the spine (Bendix et al., 1988, BurgessLimerick et al., 2000, Treaster et al., 1987). Another study concluded that back discomfort
corresponds to the amount of dorsal contact with the backrest (Vergara and Page, 2000).
Investigations of backrest inclinations consisted of trunk angles ranging from 90° to 120°. Even
though these studies made general recommendations on trunk angles, an optimal backrest inclination
for typical office work was not specified.
4.1.2
Seat Pan Inclination
A seating feature that has similar effects as the backrest inclination is the slope at which the seat pan
is tilted while seated (seat pan inclination). A positive seat pan inclination will be defined as one
38
with a forward tilt (e.g. at the extreme, the individual would be dumped out of the chair). Similarly, a
negative seat pan inclination will be defined as one with a backward tilt (e.g. at the extreme, the
individual would feel tucked in the chair). A positive seat pan inclination increases the trunk angle,
thus decreasing worker discomfort just as discussed in the above section. A concern is the extent at
which the seat pan is inclined. Backrest and seat pan inclinations can be adjusted in combination
with each other in order to adapt to the needs of the individual and task performance.
Ten studies examined the correlation between seat pan inclination and discomfort. Eight of the ten
investigations suggested the need for seat pan angle adjustability (Bendix and Biering-Sørensen,
1983; Bridger, 1988; Congleton, 1987; Naqvi, 1994; Rogers et al., 1990; Snijders, 1995; Soderberg
et al., 1986; Deursen, 2000). A positive seat pan inclination has been shown to increase
productivity, improve body comfort, and reduce buttock-thigh pressure resulting in increased blood
circulation (Congleton, 1987). Research also showed that the maximum trunk angle studied resulted
in the greatest worker comfort, increased assembly rates, and produced the least EMG activity
(Rogers et al., 1990; Naqvi, 1994; Bridger, 1988; Soderberg et al., 1986). Since none of the
reviewed research examined the effects of a negatively inclined seat pan, one of two interpretations
may be appropriate. First, negative effects result from such an adjustment or second, further studies
need to be performed to justify negative seat pan inclinations.
A more recent area of research under investigation in seating is the introduction of rotary continuous
passive motion (RCPM). Continuous passive motion (CPM) was originally used as a post-operative
treatment method used to aid recovery after joint surgery to prevent the development of scar tissue
and increase the range of motion. However, new applications of CPM to ergonomics use motorized
devices to gradually move joints through a prescribed range of motion for an extended period of time.
One study applies this method to office seating in order to address the effects of oedema (Deursen et
al., 2000). Oedema is swelling of the lower limbs resulting from an increase in venous pressure
caused by prolonged sitting. In the long run, this can lead to muscle inefficiency in the legs,
especially in the calf muscles. The study found significant differences in lower leg swelling between
the static and the dynamic seat pan. It was concluded that dynamic stimulation from RCPM during
sitting has an oedema reducing effect on the lower extremities. The dynamic stimulation also
resulted in spinal length increases among subjects as compared to normal shrinkage during an hour
of normally supported sitting. Applying CPM to office seating is a new and exciting concept offering
enormous potential for the future in reducing musculoskeletal discomfort, work-related injury, and
therefore boosting productivity (Deursen et al., 2000).
4.1.3
Lumbar Support
The correlation between lumbar supports and musculoskeletal discomfort has been an area of prime
interest in the evaluation of chair features and designs (Bendix et al., 1996; Coleman et al., 1998;
Hermans et al., 1999; Porter and Norris, 1987; Vergara and Page, 2000; Vergara and Page, 2002).
The research as a whole indicates that people have different needs for lumbar supports depending on
certain characteristics such as height, weight, health, and gender. The data on lumbar support height
collected from research is summarized in Appendix C.
Height. Taller subjects preferred higher lumbar supports and vice versa (Bendix, 1996).
Weight. Higher lumbar supports were chosen by subjects with a greater Body Mass Index (BMI)
(Coleman et al., 1998).
39
Health. Subjects with Lipoatrophia Semicircularis (LS), a condition that causes band-like circular
depressions and isolated atrophy of subcutaneous fatty tissue around the legs, showed less use of the
lumbar supports and exhibited greater pressure towards the front of the chair (Hermans et al., 1999).
Also, subjects who reported back pain or discomfort adjusted lumbar supports closer to the front of
the seat (Coleman et al., 1998).
Gender. Females, on average, preferred lumbar supports 1 cm lower than males. However, this
particular study did not control for height (stature). The average preferred in/out location was 20 mm
forwards of the backrest for both males and females (Porter and Norris, 1987).
A logical interpretation of these findings is that traditional fixed height lumbar supports are unlikely
to provide comfortable and appropriate support for the broad range of users. Table 4.2 below shows
the tested ranges of adjustability as well as the preferred lumbar heights for two particular studies
(Coleman et al., 1998; Porter and Norris, 1987). Along with lumbar support height, lumbar support
depth also plays an important role in worker comfort.
Table 4.2: Lumbar support height research data
Author
Coleman et al.,
1998
Porter and Norris,
1987
Range of Adjustability
Average Preferred Height
4.29 to 9.01 in. (11.0 to 23.1 cm) from top
7.41 in. (19.0 cm) from top of seat
of seat
At 90° trunk angle, males prefer 9.56 in.
7.22 to 12.29 in. (18.5 to 31.5 cm) from
(24.5 cm), females prefer 9.13 in. (23.4
top of seat (measured from the top of the
cm). At 120° trunk angle, males prefer
seat pan to the middle of the lumbar
9.13 in. (23.4 cm), females prefer 8.19 in.
support)
(21.0 cm)
One area explored in the research that is not fully covered in the current standards is the distance
from the front of the seat to the point of direct contact with the lumbar support of the chair (lumbar
depth). As stated above, subjects have different needs for lumbar depth and these needs should be
taken into account in chair design. In addition to the limited research on lumbar depth, the situation
is bemired by inconsistent reporting methods: (1) measurements were reported from the front edge of
the seat pan (Coleman et al., 1998) and (2) measurements were reported from the back of the
backrest (Porter and Norris, 1987) to the lumbar support (See Table 4.3 below for lumbar support
depth data). Despite the correlation between the two studies, the range of lumbar depth in seat
design is somewhat controversial. Therefore, this subject requires further research to adequately
substantiate the needs of the users.
Table 4.3: Lumbar support depth research data.
Author
Range of Adjustability
Coleman et al.,
1998
12.17 to 17.86 in. (31.2 to 45.8 cm) from
front of seat
15.09 in. (38.7 cm) from front of seat
0.39 to 1.95 in. (1.0 to 5.0 cm) forward
from backrest
0.78 in. (2.0 cm) forward
Porter and Norris,
1987
40
Average Preferred Depth
4.1.4
Arm Support
Another area associated with discomfort from seating research was arm support. Two studies
examined the correlation between arm supports and musculoskeletal discomfort (Hasegawa and
Kumashiro, 1998; Garcia et al., 1998). Both investigations concluded that some sort of arm support
during computer input tasks improved comfort. In particular, one laboratory-controlled study
concluded that the use of armrests was effective for the reduction of muscle activity in one-handed
keyboard operations such as numerical data entry (Hasegawa and Kumashiro, 1998). The other
article compared different arm supports and indicated that the presence of arm supports significantly
increased comfort, decreased effort, and decreased the rating of perceived exertion (RPE) (Garcia,
1998). The results of this study also suggest that the width of arm supports varies for the range of
users and must be accounted for when considering seating design.
4.1.5
Task Variability
One aspect of seating often overlooked in the office environment is task variability. Four studies
examined the correlation between seating characteristics, tasks, and musculoskeletal discomfort.
Research has found that task intensity is associated with neck, shoulder, and back discomfort while
hand and arm symptoms of discomfort are associated with desk comfort and VDT use. One study
provides evidence that subjects prefer sitting at different heights while performing reading and
writing tasks in order to minimize back discomfort (Mandal, 1989). Research also suggests that the
task length directly affects the extent of back discomfort while seated. Back discomfort was found to
increase over time while seated and performing the same task (Dieen, 1997). Another study
concluded that tasks affected trunk load more than chair features. Therefore, task variability may
play a role in musculoskeletal discomfort in office seating.
4.1.6
Summary of Research
In general, current research supports the CSA, BIFMA, BSR/HFES, and OSHA standards and
guidelines concerning seating in the office workplace. However, the research does not address every
chair feature covered by the standards. Research does however, stress the importance of
adjustability over fixed-position chair characteristics that the guidelines accept as is highlighted in
the next section. Also, certain ranges of adjustability recommended from research include broader
ranges than the standards presented in Table 4.4.
4.2
Standards
As seen in Table 4.4, CSA-Z412, BIFMA, BSR/HFES, ISO and OSHA guidelines include
recommendations on several different chair features. The CSA and BSR/HFES guidelines listed
proposed ranges for which specific chair features should fall under. BIFMA, ISO, and OSHA
recommend less specific guidelines for essentially the same features. Since all recommended
standards are listed for fourteen different chair features from multiple sources, many of the guiding
principles that govern chair features overlap.
41
Table 4.4: Chair guidelines outlined in ergonomic standards
Topic
Standard
BIFMA
Seat Height
• Height when combined with seat pan angle should result in a
thigh-to-torso angle not less than 90°
BSR/HFES 100
Draft
• Minimum range of 4.45 in. (11.4 cm)
• Recommended range 14.82 to 21.84 in. (38.0 to 56.0 cm)
CSA-Z412
• Low seat height: 14.82 to 17.55 in. (38.0 to 45.0 cm)
• Standard seat height: 16.38 to 19.89 in. (42.0 to 51.0 cm)
ISO 9241
BSR/HFES 100
Draft
Seat Depth
Recommendations
CSA-Z412
• Should be lower leg length and should keep the knee joint angle
greater than 90°
• If non-adjustable, no more than 16.77 in. (43.0 cm)
• If adjustable, include 16.77 in. (43.0 cm)
•
•
•
•
Shallow: 14.82 to 16.38 in. (38.0 to 42.0 cm)
Medium: 16.38 to 17.94 in. (42.0 to 46.0 cm)
Deep: Greater than 17.94 in. (46.0 cm)
If adjustable, by at least 1.95 in. (5.0 cm) to accommodate
medium seat depth
• Should ensure that the legs can be positioned without
ISO 9241
BIFMA
Seat Width
compression at the back of the knee and to enable the buttocks to
be positioned to enable full use of the back rest
• Wider than hip breadth
BSR/HFES 100
Draft
• Should be at least 17.94 in. (46.0 cm) wide
CSA-Z412
• Greater than or equal to 17.55 in. (45.0 cm)
ISO 9241
• Should exceed the seated hip width of the largest individual in the
OSHA
design range
• Must accommodate employee
• Seat pan shall either recline backward and/or forward from the
Seat Angle
BSR/HFES 100
Draft
recline
CSA-Z412
Seat Cushion
horizontal
• Should have an adjustable range of at least 6° including 3° of
BSR/HFES 100
Draft
OSHA
• If the seat angle is adjustable independent of the backrest, a
minimum of 3° forward and 4° rearward
• Front edge of seat pan should be rounded
• Cushioned and has waterfall front
42
Table 4.4 (con’t): Chair guidelines outlined in ergonomic standards
Topic
Standard
Movements of
the Seat Pan
and Back
Support
BSR/HFES 100
Draft
CSA-Z412
Recommendations
• Backrest and seat pan should be adjusted to allow for a 90° to
105° torso to thigh angle
• Independent: see backrest angle and seat angle
• Concurrent tilt: seat minimum 10° with minimum concurrent
backrest tilt of 15° (1.5:1 ratio)
BIFMA
BSR/HFES 100
Draft
Lumbar
Support
• Adequate lumbar support, fit user’s lumbar curve
• Lumbar support should be located between 5.85 and 9.75 in.
(15.0 and 25.0 cm) above compressed seat height
• Adjustable by 1.95 in. (5.0 cm) from 5.85 to 9.75 in. (15.0 to 25.0
CSA-Z412
cm) above seat
• If fixed, within 5.85 to 9.75 in. (15.0 to 25.0 cm)
OSHA
BSR/HFES 100
Draft
• Provides support to lumbar area
• At least 17.55 in. (45.0 cm) above compressed seat height
• Standard back: 17.55 to 21.45 in. (45.0 to 55.0 cm) from upper
Backrest
Height
CSA-Z412
•
surface of seat cushion
High back: greater than 2.93 in. (7.5 cm) higher than standard
back
• Minimum lower boundary should be buttock height above seat
ISO 9241
Backrest Width
level where max upper boundary is height of the bottom corner of
the scapula
BSR/HFES 100
Draft
• At least 14.04 in. (36.0 cm)
CSA-Z412
• At least 13.65 in. (35.0 cm)
• If the backrest in adjustable, it should adjust 10° within range of
93° to 113°
If the backrest is fixed, the angle should not be less than 93° or
greater than 103°
Backrest Angle
CSA-Z412
Backrest Lock
CSA-Z412
• If the backrest angle is adjustable, it should be lockable at various
Chair Tilt Lock
CSA-Z412
• Chair lockable at various positions within the tilt range
Arm Supports
OSHA
•
positions within the backrest adjustment range
• Support forearms and does not interfere with movement
43
Table 4.4 (con’t): Chair guidelines outlined in ergonomic standards
Topic
Standard
BIFMA
BSR/HFES 100
Draft
Arm Support
Height
Recommendations
• Support forearms without interfering with tasks, avoid lifting
shoulders or leaning to side
• Should adjust in height from 7.02 to 10.53 in. (18.0 to 27.0 cm)
above compressed seat height
• If fixed, within the range of 7.41 to 9.75 in. (19.0 to 25.0 cm)
CSA-Z412
above compressed seat height
• If adjustable: at least by 1.95 in. (5.0 cm) including 7.41 to 9.36 in.
(19.0 to 24.0 cm) range
• Should be at seated elbow height. However, thickness of work
ISO 9241
Arm Support
Width
Arm Support
Length
Arm Support
Setback
Inside Distance
between
Armrests
4.3
CSA-Z412
BIFMA
surface, thigh height, and armrest separation should also be taken
into account
• At least 1.76 in. (4.5 cm)
• Allows user to sit close to workstation while maintaining contact
with backrest
CSA-Z412
• At least 7.02 in. (18.0 cm)
CSA-Z412
• At least 5.85 in. (15.0 cm) from front of seat
CSA-Z412
• At least 17.55 in. (45.0 m)
ISO 9241
• Should accommodate for the maximum elbow to elbow breadth of
the design population
Recommendations
How does backrest inclination affect worker discomfort?
Backrest inclination has a significant effect on worker discomfort. Increasing the angle between the
trunk and thighs decreases stress on the spine. Therefore, backrest inclination should be adjustable
in order to allow for increased angles between the trunk and spine. Also, chairs with adjustable
backrests are preferred and cause less discomfort than conventional non-adjustable office chairs.
Additionally, backrest inclination should be pressure adjustable in order to provide adequate support
for different body masses.
How does seat pan inclination affect worker discomfort?
Again, increasing the angle between the trunk and thighs decreases the stress and load placed on the
spine. Therefore, seat pan inclination should be adjustable in order to allow for increased angles
between the trunk and spine. In combination with the adjustability of backrest inclination, a
positively inclined seat pan moves the spine toward the natural curvature of the spine. Also, chairs
with adjustable seat pan inclinations are preferred and cause less discomfort than conventional nonadjustable office chairs.
44
How does the lumbar support affect worker discomfort?
As shown in the research, people have different needs for lumbar supports depending on certain
characteristics such as height, weight, health, and gender. When these needs are not met, pain and
discomfort are a direct result of inadequate lumbar support, especially in the low back region.
Therefore, it can be concluded that traditional fixed height lumbar supports are unlikely to provide
comfortable and appropriate support for the broad range of users. At the bare minimum, the backrest
(lumbar support) should accommodate both body weight and size. Ideal lumbar support adjustability
includes lumbar height, depth, and applied pressure capabilities. However, additional research is
needed to fully understand the effects of lumbar support depth and applied pressure. Based on
covered research, the lumbar support depth can be compensated for in two different ways with
respect to chair design adjustability.
• Adjustable seat pan – The capability of a seat pan to slide forward (away from the backrest)
and backward (toward the backrest) as shown in Figure 4.1a.
• Adjustable backrest depth - The capability of a backrest to slide forward (toward the front of
the seat) and backward (away from the front of the seat) as shown in Figure 4.1b.
(a)
(b)
Figure 4.1: Chair adjustability (a) seat pan depth (b) backrest depth
How do arm supports affect worker discomfort?
Research concluded that some sort of arm support during computer input tasks improved comfort,
reduced muscle activity, and decreased perceived exertion. Therefore, it can be concluded that the
presence of arm supports are essential in chair design. And in order to meet the needs and
preferences of the range of users, adjustable arm supports are more effective in reducing worker
discomfort than fixed arm supports. See Appendix C for specific recommendations for arm support
characteristics.
Why should task variability be considered in office seating?
Task variability is an important consideration in chair design. The postures, ranges of motion, and
work intensity are different for various tasks. Hence, an employee performing only one or two tasks
may not need the same range of adjustability of an employee performing five or more tasks.
Therefore, chair capabilities need to be assessed according to the variety of tasks performed in the
office workplace. However, the fact that few laboratory experiments exist where multiple tasks were
performed suggest that further research needs to be undertaken to determine the scope of task
variability and office seating.
45
Overview of Recommendations
Research does not fully cover all chair features, therefore recommendations for these features are
based on integrating current standards and guidelines discussed above. The recommendations below
that reference a particular standard (i.e. CSA, BIFMA, etc.) are purely based on the assessment of
available guidelines. However, the recommendations that reference research are based on collective
scientific findings from ergonomic investigations along with any noted standards.
The following recommendations for office seating, listed in Table 4.5, are based on minimizing
discomfort, muscle activity and fatigue, increasing productivity, and maintaining task variability for
the broad range of users in a seated office environment.
2
7
8
11
12
13
14
10
9
3
1
Front View
Side View
1
vertical distance from floor to top of seat pan
horizontal distance between the front of the seat pan to the back of the seat pan
3
horizontal distance between outermost edges of seat pan (left and right edges)
4
angle of the center of the seat pan with respect to the horizontal (seat pan)
5
vertical distance between compressed seat pan and center of lumbar support
6
standard method of measurement needs to be determined
7
vertical distance between upper surface of seat cushion and top of backrest
8
horizontal distance between outermost edges of backrest
9
angle between horizontal axis of the seat pan and the center line of the backrest (trunk angle)
10
vertical distance between compressed seat pan and top of arm support
11
horizontal distance between inside edge of arm (closest to body) support and outer edge
12
horizontal distance between the front edge of arm support and back edge of arm support
13
horizontal distance between the front edge of seat pan and front edge of arm support
2
14
horizontal distance inner edges of right and left arm supports
Figure 4.2: Chair, illustration of dimensions in Table 4.5
46
Research
BIFMA
BSR/HFES
Table 4.5: Chairs: Recommendations based on ergonomic standards and research
To accommodate the range of
users to ensure that the thighto-torso angle is not less than
90° and the knee joint angle is
greater than 90°
Seat Depth2
Adjustable, about the range
14.82 to 17.94 in. (38.0 to 46.0
cm)
To accommodate upper leg
length for the range of users so
that the legs can be positioned
without compression at the
back of the knee and enable
the buttocks to be positioned to
enable full use of the backrest
Seat Width3
At least 17.94 in. (46.0 cm)
To accommodate hip breadth
for the range of users
Seat Height1
Seat Pan
4
Angle
ISO
14.82 to 21.84 in. (38.0 to 56.0
cm)
OSHA
CSA
Recommendations Given
Current Standards and
Research
Chair Features
Minimizes load placed on the
Adjustable, about the range 0°
trunk resulting in less
to 4° negatively inclined (CSA)
discomfort and the spine
and 0° to 25° positively inclined
moves toward lumbar lordosis
Seat Cushion
Cushioned and has waterfall
front
Front edge of seat pan should
be rounded
To minimize leg discomfort due
to direct seat contact
Movements of
the Seat Pan
and Back
Support
Independent: see backrest and
seat pan angle. Backrest and
seat pan should be adjusted to
allow for 93° to 120° angle
between torso and thighs
Reduces trunk flexion and back
load resulting in increased
comfort
Lumbar
Support Shape
Adequate lumbar support, fit
user’s lumbar curve. Provides
support to lumbar area
To provide adequate lumbar
support for the range of users
Lumbar
Support
5
Height
To provide adequate lumbar
Adjustable, about the range 3.9 support and minimize low back
to 11.7 in. (10.0 to 30.0 cm)
discomfort for the range of
users
Lumbar
Support
6
Depth
Adjustable, optimal ranges not
available: needs further
research to determine ranges
Backrest
Height7
Standard back: 17.55 to 21.45
in. (45.0 to 55.0 cm) from upper
To provide adequate back
surface of seat cushion
support for the range of users
High back: At least 2.93 in. (7.5
cm) higher than standard back
Backrest
Width8
At least 14.04 in. (36.0 cm)
47
To accommodate lumbar
region for the range of users
To accommodate the back
width for the range of users
Research
BIFMA
BSR/HFES
Table 4.5 (con’t): Chairs: Recommendations based on ergonomic standards and research
Adjustable, about the range of
93° to 120°
Minimizes back discomfort and
more pressure is transferred to
backrest, less on spine
Backrest Lock
Lockable at various positions
within the backrest adjustment
range
Limits the range of motion for
user preferences
Chair Tilt Lock
Chair lockable at various
positions within the tilt range
Limits the range of motion for
user preferences
Arm Supports
Supports forearms and does
not interfere with movement or
tasks, avoid lifting shoulders or
leaning to side
Minimizes strain on shoulders
and low back during seated
office tasks
Arm Support
Height10
7.02 to 10.92 in. (18.0 to 28.0
cm)
To accommodate seated elbow
height for the range of users
performing a variety of tasks
Arm Support
Width11
At least 1.76 in. (4.5 cm)
To accommodate forearm
width for the range of users
Arm Support
12
Length
At least 7.02 in. (18.0 cm).
Maintain backrest contact
To allow the range of users to
sit close to workstation while
maintaining contact with
backrest
Arm support
Setback13
At least 5.85 in. (15.0 cm)
Allows users to sit close to
workstations
16.77 to 20.67 in. (43.0 to 53.0
cm)
Should accommodate for the
maximum elbow to elbow
breadth of the design
population
Inside
Distance
between
14
Armrests
ISO
Backrest
9
Angle
OSHA
CSA
Recommendations Given
Current Standards and
Research
Chair Features
As illustrated in Figures 4.3 and 4.4
1
vertical distance from floor to top of seat pan
2
horizontal distance between the front of the seat pan to the back of the seat pan
3
horizontal distance between outermost edges of seat pan (left and right edges)
4
angle of the center of the seat pan with respect to the horizontal (seat pan)
5
vertical distance between compressed seat pan and center of lumbar support
6
standard method of measurement needs to be determined
7
vertical distance between upper surface of seat cushion and top of backrest
8
horizontal distance between outermost edges of backrest
9
angle between horizontal axis of the seat pan and the center line of the backrest (trunk angle)
10
vertical distance between compressed seat pan and top of arm support
11
horizontal distance between inside edge of arm (closest to body) support and outer edge
12
horizontal distance between the front edge of arm support and back edge of arm support
13
horizontal distance between the front edge of seat pan and front edge of arm support
14
horizontal distance inner edges of right and left arm supports
48
4.4
Limitations
Researchers are dependent upon the subject’s opinion when quantifying musculoskeletal discomfort
and fatigue while seated. Personal preference and behavioral variations are also significant factors in
determining the relationship between different chair features and worker discomfort. Therefore,
those field studies that relied solely on subjective measures as a means of evaluation subject
themselves to inferences based on opinion rather than objectivity.
Other limitations in current research involved certain oversights in data collection such as body fat
percentages, detailed anthropometric measures (i.e. popliteal height), and separation of tasks that
were not performed that could have been beneficial in supporting specific claims. In addition, very
few studies concerning office seating examined the effects of chair features on performance measures
and productivity. This could be due to the fact that many tasks performed in an office environment
are not easily submitted to productivity measures. Beyond simple data entry tasks, it is difficult to
systematically quantify worker productivity in direct relation to chair features. Performance may be
influenced by feelings of boredom, stress, or fatigue among subjects rather than chair design and
posture. However, the problems associated with quantifying worker productivity could be addressed
with improved control groups.
4.5
Future Research
Future seating research would greatly improve with the following steps:
• Include control groups
• Use well controlled and measurable tasks
• Broader range analysis for chair feature adjustability
Control Groups. Future research should incorporate control groups to combat the limitations listed in
section 4.5.
Task Selection. In field and laboratory research, choose jobs that are simple and include
standardized tasks that vary little from employee to employee. A variety of seated office tasks need to
be investigated in order to determine the scope of task variability and its effects on the body.
Adjustability Range Analysis. Increasing the ranges of adjustability for the multitude of chair
features would provide a stronger basis for optimal postures during office work. In particular, lumbar
support depth, posterior seat pan inclination, and arm support breadth need further investigation to
assess their impact on risk, comfort, and productivity. One other research necessity is to develop a
standard method for measuring lumbar depth. A common reference point must be established among
future researchers in order to compare the values between studies.
49
50
5 Lighting
Commonly overlooked in the office environment, lighting is a part of every work task. A worker’s
eyes receive light directly from light sources and indirectly from surface reflections. Direct lighting,
also called workplace illumination, is measured in units of luminaires (lux). Illumination is
measured in both the horizontal and vertical planes and is often described as a ratio of horizontal and
vertical illuminances. Lighting bouncing off reflective surfaces, called indirect lighting, may cause
glare on desks and computer monitors. Glare lowers the contrast on a reflective surface, thereby
impeding visibility. Luminance, measured in candelas per unit of area squared, describes the
amount of indirect light reflected off a surface.
Research links glare to increased risks of visual discomfort among employees (Sheedy and Bailey,
1995). Employee symptoms of visual discomfort may include dry eyes, blurry vision, or headaches.
To reduce these symptoms, current research examines the light sources responsible for glare
including natural light, overhead lights, and task lights. The following will discuss the advantages
and disadvantages associated with each of these light sources.
5.1
Research
The eleven articles chosen for review focus on three types of glare producing sources:
• Natural lighting – Illumination produced by the sun versus a man made source
• Overhead lighting – Commonly ceiling mounted fluorescent tubes
• Task lighting – Tabletop or shelf mounted light sources
The design characteristics of these light sources as well as the position of the workstation in relation
to these light sources determine VDT and work surface glare levels.
Research in this area attempts to answer the following two questions:
1. How should the lighting-workstation system be designed to reduce glare?
2. What other aspects of natural, overhead, and task lighting may cause discomfort?
5.1.1
Natural Lighting
Illumination produced by the sun consists of light energy across all spectral frequencies (Hedge,
2000). The amount of natural lighting in an office environment fluctuates from day to day and even
from hour to hour (Begemann et al., 1996). The reasons for this variability include:
• Window location – Compass direction determines light levels throughout the day
• Window size – Larger windows let in more natural lighting
• Floor number – Lighting levels change as much as 80 lux per story (Wilkens et al., 1989)
• Geographic location – Natural lighting intensity varies across the country
• Season – Natural lighting intensity varies depending on the time of year
• Weather – Natural lighting levels vary daily due to changes in cloud coverage
• Window treatments – The thickness and coverage of shades dictate light levels
Despite these fluctuations, workers generally prefer working under natural lighting conditions (Roche
et al., 2000). However, in the presence of natural lighting, workers favor higher artificial lighting
levels than standards dictate (Begemann et al., 1996). On average, office workers add 800 lux of
51
artificial light to natural lighting levels for a total desk illuminance of greater than 1000 lux. Some
researchers suggest workers may prefer natural lighting due to the biological effects of lighting below
1000 lux (Begemann et al., 1996). This condition called “biological darkness” is thought to be
linked to performance difficulties, alertness, and sleeping disorders. Research has shown that
introducing natural lighting reduces the incidence of headaches among office workers (Wilkens et
al., 1989).
5.1.2
Overhead Lighting
Though incandescence and halogen lighting systems are found in some offices, fluorescent lighting
systems are more common. Energy efficient and long lasting, fluorescent lights comprise of
phosphor-coated tubes containing low-pressure mercury vapor. It is popularly believed that
fluorescent lighting causes eyestrain and headaches (Stone, 1992). However, researchers have been
unable to determine which aspects of fluorescent lighting, if any, are to blame. Lighting direction,
flicker rate, luminous and chromatic modulation, spectral power distribution, intensity, and line of
sight are all suspected of causing visual discomfort.
Lighting Direction. In overhead lighting, direction of the light source is an important factor in worker
comfort. The two directions covered by current research include parabolic downlighting and lensedindirect lighting.
Parabolic downlighting directs more of the light output directly downwards to reduce the light output
at increasing horizontal angles from the luminaire. Although parabolic downlighting may potentially
reduce screen glare problems, it may also produce distinct shadows and harsh contrasts causing
excessive strain on the eyes. In general, this type of lighting is considered a good solution in glare
screen reduction for VDT users (Hedge, 1995).
Lensed-indirect lighting is a form of uplighting that uses an engineered optical system to distribute
light evenly over a greater area of the ceiling. In turn, less pronounced shadows and softer contrast is
admitted from the lighting, reducing the effects of screen glare and worker discomfort (Hedge, 1995).
In one study, eighty-nine workers exposed to a lensed-indirect and a parabolic downlighting system
preferred the lensed-indirect system. The system was more favorably rated on several subjective
lighting impressions scales. In addition, VDT workers reported fewer screen glare problems, and
fewer and less frequent problems with tired eyes and eye focusing. Lensed-indirect lighting was
associated with better productivity, as well as, higher satisfaction and lighting quality ratings.
Overall, some two-thirds of workers preferred working under lensed-indirect lighting system to
parabolic downlighting (Hedge, 1995).
Flicker Rate. The flicker rate of fluorescent lights varies depending on the ballast technology.
Traditional core-coil magnetic ballasts produce a flicker rate of 120 Hz while newer electronic
ballasts range between 20 to 60 kHz. Though humans do not consciously perceive flicker rates
greater than 60 Hz, researchers have found that the human visual system reacts to flicker rates as
high as 147 Hz (Veitch and McColl, 1995). Flicker rates above visual reaction levels may reduce the
incidence of headaches and eyestrain among workers. However, observed effects of flicker rate on
worker visual discomfort have been small and inconsistent between studies (Fleming et al., 2000;
Kuller and Laike, 1998; Veitch and McColl, 1995; Wilkins et al, 1989). Flicker rates tested in the
literature are listed in Table 5.1.
52
Table 5.1: Flicker rates tested in the literature
Improved Visual Comfort
and Performance at:
Study
20-60 kHz
20-60 kHz
Veitch and McColl, 1995
32 kHz
Flicker Rates Tested
120 Hz
100 Hz
1
Low Level
1
Low Level
1
32 kHz
Wilkins et al., 1989
1
No Difference
Fleming et al., 2000
1
No Difference
Kuller and Laike, 1998
High Level
High Level
No numerical information on flicker rates was given
Luminous and Chromatic Modulation. Light intensity and color vary due to the differing decay rates
of phosphors used in fluorescent lights. These variations in light and color are referred to as
luminous and chromatic modulation. However, observed effects of modulation on worker visual
discomfort have been small and inconsistent between studies (Fleming et al., 2000; Veitch and
McColl, 1995).
Spectral Power Distribution.
Fluorescent lights emit different levels of radiant energy that
determines the color and spectral range of the light produced. Manufacturers of full-spectrum
fluorescent lighting claim that broader spectrum lighting is better because it simulates natural
lighting. Currently, fluorescent lighting color has not been shown to affect visual performance or
comfort (Veitch and McColl, 1995; Veitch and McColl, 2001). Though some recent preliminary
research suggests that spectral distribution is an important factor of fluorescent lighting and should
be examined more closely (Fleming et al., 2000).
Intensity. Some research suggests that worker discomfort increases as the intensity of overhead
lighting increases (Ngai and Boyce, 2000). The study showed that larger luminaire area and
increased ambient illuminance, from 280 to 506 lux, increased subject discomfort.
Line of Sight. Though the location of overhead lights in relation to the workstation can play a role in
reducing glare, little research has looked at this area. However, one study did find that increases in
deviation of light source from the line of sight reduced employee discomfort (Ngai and Boyce, 2000).
5.1.3
Task Lighting
Workstation lighting may be supplemented by additional desk top or shelf lights. Researchers have
been primarily concerned with characteristics of task lighting that cause glare including luminance
level, position (line of sight), and polarization.
Luminance Level. Luminance level significantly effects task performance. One study has shown that
as the level of luminance increases from a minimum of 7.75 cd/m2 to a maximum of 277 cd/m2, dataentry performance increased linearly. The lowest luminance resulted in the most discomfort, thus
hindering productivity among subjects (Eklund, 2001). As with overhead lighting, other research
found that subjects preferred desktop luminance levels greater than the Illuminating Engineering
Society of North America (IESNA) recommended specification of 40 cd/m2 (Veitch, 2000). These
findings suggest that current luminance levels from task lighting are not bright enough to maximize
53
performance and reduce worker discomfort. Further research is needed to determine ideal ranges of
adjustability for these luminance levels.
Line of Sight. The position of task lighting in relation to the worker is critical in providing
comfortable lighting environments. One study found that due to stature differences almost all of the
male subjects in the study looked directly into cubicle shelf lights while most females were exposed
to direct glare from the same lighting (Japuntich, 2001). Offices should avoid fixed lighting to meet
the needs of multiple users.
Polarization. Polarized shelf lights have been show to reduce glare (Japuntich, 2001). However, the
benefits of polarized lighting diminish as ambient lighting increases.
5.2
Standards
Only the CSA-Z412 and ISO 9241 standard gives lighting recommendations as seen in Table 5.2.
The lighting research reviewed above cannot confirm the recommended lighting levels given by the
CSA-Z412 standard. The research has focused on total illuminance levels while the
recommendations are given in luminance ratios. In addition, the research on office illuminance has
studied broad office environments instead of specific office tasks. The research does support careful
workstation positioning for reducing glare. However, the research suggests that these strategies
should be tried before daylight is removed using blinds or shades.
Table 5.2: Lighting guidelines outlined in ergonomic standards
Topic
Standard
Horizontal
Illuminance
CSA-Z412
From IESNA
•
•
•
•
Filing: 500 lux
Open office w\ intensive VDT use: 300 lux
Open office, intermittent VDT use: 500 lux
Private office: 500 lux
Vertical
Illuminance
CSA-Z412
From IESNA
•
•
•
•
Filing: 100 lux
Open office w\ intensive VDT use: 50 lux
Open office, intermittent VDT use: 50 X lux
Private office: 50 lux
CSA-Z412
From IESNA
•
•
•
•
•
•
3:1 between paper task and adjacent dark computer screen
1:3 between paper task and adjacent light computer screen
3:1 between task and adjacent dark surroundings
1:3 between task and adjacent light surroundings
10:1 between task and more remote dark surfaces
10:1 between task and more remote light surfaces
•
Glare should be reduced from outside sources using blinds,
curtains, etc. and from electric sources using shields, removing
the light source, or relocating the workstation
•
Glare should be reduced by correctly locating the VDT and
workstation, by implementing glare reducing equipment features,
or by adjusting the artificial and natural lighting
Luminance
Ratios
Recommendations
CSA-Z412
Glare
ISO 9241
54
5.3
Recommendations
How should the lighting-workstation system be designed to reduce glare?
When possible, adjustable task lighting with a filter or polarized lenses should be used in office
environments with multiple users. The best way to ensure that workers are comfortable with the
lighting conditions is to give workers control over lighting levels and locations.
What other aspects of natural, overhead, and task lighting may cause discomfort?
Whenever possible, office workers should be given access to natural lighting. Current research
suggests that there are physiological benefits for exposing workers to natural lighting throughout the
workday. Artificial lighting should be used to raise the total illuminance above 1000 lux.
When using fluorescent lighting, choose electric ballasts with a flicker rate above 20 kHz. Though
researchers’ conclusions and recommendations are not unanimous on the benefits of higher flicker
rates, high flicker rates have not been shown to decrease employee comfort and in several instances
they have been shown to improve comfort. In addition, uplighting systems appear to be preferable to
downlighting systems.
In summary, to improve the visual comfort of office workers:
• Use a combination of natural light and artificial lighting
• Use electronic ballasts with high flicker rates (> 20kHz)
• Use adjustable task lighting with a filter or polarization
• Allow workers to control lighting levels
5.4
Research Limitations
Researchers are dependent upon subjects’ opinions when quantifying visual discomfort and fatigue.
Though some technologies exist that measure the physiological response of the human eye, no
technology exists to measure perceived discomfort (including eyestrain). Although medical science
has not yet determined if repeated exposure to eyestrain has any long-term effects, employee visual
comfort should not be ignored.
5.5
Future Research
Fluorescent lighting has been linked to increased complaints of headaches and visual strain (Stone,
1992). However, current research has been unable to pinpoint the exact characteristics of overhead
fluorescent lighting that are causing this discomfort. Therefore, more systematic research of flicker
rate, modulation, spectral range, intensity, and line of sight should be performed to find the best
fluorescent lighting designs.
The interaction between working time and lighting is not well understood. Some researchers suggest
that natural lighting is preferred by people due to the gradual change of lighting levels throughout the
day (Begemann et al., 1996). Researchers should further examine the influence of gradual lighting
shifts on workers as well as whether or not this natural lighting shift can be duplicated with artificial
lighting.
55
56
6 Breaks
Even with the benefit of a well-designed workstation and three breaks a day, VDT operators may still
suffer from musculoskeletal, visual, and mental fatigue (Swanson et al., 1989). Musculoskeletal
fatigue is caused by the static postures and repetitive movements required by many VDT tasks
(Atiken, 1994). Static postures and repetition produce a buildup of waste products in the muscles.
Without adequate rest, muscles may become fatigued and sore. Rest allows the circulatory system
time to remove waste products and provide muscles with fresh nutrients. Rest also gives the
musculoskeletal system time to repair damaged tissue. When a muscle is fatigued and continues to
work, the risk of injury to that working muscle increases (Rodgers, 1997).
The rest break schedule used in many offices includes a fifteen-minute break in the morning and
afternoon and a thirty-minute lunch. This traditional rest break schedule may not provide adequate
recovery time for the musculoskeletal and circulatory systems. Some companies have been
encouraging employees to take 20 seconds to 2 minutes rest breaks between regularly scheduled
breaks. These short breaks, commonly referred to as microbreaks or micropauses, have yielded
positive results for some companies. In addition to microbreaks, medical experts encourage
employees to exercise during breaks (Aitken, 1994). Companies introducing supplementary break
and exercise programs have made claims of 10% increases in productivity and 50% decreases in
work-related injuries (Aitken, 1994). Researchers are attempting to determine the length, frequency,
and type of rest breaks needed to reduce the risk of injury while increasing productivity.
6.1
Research
The sixteen articles chosen for review examine task scheduling in office environments using VDTs.
Break research strives to answer one or more of the following questions:
1. Do breaks affect worker discomfort?
2. Do breaks affect productivity and costs?
3. Do breaks affect worker mental state or mood?
4. Should stretching exercises be performed during breaks?
5. Should breaks be regimented or self-determined?
Answering these questions will allow researchers to develop optimal break schedules for office
environments. The following sections explore each of these research questions.
6.1.1
Muscle Activity and Discomfort
Work-rest cycle research focuses on the connection between rest breaks and musculoskeletal as well
as visual discomfort. As previously mentioned, discomfort is commonly qualified through discomfort
surveys, in which the subjects subjectively rate the amount of pain felt in one or more body parts.
EMG measurements are also used to measure muscle activity and fatigue. Higher levels of muscle
activity are associated with faster muscle fatigue and increased discomfort. A breakdown of the
measurement methods used in the studies discussed below is listed in Appendix D.
Extensive research has been done on work-rest cycles involving light, repetitive, industrial tasks.
Many of the stressors in industrial tasks are found in VDT work. Therefore, some conclusions drawn
from industrial research may be applied to office environments. A generally accepted conclusion
among break researchers in industry is that short, frequent breaks are more effective than longer,
57
more infrequent breaks for decreasing employee discomfort (Swanson et al., 1989). Breaks research
preformed on VDT operators support this finding.
Supplemental breaks added to a conventional break schedule reduced muscle and visual eye
discomfort among VDT users (Galinsky et al., 2000; Henning et al., 1993; McLean et al., 2001).
Current research does not clearly indicate an optimal break length for minimizing musculoskeletal
discomfort as well as eyestrain. The shortest work-rest cycle examined, yielding positive results, was
a 30 second break every 20 minutes introduced in addition to the conventional break schedule
(McLean et al., 2001). While some studies indicate that break length does not make a difference, no
study has systematically compared more than three work-rest cycles under the same conditions
(Boucsein and Thum, 1997; Lim et al., 1990). Rest breaks taken too frequently may interfere with
worker productivity by disrupting work cycles (Swanson et al., 1989). In fact, employees seem to
prefer longer, less frequent breaks for this reason (Helander and Quance, 1990; Henning et al.,
1997).
The effectiveness of rest breaks may depend on some yet unidentified organizational factors. One
study found that adding four additional microbreaks per hour coincided with a reduction in eyestrain
and lower extremity discomfort for a small work site yet made no difference for a large work site
(Henning et al., 1997). Psychosocial or organizational factors may influence the optimal rest break
schedule.
6.1.2
Productivity and Costs
Researchers have used workers’ compensation records to track costs of injuries, while the number of
completed task cycles are used to track productivity. In the case of data entry tasks, productivity has
been calculated based on typing speed, keying errors, and correction counts. A breakdown of the
measurement methods used in the studies discussed below is listed in Appendix D.
Researchers disagree on the effect of rest breaks on productivity. Most studies find no significant
correlation between breaks and productivity (Balci et al., 1998; Galinsky et al., 2000; Henning et al.,
1997; Mclean et al., 2001). However, other researchers have observed some productivity gains when
supplemental breaks were added to a conventional break cycle (Henning et al., 1997; Henning et al.,
1993; Swanson et al., 1989). One study reported a 25% increase in productivity and a drop from
twelve workers’ compensation claims to zero over a one year period in which supplemental breaks
were added to the conventional break schedule (Thompson, 1990). However, because the study did
not use a control group and a pay mechanism change occurred during the data collection period, the
results are of limited use.
6.1.3
Emotional State
The mind as well as the body can become fatigued over time; both the physical and psychological
demands of a job are thought to influence an optimal break schedule design. The physical demands
of the job include activity diversity, cycle length, repetition, working posture, and working muscle
force. Workload, work hours, overtime, co-worker interaction, corporate climate, and stress are all
psychological demands associated with a job. Surveys are used to measure a subject’s emotional
state or mood. In addition to surveys, physiological measurements such as heart rate are used to
measure stress levels. Higher heart rates correlate with higher perceived stress levels (Boucsein and
58
Thum, 1997). A breakdown of the measurement methods used in the studies discussed below is
listed in Appendix D
Most studies have found no correlation between rest breaks and mood defined in terms of emotional
strain, stress, or boredom (Galinsky et al., 2000; Henning et al., 1997, Henning et al., 1993).
However, a correlation between breaks and mood has been observed when the time of day is
considered. One study found that short frequent breaks reduced mental and emotional strain during
the early afternoon, while longer less frequent breaks were more effective in the late afternoon
(Boucsein and Thum, 1997). When subjects defined the length of breaks, subjects took longer
microbreaks, as subjects became bored with the work. Therefore, break schedules may need to vary
based on the time of day.
6.1.4
Exercise Breaks
Medical experts promote exercise breaks as being more effective then rest breaks alone (Aitken,
1994). Exercise increases circulation and stretches muscles to prevent stiffness. Research has been
mixed on the effectiveness of exercise breaks on productivity and comfort (Anon, 1999). While
research has shown that exercise breaks may reduce muscle strain and discomfort as well as improve
productivity, other research has shown that exercise breaks have little or no effect on the incidence of
MSDs in the workplace. In some cases, exercises may exacerbate existing physical problems or
cause new hazards (Anon, 1999). As a result, exercises work best when:
• Tailored to meet an individual’s needs
• Part of a comprehensive ergonomics program
• Designed for the specific work task
• It is easy to learn and not embarrassing
• Supported by management but “owned” by employees (Anon, 1999)
A breakdown of the measurement methods used in the studies discussed above is listed in Appendix
D
6.1.5
Break Enforcement
Computer programs are available to remind VDT users to take rest breaks based on a timer
mechanism or a keystroke count. Without a reminder, users work beyond the point productivity
begins to drop or end breaks before completely recovered (Henning et al., 1989; Swanson et al.,
1989). As a result, regimented breaks tend to reduce muscle discomfort better than self-determined
breaks (Boucsein and Thum, 1997; McLean et al., 2001). However, regimented breaks may
interrupt workflows and cause employees undue stress (Boucsein and Thum, 1997, Swanson et al.,
1989). One compromise may be to use a compensatory or continuous feedback system. These
systems track user breaks and only force users to take a break if they do not reach some target goal.
Such systems have been shown to be as effective as regimented break schedules and tend to be
preferred by users (Henning et al., 1994; Henning et al., 1996). However, such programs are only
useful for tasks involving intensive keying.
59
6.2
Standards
As seen in Table 6.1, the CSA-Z412 standard and OSHA guideline include brief recommendations
on task scheduling. The CSA standard recommends frequent short breaks and suggests that workers
should get up from the computer hourly. OSHA also recommends microbreaks for workers using
VDT, but does not offer a time schedule for taking microbreaks. Although non-specific, the
recommendations are in line with the findings of current research.
Table 6.1: Break guidelines outlined in ergonomic standards
Topic
Standard
CSA-Z412
Breaks
OSHA
6.3
Recommendations
• Workers should have control over when breaks are taken
• Frequent short breaks over infrequent long breaks
• Workers should get up from the computer hourly
• VDT tasks should be organized in a way that allows employees to
vary VDT tasks with other work activities, or to take micro-breaks
or recovery pauses while at the VDT workstation
Recommendations
Regular, frequent, short rest breaks are recommended in order to reduce musculoskeletal discomfort.
Employees should be reminded to get up from the computer for at least 30 seconds every 60 minutes
for normal VDT work and at least 30 seconds every 30 minutes for intensive VDT work. Adding
short frequent rest breaks to a conventional break schedule is recommended to increase worker
productivity. However, employees may not remember to take rest breaks on their own. Instead, a
computer form of reminding or incentive system may be helpful to improve vigilance of break
schedules. Exercise breaks should be:
• Tailored to meet an individual’s needs,
• Part of a comprehensive ergonomics program,
• Designed for the specific work task,
• Easy to learn and not embarrassing, and
• Supported by management but “owned” by employees (Anon, 1999).
6.4
Limitations
Fatigue research in the office environment must not only consider the physical strain that comes from
performing repetitive office tasks but also the mental strain that may comes from concentrating for
long periods of time. The inability to accurately quantify the emotional state of workers limits the
accuracy of work-rest cycle research. Productivity as well as subjective ratings of discomfort may be
influenced by feelings of boredom or stress among subjects. In addition, many task performed in an
office environment are not easily submitted to productivity measurements. Beyond simple data entry
tasks, it is difficult to systematically quantify worker productivity. Both the problem of quantifying
worker emotional states and quantifying worker productivity could be addressed with control groups.
60
6.5
Future Research
Current research lacks a systematic approach to work-rest schedules for the office environment. Few
researchers test more than three different work-rest cycles under the same conditions. Researchers
are attempting to pinpoint the optimal break frequency and break length that minimizes employee
discomfort and emotional strain while improving productivity. To find these optimal work-rest
cycles, breaks research would ideally be conducted using a systematic approach. First, only one
characteristic of the work-rest schedule should be changed at a time (e.g. break frequency or break
length). Next, variables should be tested in a series of experiments. For example, when testing
break length, two break lengths of 10 minutes and 30 seconds could be tested. If 30 seconds yields
better results than two new quantities of either side of 30 seconds would be added to the study. Now
break lengths of 15 seconds, 30 seconds, and 5 minutes would be tested. Researchers would repeat
this process until honing in on an optimal value.
The work-rest research investigations can be further improved through careful task selection. In the
laboratory, select tasks that offer well-controlled productivity measures for easier comparison
between subjects. In the field, choose jobs that are simple and involve standardized tasks that vary
little from employee to employee. In addition, both laboratory and field work should incorporate
control groups to combat the limitations listed in section 6.5.
The type of rest breaks used by employees should also be explored in future research. Does exercise
or stretching during breaks reduce employee discomfort more than just standing up at the
workstation? Do the types of breaks needed depend on the type of task being performed? Does the
design of the workstation influence the frequency or duration of necessary breaks? All of these
questions must be answered to design a truly comprehensive rest break schedule.
61
62
7 Expectations and Opportunities
Information about CTD and MSD injury development is in its infancy. And, the requested
quantification of the dose-response relationship (between exposure and injury development) is not
expected in the immediate future. Instead, injury prevention is based on general recommendations,
voluntary standards, and guidelines on workplace design.
Guidance must come from assimilating recommendations from multiple sources including ad hoc,
disjointed, and narrowly focused research initiatives. Such synthesis is time sensitive and time
consuming. This white paper provides a time sensitive amalgamation of office environment
ergonomics research and formulated concise, cohesive recommendations. A part of this process was
the development of a method for future information review and assimilation. To remain current, this
analysis should be updated every two to four years.
By systematically reviewing current research and assimilating new information into designs, work
practices, and evaluation processes, we arm ourselves with the best defense against injury possible.
It is with this information that designers, engineers, and managers have the opportunity to develop
the next generation of products, processes, and work environments.
63
64
Glossary
Chromatic Modulation
Variation of light color due to the differing decay rates of phosphors used
in fluorescent lights
Dependent Variable
Variables that are influenced by the independent variable
Dorsal Contact
The contact area between an individuals back (dorsal side of the torso)
and an object such as a chair backrest
Dose-response
The trade-off between exposure levels and risk
Electromyography
Also called EMG, measures muscle activity and muscle fatigue
EMG
See electromyography
Epidemiology
The study of the incidence and distribution of disease and other factors
relating to health.
Extension
Increases the angle between the bones of the limb at the joint during
bending (e.g. wrist extension moves wrist backward)
Flexion
Decreases the angle between the bones of the limb at the joint during
bending (e.g. wrist flexion move wrist forward)
Guideline
A set of recommendations compiled by the government or a professional
organization to assist companies in achieving a specific outcome
Independent Variable
Variables systematically changed by the experimenter in order to record
reactions in the dependent variable
Luminous Modulation
Variation of light intensity due to the differing decay rates of phosphors
used in fluorescent lights
Makeforce
The amount of force required to depress the key on a keyboard to its
activation point
Ocular Surface Area
The exposed portion of the eye not covered by the eyelid (also called
OSA)
Pronation
For the hand/wrist, the wrist is rotating the hand palm side down
p-value
Describes the strength of the correlation between the independent and
dependent variable. A p-value of 0.05 is the maximum p-value allowable
for statistical significance
Radial Deviation
Bending wrist towards the thumb side of the forearm
Rest Break
A temporary cessation of work without active exercise or stretching
Spectral power
distribution
Representation of an electrical signal in the frequency domain, including
power distribution into the appropriate frequencies for a sinusoidal
representation
Standard
A set of rules officially endorsed by a professional or governmental
standards organization
65
Statistical Significance
It is reasonable to believe that a correlation between the independent and
dependent variable exists
Supination
For the hand/wrist, the wrist is rotating the hand palm side up
Quantitative
Objective, numerical results not influenced by personally biases
Qualitative
Subjective, personal biases of researcher or subjects may influence
results
Ulnar Deviation
Bending wrist towards the little-finger side of the forearm
66
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73
74
Appendix A – Keywords and Search Results
NOTE: The following symbols are used in the searching process:
(*)
A wildcard operator included in search terms to represent unknown
characters
(")
Used to combine separate keywords into a single term
(<>)
Signifies the NOT Boolean operator used to exclude search terms
(AND)
Both terms must be present in the target
(OR)
One or the other term must be present in the target
Table A.1: Video display terminals (VDTs) keywords
Primary Keyword
Eliminators
Modifiers
Placement* OR Position* OR Location* OR
Height* OR Angle* OR Office* OR Discomfort
OR Health OR Symptom* OR Ergonomic*
VDT
Visual Display
Terminal*
Proceedings
(Under Source)
VDU
Placement OR Position OR Location OR Height
OR Angle
Visual Display Unit*
Computer OR Placement OR Position OR
Location OR Height OR Angle
Monitor
Viewing Angle
Table A.2: VDTs search results
Date
Keywords
No. Hits
A
VDT <> Eliminators
528
B
437
234
234
E
A AND Modifiers
B <> (in Source) Japanese OR Advances OR "Work with
Display Units"
"Visual Display Terminal*" <> A
71
71
F
VDU AND Modifiers <> A OR E
233
G
VDU <> A OR E
183
183
H
Visual Display Units* AND Modifiers <> G
54
54
I
Monitor AND Modifiers <> H
720
J
Viewing Angle* AND Modifiers <> I
19
C
10/28/2002
No.
Downloaded
ID
75
19
Table A.3: Input device keywords
Primary Keyword
Eliminators
Modifiers
Workload OR Discomfort OR Productivity OR
Posture OR CTS OR Carpal OR "Muscle
Load" OR EMG
Input Device OR
Input Devices
Trackpad
Trackball
Touch Pad
Voice Recognition
Software
Voice Activation
software
Keyboard
Mouse OR
Mousing OR Mice
Typing
Keying
Activation Force
Tray
Proceedings (From Source) OR Symposium
(From Source) OR CAD
Table A.4: Input devices search results
Date
ID
Keywords
10/22/2002
A
B
C
E
F
G
H
I
J
K
"Input Device" OR "Input Devices" <> Eliminators
A AND Modifiers
Trackball <> Eliminators
Trackpad <> Eliminators
Touch pad <> C
"Voice Recognition Software" <> Eliminators
"Voice Activation Software" <> Eliminators
Keyboard* <> Eliminators
Keyboard Tray* <> Eliminators
Activation Force* <> Eliminators
No.
Hits
1276
121
40
0
0
1
0
756
1
6
No.
Downloaded
Table A.5: Seating keywords
Primary Keyword
Chair OR Chairs
Eliminators
Wheelchair* OR Industrial OR School* OR
Elderly OR Dental OR Theatre* OR Plane*
OR Helicopter* OR Flight* OR Vehicle* OR
Fork Lift* OR Fork-Lift OR Car* OR
Automobile* OR Ejection OR Bus OR Truck
OR Crane* OR Subway* OR "Mass Transit"
OR Boat* OR Train* OR Crew* OR Rail* OR
Vibration* OR Children* OR Spectator* OR
Sport* OR Seat-Belt OR Gz OR Shower OR
Disabled OR Sewing OR Pregnant OR
Aviation* OR Rehab* OR Supermarket* OR
Gravity OR Proceedings (under source)
Seat OR Seating
Sitting
Armrest*
Backrest*
Headrest*
Caster*
76
Modifiers
Office OR Ergonomics
121
40
1
1
6
Table A.6: Seating search results
Date
ID
Keywords
10/28/2002
A
B
C
D
E
F
G
H
Chair OR Chairs
Chair OR Chairs <> Eliminators
Seating <> A
Sitting <> B
Armrest* <> C
Backrest* <> D
Headrest* <> E
Caster* <> F
No.
Hits
424
126
204
107
2
0
1
0
No.
Downloaded
0
126
204
107
2
0
1
0
Table A.7: Lighting keywords
Primary Keyword
Lighting
"Refresh Rate"
Flicker
"Screen reflection"
Eliminators
Proceedings (From Source) OR Traffic OR
Highway* OR Street* or Automobile* or Car*
or Transportation* OR Headlamp* OR Driver*
OR roadway* OR Emergency OR Escape OR
School* OR Security OR Mine* OR Industry
OR Industrial OR Floodlighting OR Dental
Modifiers
Glare OR Position OR Placement OR Office*
OR VDT* OR Discomfort OR Performance OR
Productivity OR Strain OR Health
Light*
Table A.8: Lighting search results
Date
ID
Keywords
10/24/2002
A
B
C
D
E
Lighting <> Eliminators
B AND Modifiers
Refresh rate <> Eliminators
Flicker <> Eliminators AND Modifiers
Screen reflection <> Eliminators
No.
Hits
562
333
24
32
0
No.
Downloaded
333
24
32
Table A.9: Breaks keywords
Primary Keyword
Rest Break
Rest Period
Eliminators
Proceedings
Proceedings (From Source) OR Vibration OR
Cold OR Industry
Rest Cycle
Office, Typing
Office
Office OR Typing OR "data entry" OR
Keyboarding
Rest Pauses
Work Break
Work Pauses
Exercise Break
Microbreak
Modifiers
Office
Proceedings (From Source) OR Vibration OR
Cold OR Industry OR Laboratory
77
Table A.10: Breaks search results
Date
ID
Keywords
10/24/2002
A
B
C
D
E
F
G
H
I
J
"Rest Break" OR "Rest Breaks"
A <> Eliminators
"Rest Period" OR "Rest Periods" <> B
"Rest Cycle" OR "Rest Cycles" <> C
"Rest Pause" OR "Rest Pauses" <> D
"Rest Pause" OR "Rest Pauses" <> D AND Modifiers
"Work Break" OR "Work Breaks" <> E
"Work Pause" OR "Work Pauses" <> G
"Exercise Break" OR "Exercise Breaks" <> H
Microbreak*
78
No.
Hits
74
48
87
3
548
80
2
2
3
9
No.
Downloaded
48
87
3
80
2
2
3
9
Appendix B – Input Device Research
Table B.1: Advantages/disadvantages of alternative keyboards and standard designs?
Article
Gerard et al 1994
Keyboard Type
• Standard keyboard
• Kinesis keyboard with split
design with tilted and
concave keys
Gilad et al. 2000
•
•
•
•
Hoffmann et al. 1995
• Simulated keyboard taking
subject anthropometry into
account
Serina et al. (1999)
Smith et al. 1998
• Adjustable/split keyboard
Swanson et al. 1997
• Alternative designs
Zecevic et al. 2000
• Standard
• Contoured split keyboard
• Adjustable split keyboard
Results
• 72% of normal speed proficiency had been achieved
in 115 min
• 97% of normal accuracy had been achieved in 65
minutes.
• Negative keyboard requires less effort than Apart or
Tony
• The Tony and apart designs created more shoulder
effort than the flat design
• Clear preference for flat design followed by negative
tilt
• Shows a clear reduction in performance with the Tony
and Apart designs
• Improvement in quality when using the negative
condition
• Supports assumption that Main wrist strain is a result
of extension not Ulnar deviation
o
• 10 negative tilt creates a more natural wrist position
• Minimum movement times occurred when the interkey spacing was approximately equal to the finger pad
size.
• On an ‘ideal’ workstation set-up, a majority of subjects
typed with a mean wrist extension angle greater than
o
15 with more than one-quarter of subjects typing with
o
a mean wrist ulnar deviation angle greater than 20
• In general, wrist postures and motions were not
predictable based on body size, elbow height, hand
length, shoulder width, or wrist dimensions.
• It took ~2 hours of use before the subjects
performance on a split keyboard equaled that of a
traditional keyboard
• Subjects experienced pain toward the end of the trial
regardless of the keyboard they used.
• The split keyboard reduced wrist/hand pronation.
o
• Overall performance for keyboard with 9 lateral
inclination, 7.5" between G and H keys was poorer
o
than keyboard with 12 lateral inclination, 1.5"
distance between G and H or the standard keyboard
• The decrease in productivity on the alternative
keyboards seen in the first day was recovered by the
third day of the trial
• There were no differences between keyboard
conditions on ratings of overall discomfort, which were
of minimal magnitude throughout the study.
• Typists were most productive when using the
standard keyboard. Next, the contoured split
keyboard, followed by the adjustable split keyboard.
• Both alternative keyboards reduced wrist extension
compared to the standard design.
• The contoured split keyboard significantly reduced
ulnar deviation while the adjustable split keyboard
significantly increased radial deviation.
• More time was spent in neutral and moderate ranges
of extension/flexion and radial/ulnar deviation when
typing on the contoured split than on the adjustable
split or standard keyboards.
• Hand position in terms of pronation and supination:
The alternative designs reduced pronation compared
to the standard design.
Standard keyboard
Tony design
Negative tilt keyboard
Split adjustable keyboard
79
Table B.2: Are wrist rests beneficial during keying tasks?
Article
Wrist rest
Smith et al. 1998
• Compared typing with wrist
rest against typing without.
Keller et al., 1998
• Compared typing with wrist
rest against typing without.
Parsons, 1991
• Compared different types of
wrist rests
Fernstrom et al., 1994
• Impact of palm rest on
muscular activity studied
Results
• When a wrist rest was not used, performance on the
first and second days of using the keyboard was
very similar. However, when a wrist rest was used,
performance on the second day of using the
keyboard was considerably higher.
• More left hand extension was found in users who
did not use the wrist rest
• Participants using wrists rests reported feeling more
in control of their typing than did those who did not
use wrist rests
• Participants who did not use a wrist rest reported
more pain in the front of their shoulders and more
pain at the outside of their elbow/forearm than those
who did use a wrist rest
• Muscle activity for wrist rest users was half the
amount seen in non-wrist rest users
• Evaluated nine different types of wrist rests. At the
end of the study only four of the forty subjects found
the wrist rests useful in decreasing discomfort, the
remaining thirty-six subjects did not find them
useful, and seven found that discomfort increased
while using the wrist rests
• Palm rest did not decrease strain in the forearms
• Palm rest did not decrease strain in the shoulders
Table B.3: What effects does keystroke force have on the musculoskeletal system?
Article
Keystroke Force
Armstrong et al. 1994
• Keystroke makeforce
between keyboards varied
from 0.53N to 0.89N
Rempel et al. 1997
• Keystroke makeforce at
values 0.34N, 0.47N, 1.02
N
Thompson 1999
• Compared keyboards with a
linear-spring (no tactile
feedback), a collapsing
(snap) spring, and a rubber
dome mechanism
80
Results
• Subjects use 2.5 to 3.9 times the required activation
force.
• Average keying force was lowest for the keyboard
with lowest required activation force.
• Keyboard reaction forces appear to be an indicator
of finger forces for keying tasks.
• No differences in applied fingertip force or finger
flexor EMG were observed during typing on
keyboards with switch makeforce of 0.34 or 0.47 N
• Applied fingertip force increased by approximately
40% when the keystroke makeforce was increased
from 0.47 to 1.02 N
• EMG activity increased by approximately 20% when
the keystroke makeforce was increased from 0.47 to
1.02 N
• The overforce applied is 2 to 3 times that simply
required to depress the key to the bottom.
• Keyboarders apply substantially more key force to
keyboards without tactile feedback than to those that
do have feedback.
Table B.4: What is the optimal location for the keyboard?
Article
Location Parameter
Sauter et al., 1991
• Height
Wolstad et al., 1993
• Simultaneous height and
slope adjustments
Black et al., 1997
• Height
Simoneau et al., 2001
• Height and slope
Gilad, 2000
• Negative keyboard slope
Results
• Arm discomfort increases as the keyboard is raised
above elbow level
• Minimum deviations were seen when higher
keyboard heights were coupled with positive slopes
• Least deviation for lower keyboard heights was
observed when coupled with negative slopes
• As the keyboard height was raised the muscle
activity of the trapezius and shoulder flexors
increased
• The biceps and deltoids had the lowest EMG values
at elbow height
• Increase in trapezius muscle activity correlated with
discomfort levels
• Wrist extension angles decrease as keyboard height
increases above elbow height
• Wrist extension angles reached a minimum of 9° at a
negative 15° keyboard slope, supporting a negatively
tilted keyboard
• Downward tilting keyboard with a negative 10° slope
improved the quality of the work and decreased
forearm muscle activity
• Users preferred the flat keyboard position to the
negative tilt position
Table B.5: Does the duration of keyboard use affect musculoskeletal discomfort?
Article
Hanson et al. 1997
Smith et al. 1998
Swanson et al. 1997
Duration of Use
• Subjects answered survey
questions which required
listing the amount of hours
spent on the keyboard each
week
• Subjects typed for 5 hours a
day for 5 days
• Three days of testing with
each day divided into four 75
minute sessions
81
Results
• After adjusting for age and gender, the most
significant factor associated with symptoms of upper
limb disorders was the length of time the subject
spent at the keyboard during a week.
• Subjects experienced pain toward the end of the trial
regardless of the keyboard they used.
• There was a significant increase in discomfort
across the workday
• There were also significant increases in discomfort
across the three days of study (left and right upper
arm/shoulder and neck)
82
Appendix C – Seating Research
Table C.1: Chair features addressing discomfort – backrest inclination
Article
Chair – Backrest Inclination
Bendix, A. et al., 1988
• Adjustable backrest
• Ht adjust (47-55 cm)
Burgess-Limerick et al.,
2000
• Trunk angle (100°-110°)
• Monitor position (65° below
to 30° above horizontal eye
sight)
Treaster et al. 1987
• Adjust backrest (90°-120°)
Vergara & Page, 2000
• No ranges given
• No ranges given
Results
• Subjects preferred tiltable chairs over Balans for long
periods of time
• No effect of spinal shrinkage
• Optimal location for visual targets is at least 15º below
horizontal eye level. Preferred gaze angle for subjects
ranged from 19-36º below ear-eye line (avg. = 27º).
There was a strong relationship b/n backrest
inclination & cervical flexion. Increases in trunk angle
induced by changes in backrest inclination were
associated with corresponding decreases in neck
angle, indicating increased curvical flexion. Thus,
backrest inclination has no significant influence on
gaze angle; only the monitor position causes the neck
angle to decrease
• As angle b/n torso & thighs increases w/ increased
inclination of backrest, more pressure transferred to
backrest, less on spine
• Periods with greater lumbar discomfort correspond to
just dorsal contact or no contact with the backrest.
Backrest inclination of the chair corresponds with the
amount of lumbar contact
• Great changes of posture are a good indicator of
discomfort, and that lordotic postures w/ forward
leaned pelvis & low mobility are principal causes of
discomfort
Table C.2: Chair features addressing discomfort – arm support
Article
Garcia et al., 1998
Hasegawa &
Kumashiro, 1998
Chair – Armrests
• Armrest Ht, range = up to 7
cm above work surface, arm
support attaches to work
surface, not chair
• Breadth, range=17-21" =
482.6-533.4 mm
• Results indicated that arm supports had a significant
effect on comfort, effort required, and RPE. Type of
arm support did not statistically effect muscle activity.
However, subjective results suggest that the Ergorest
arm support is recommended
Results
• Armrest Ht, range = 22-28
cm
• Breadth, N/A
• Use of armrests is effective for alleviation of muscles
in one-handed keyboard operation. A chair with
height-adjustable armrests is considered desirable
when used by several people
Table C.3: Task variability addresses discomfort
Article
Dieen et al., 1997
Dieen et al., 2001
Mandal, 1989
Nelson & Silverstein,
1998
Task
Results
• Back load and discomfort were lower when working
on chair. However, back discomfort increased
throughout work on chair
• Tasks effected trunk load more than the chair features
• Tasks: Sitting and kneeling
• Tasks: Typing, CAD, reading
• Tasks: reading & writing
• Work surface inclination,
range = 0 to 20 degrees
• Work surface height, range =
72-92 cm
• Seat ht = 158-175 cm
• Subjects preferred sitting 10-20 cm higher than
recommended current standards when reading &
writing
• Hand/arm symptoms associated w/ desk comfort,
satisfaction, and VDT use. Neck/shoulder/back
symptoms associated w/ intensive work in chair/desk
• Tasks: VDT work, intensive
office work
83
Table C.4: Chair features addressing discomfort – seat pan inclination
Article
Bendix & BieringSørensen, 1983
Bridger, 1988
Congleton, 1987
Chair – Seat Pan
Results
• With increasing forward inclination of the seat, the
spine moved toward lumbar lordosis
• 5° forward inclination & horizontal seats were
preferred based on comfort
• Seat pan inclination, range =
0° to 15° anteriorly
• No backrests were used
• Seat pan slope = forward 25°
• Chair type: Balans, kneesupported
• Work surface inclination =
15°
• The neutral posture chair is a
combination of a forwardsloping cultivator seat and an
English saddle, with wrap
around leg trough support.
• Forward-sloping chair and tilted work surface used in
combination resulted in reduced trunk flexion and
more comfort
• The neutral posture chair was found to increase
productivity, improve body comfort, and reduce
buttock-thigh pressure
• Backrest and seat pan angles were plotted over time
and analyzed using both graphical and statistical
methods. The resulting scatter plots do not support
the industry standard, 1:2 or 1:3 ratio, of changes in
seat pan to backrest angles. The possibility of a
variable linkage is discussed; however, problems
associated with such a solution raise the possibility
that control issues might be best addressed through
training and exploration
• Seat and backrest inclination which reduces shear
forces on the seat in passive seating forms the center
of attention
• For chairs it was found that when little shear is
accepted, a fixed inclination between seat and
backrest can be chosen between 90°and 95°
• To avoid neck problems, a chair with a forward incline
greater than 5° should not be used and to avoid lower
back problems a chair with a seat angle of
approximately 10° be used. For comfort it is
recommended to use a chair with a seat pan that
inclines to approximately 15° (forward)
• Non-significant trends indicate that light assembly
production rose when using the ergonomic chairs an
average of 7% in the final 30 minutes of each session.
Overall comfort ratings favored the ergonomic chairs
• Nonsignificant trends indicated that assembly rates
rose 5-7.8% when using an 185º or 155º angle in the
final 30 minutes and 7.5-9% in the final 15 minutes
• Muscles showed practically no activity in
unconstrained erect posture. During unconstrained
sitting both oblique abdominals are active. In most
subjects the activity of the oblique abdominals was
significantly smaller when sitting on a soft car seat
than when sitting on an office chair with a hard seat.
Contraction of abdominals in unconstrained standing
and sitting may help in stabilizing the basis of the
spine and particularly the sacroiliac joints. During
standing and sitting the abdominal muscles have a
significant role in sustaining gravity loads
Gardner, 1995
• Specifics discussed in article
Goossens & Snijders,
1995
0° to 20°
Naqvi, 1994
5°, 10°, 15° anteriorly
Rogers et al., 1990
• Thigh work-surface angle =
185° and 155° angle
Snijders, 1995
• Trunk angle, range = 0º
(erect) to 65º, max stooping
Soderberg et al., 1986
0-20° anteriorly
• Chair type: Balans, kneesupported, 100º to 110°
• Greatest inclination of seat pan 20º anteriorly was
most comfortable & produced least EMG activity
Van Deursen et al.,
2000
• Rotary continuous passive
motion of the seat pan
• Measurements with and
without dynamic stimulation
of seat pan.
• Significant differences (0.014 less than p<0.129) in
lower leg swelling were found between the static and
the dynamic situation. Female subjects showed an
increased response in time-related leg swelling and
the age of subjects also influenced the results. It is
concluded that dynamic stimulation during sitting has
an oedema reducing effect on the lower extremities
84
Table C.5: Chair features addressing discomfort – lumbar support
Article
Bendix, T. et al., 1996
Coleman et al., 1998
Chair – Lumbar Support
• 7 cm curvature b/n top &
middle
• A: no backrest, B: 20 cm
vertically, 36 cm
transversally; C: 30 cm
vertically, 32 cm
transversally
• Lumbar Ht: Range=110231mm from top of seat,
mean=190 mm
• Lumbar Depth: Range=312458mm from front of seat,
mean=387 mm
Hermans et al., 1999
• Lipoatrophia semicircularis
(LS) – band-like circular
depressions & isolated
atrophy of subcutaneous
fatty tissue
Porter & Norris, 1987
• Seat ht = 874(42) - 942(29)
• Lumbar Ht: 185-315 mm
• Lumbar Depth: 10-50 mm
forward from backrest
• No ranges given
• No ranges given
Results
• Traditional idea that backrest facilitates lordosis was
found NOT true. Backrests facilitate stabilization of
lumbar spines by providing lower backs w/ support,
resulting in relative kyphotic increases
• Higher lumbar supports were chosen by subjects with
greater Body Mass Index (BMI)
• Subjects who reported back pain or discomfort
adjusted lumbar supports closer to front of seat
• Taller subjects preferred higher lumbar supports &
vice versa
• Traditional padded fixed-ht lumbar supports are
unlikely to provide comfortable & appropriate for the
range of users
• Remarkable posture differences b/n LS group & nonLS group. Less use of lumbar supp, static posture,
excessive seat height observed from LS group
• Pressures on front of chair for LS group were on avg.
30% higher than non-LS
•
LS group had more static sitting posture as
compared to others – locking back
• Females preferred lumbar support 10 mm lower than
males. The preferred in/out location was 20mm
forwards of backrest
• Four types of backrest use were detected determining
lumbar curve. & pelvic incl angles. The simple system
to measure backrest use is successful. Periods with
greater lumbar discomfort correspond to just dorsal
contact or no contact with the backrest
• Great changes of posture are a good indicator of
discomfort, and that lordotic postures w/ forward
leaned pelvis & low mobility are principal causes of
discomfort
Table C.6: Methods for quantifying chair features
Backrest Inclination
Seat Pan Inclination
Lumbar Support
Arm Support
85
Heart Rate
X-ray
Changes in Leg Volume
Infrared Emitting Diodes:
Optoelectronic movement
Statometric Measures of
Posture
Pressure Mapping
Strain Gauges
Electromyography (EMG)
Chair Features
Discomfort Surveys
Methods of Quantification
86
Appendix D – Breaks Research
Table D.1: Rest-break research addressing discomfort
Article
Variable(s) Tested
(Break in min. / Work in min.)
Task
Results
Balci et al.,
1998
Over 3 hours subjects took:
• Short Break (4 /30)
• Long Break (10 /60)
Students
performing
intensive
typing task
• Short Break reduced musculoskeletal
discomfort (measured by EMG of
arm/neck and by discomfort survey)
Boucsein &
Thum, 1997
Lunch Break plus:
• Short Break (7.5 /50)
• Long Break (15 /100)
• Scheduled Breaks
• Nonscheduled Breaks
Patent
Examiners
writing reports
• Break length did not influence physical
strain (measured by EMG levels of the
neck.)
Galinsky et
al., 2000
Conventional Breaks plus:
• No Additional Breaks
• Four, 5 min Breaks (5 /60)
Data entry
operators
performing tax
processing
• Supplemental breaks reduced muscle
discomfort (measured by discomfort
survey)
• Supplemental breaks reduced eye
soreness and blurring (measured by
discomfort survey)
Helander &
Quance,
1990
• Infrequent (40 /240)
• (20 /120)
• (10 /60)
• Very Frequent (5 /30)
Students
performing
intensive
typing task
• Less spinal shrinkage occurred with the
20 or 40 min break condition
• Subject preferred the 20 min break
condition
• Frequent Breaks (0.5 or 3/15)
• Exercise Breaks (0.5 or 3/15)
Insurance
claims
processors
• Visual discomfort (measured by
discomfort survey) improved with both
break conditions
• Exercise Breaks reduced visual
discomfort the most
• Breaks had no significant effect on
muscle discomfort
Lim et al.,
1990
• User defined
Governmental
agency
including
professionals
and clerical
VDT users.
• Frequency and length of rest breaks do
not affect self-reported back pain
McLean et
al.,
2001
• Frequent Breaks (0.5/20)
• Infrequent Breaks (0.5/40)
• User Defined 0.5 min Breaks
Office workers
performing
data entry
tasks
• Breaks reduced musculoskeletal
discomfort (measured by myoelectric
signal of neck, back, shoulder, and upper
extremities and by discomfort surveys)
• Frequent breaks reduced
musculoskeletal discomfort the most
Henning et
al., 1993
87
Table D.2: Rest-break research addressing productivity and/or costs
Article
Variable(s) Tested
Task
Results
Students
performing
intensive
typing task
• Break length did not affect productivity or
error rates
Henning et
al.
1997
Conventional breaks plus:
• No additional breaks
• Frequent breaks (0.5 or 3/15)
• Exercise breaks (0.5 or 3/15)
Computer
operators at
two worksites
• No improvement in productivity
(measured by claims/hour) at the larger
worksite
• Exercise breaks improved productivity at
the smaller worksite
• Low compliance only completed ½ of
added breaks
Henning et
al., 1993
Insurance
claims
processors
• Breaks improved productivity (measured
by claims/hour) with exercise breaks
improving productivity the most.
Henning et
al. 1989
• Frequent break (10/40) and micro
break (user set /20)
Galinsky et
al., 2000
• Four, 5 min breaks (5 /60)
Mclean et al.,
2001
• Frequent breaks (0.5/20)
• Infrequent breaks (0.5/40)
• User defined 0.5 min breaks
Office workers
performing
data entry
tasks
• Conventional breaks
• Frequent breaks (10/60)
Clerical
workers
• Hourly breaks
VDT Task
• 20-30 sec breaks as needed and
two (5/60) exercise breaks
Data-entry
operators
Balci et al.,
1998
• Short break (4/30)
• Long break (10/60)
Experienced
Typist
performing
intensive data
entry task
Data entry
operators
performing tax
processing
• Micro breaks grew longer as
performance dropped
• Break frequency had no significant effect
on productivity (measured by word
counts and percent accuracy)
• Break frequency had no significant effect
on productivity (measured by word
counts)
In Swanson
et al., 1989
• Bhatia and
Murrell,
1969
• Horie et
al., 1987
Thompson,
1990
88
• Frequent breaks were preferred by
workers and showed greater productivity
gains.
• Hourly breaks improved worker
productivity
• 25% increase in productivity
• No new claims filed over a 1 year period
• Less overtime taken by employees
Table D.3: Rest-break research addressing subject emotional state
Article
Variable(s) Tested
Task
Results
Boucsein &
Thum, 1997
Lunch break plus:
• Short break (7.5 /50)
• Long break (15 /100)
• Scheduled breaks
• Nonscheduled breaks
Patent
Examiners
writing reports
using VDT
• Short breaks were more effective for
reducing emotional and mental strain
(measured by electrodermal activity and
heart rate variability respectively) until
late afternoon.
• Long breaks were more effective for
reducing emotional strain in the late
afternoon.
Galinsky et
al., 2000
• Four, 5 min breaks (5 /60)
Data entry
operators
performing tax
processing
mood (measured by survey).
Computer
operators at
two worksites
Insurance
claims
processors
• Breaks did not influence employee
Experienced
Typist
performing
intensive data
entry task
• As boredom increased (measured by
survey) subjects look longer
microbreaks.
Henning et
al., 1997
Henning et
al., 1993
Henning et
al. 1989
• Frequent break (10/40) and
Microbreak (user set /20)
89
Table D.4: Rest-Break Research Addressing Exercise Breaks
Article
Variable(s) Tested
Task
Results
Reviewed by
Anon, 1999
• Lee et al.,
1985
• Exercise breaks
• VDT work
• Lee et al,
1992
• Exercise breaks
• Not listed
• Swanson et
al., 1993
• Passive breaks
• Exercise breaks
• Not listed
• Thompson,
1993
• Wrist exercise
• No wrist exercises
• Williams et
al., 1989
• Before 10 week program
• After 10 week program
• Lee et al.,
1992
• Exercises in general
Henning et al.,
1997
Computer
operators at
two worksites
Henning et al.,
1993
Insurance
claims
processors
Thompson,
1990
• 20-30 sec breaks as needed and
two (5/60) exercise breaks
Data-entry
operators
• Operators
• Garment
workers
• Not listed
90
• Exercise reduces strain of the neck and
trapezius muscles caused by rigid work
postures
• Exercise reduces muscle fatigue
• No significant differences on comfort
and mood
• Exercises had no significant effect on
carpal tunnel syndrome symptoms
• No significant change in carpal tunnel
syndrome incidence or symptoms after
10-week exercise program
• May exacerbate existing problems or
create new hazards
• Exercise breaks improved eye, leg, and
foot comfort (measured by survey)
better than frequent breaks at the
smaller worksite
• Greatest increases in eye comfort
(measured by survey) occurred with
exercise breaks
• Leg comfort (measured by survey)
increased only during exercise breaks
• 25% increase in productivity
• No new claims filed over a 1-year period
• Less overtime taken by employees
Table D.5: Rest-break research addressing regimented vs. self-determined breaks
Article
Boucsein &
Thum, 1997
Henning et al.,
1996
Henning et al.,
1994
Variable(s) Tested
Break (min.) / Work (min.)
Lunch Break plus:
• Short break (7.5/50)
• Long break (15/100)
• Regimented breaks
• Breaks due to co-worker
interruptions and system
breakdown
Over 65 minutes subjects took:
• Regimented breaks (0.5/10)
• Continuous feedback (only force
break if target of 20 sec. not
reached by user)
Over a 48 minutes subjects took:
• Regimented breaks (0.33/5)
• Compensatory (0.33/5) 20
seconds minus spontaneous rest
pauses longer than 3 sec.
Task
Patent
examiners
writing
reports
Students
performing
intensive
typing task
Students
performing
intensive
typing task
Henning et al.,
1989
• Frequent break (10/40) and Micro
break (user set /20)
Experienced
typist
performing
intensive
data entry
task
McLean et al.,
2001
• Frequent breaks (0.5/20)
• Infrequent breaks (0.5/40)
• User defined 0.5 min breaks
Office
workers
performing
data entry
tasks
91
Results
• Less physical stain of the neck (measure
with EMG) occurred during scheduled
rest breaks
• Emotional strain (measured by
electrodermal activity) increased with
rigid scheduled breaks versus
nonscheduled breaks
• Continuous feedback improved selfmanagement with no effects on
performance (measured by keystroke
rate, error rate, and correction rate) or
mood (measured by cardiac response
and by survey)
• Subjects preferred continuous feedback
(measured by survey)
• Users took 20 seconds of micro breaks
on their own with out having to be forced
by computer
• No differences in performance
(measured by error rate, correction rate,
keystroke rate), mood (survey), or
acceptance (survey) between systems
• Back discomfort was lower in the
compensatory condition
• Employees terminated micro breaks
before recovery could occur
• Scheduled breaks reduced muscle
discomfort (measured by myoelectric
signal of neck, back, shoulder, and
upper extremities and by discomfort
surveys) better than self determined
breaks
TheErgonomicsCenter
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T
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Advancing the Science of Ergonomics in the Workplace SM
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Effects of Daylight - The Ergonomics Center Of North Carolina

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