Handgrip and Box Tilting Strategies in Handling: Effect on Stability

INTERNATIONAL JOURNAL OF
OCCUPATIONAL SAFETY AND ERGONOMICS 1996, VOL. 2, NO. 2, 109-118
Handgrip and Box Tilting Strategies in Handling:
Effect on Stability and Trunk and Knee Efforts
Alain Delisle
Micheline Gagnon
Pierre Desjardins
Universite de Montreal, Canada
The purpose o f this study was to evaluate the effect o f fou r handgrip/box tiltin g strategies (right,
left, backward, and no tilt o f the box) on trunk and knee efforts, body posture, and the stability
o f 14 participants w ith lim ited experience in handling. The tasks consisted of transferring a
lo w -lying box placed in fro n t o f the participant to a shelf o f the same height at the participant's
left. It was hypothesized that tiltin g the box could reduce trunk and knee efforts as w ell as body
asym m etry and im prove stability. A tridim ensional dynam ic rigid body model was used to
estimate the triaxial net m uscular m om ent magnitudes at the trunk (L5/SI) and at the knees. An
approach to quantify the participants' dynam ic stability was also included. Finally, five angles
were com puted to characterize body asymmetries. The results showed that tiltin g the box af­
fected specific trunk efforts, but did not succeed in reducing trunk asym m etric efforts. However,
the tilts were executed in a single direction, and it may be possible that com bined tilts o f the
box could help reduce trunk asym m etric efforts. Tilting the box had little effect on knee load­
ings, and the left tilt strategy reduced participants' stability. This study showed the im portance
o f considering the position o f the box when assessing the risks encountered in asym m etrical
handling.
handgrip
box tilts
manual handling tridim ensional reaction m om ents
low back knees stability
1. INTRODUCTION
For the last two decades, low-back pain has received much attention in research; yet it remains
a major health problem in industry. Asymmetrical manual materials handling has long been
associated with low-back pain. In vitro and simulation studies have produced results that
emphasize the strain supported by the soft tissues of the spine when forward bending is
coupled with torsion or lateral bending (Adams & Hutton, 1985; Gordon et al., 1991; Hickey
& Hukins, 1979; Shirazi-Adl, 1989). However, asymmetric hand positions and the box move­
ments encountered while handling also constitute a form of asymmetry and have received little
attention in occupational biomechanics. The impact of this form of asymmetry on joint load­
ings is not known.
Hand position has been studied in the presence of handles only (Bishu & Wei, 1992; Coury
& Drury, 1982; Deeb, Drury, & Begbie, 1985). Asymmetrical handle position stabilized the box
horizontally and vertically and minimized the physiological stress perceived by the participants
This work was funded by the Institut de Recherche en SantS et en Security du Travail du Quebec and the Natural
Sciences and Engineering Research Council of Canada. A. Delisle is supported by a PhD studentship from Fonds pour
la Formation de Chercheurs et d’Aide 4 la Recherche and from the Faculty des Etudes superieures de I’Universite de
Montreal.
Relevance: Biomechanical analyses of the impact of handling parameters such as the handgrip and box tilts are
essential for understanding body loadings encountered in asymmetrical handling.
Correspondence and requests for reprints should be sent to Alain Delisle, D6partement d’Education Physique,
University de Montreal C.P. 6128, succ. Centre-Ville Montreal (Quebec), Canada H3C 3J7. E-mail: <delislea@ere.
umontreal.ca>
109
110
A. DELISLE, M. GAGNON, AND R DESJARDINS
(Coury & Drury, 1982), whereas symmetric hand position was shown to minimize hand forces
(Deeb et al., 1985). These results are difficult to apply in industry, because field studies have
shown that most of the boxes are not provided with handles (Drury, Law, & Pawenski, 1982;
Kuorinka, Lortie, & Gautreau, 1994; Lortie, Baril-Gingras, & Authier, 1993). These field studies
also showed that handlers developed different grip and tilt strategies that may be considered
as a means for compensating the absence of handles. Recently, the impact of the handling
context on the choice of grip and box movement in expert and novice handlers was studied
(Authier, Lortie, & Gagnon, 1996). The results showed that experts more frequently tilted the
box during transfer. Right/left tilts were the most frequent, and backward/forward tilts were
also important. At deposit, the boxes remained tilted in 43% of the cases. Furthermore, the
grips most often used by experts could be associated with specific tilts of the box. Novices
generally carried the boxes without tilting them, and they systematically performed deposit
with the box held flat. Similar ergonomic observations were made in a biomechanical study
comparing expert and novice strategies in the free handling of low-lying loads (Gagnon,
Plamondon, Gravel, & Lortie, in press). The results of the ergonomic observations (Delisle &
Gagnon, 1993) generally confirmed those of Authier et al. (1996). Experts tilted the box more
frequently than novices during transfer and performed the deposit with the box most fre­
quently tilted. The most frequent grip for both groups was asymmetric with the hands on
diagonally opposed corners (the 3/7 hand position, Figure 1), as was the case in other studies
(Authier et al., 1996; Drury et al., 1982). However, the second most frequent handgrip for
experts consisted of a symmetrical hand position on the middle of diagonally opposed edges
(the 8/2 grip, Figure 1). Furthermore, different grips could be associated with different tilts
during transfer, resulting in different types of deposit.
The aim of this study was to investigate the effect of different handgrip/box tilting strategies
resulting in different types of deposit, as used by expert and novice handlers, on trunk and knee
efforts as well as on the stability condition. It was believed that deposits performed on edges
of the box (resulting from different tilts of the box), as executed by expert handlers, would
reduce trunk and knee efforts compared to deposit performed with the box kept flat, as
executed by novice handlers. Moreover, it was hypothesized that expert handgrip/box tilting
strategies could help reduce trunk asymmetric efforts and offer better stability and could also
reduce body posture asymmetries. Because a new method is presented for measuring body
asymmetry of posture, its validity for characterizing trunk asymmetry of posture is, therefore,
assessed by confronting it with trunk angles computed from an approach similar to Grood and
Suntay (1983). This study may help understand workers’ choices of handling strategies in order
to develop more appropriate training programs.
Figure 1.
The hand position convention, based on Drury et al. (1982).
HANDGRIPS AND BOX TILTS IN HANDLING
111
2. METHODS
Fourteen healthy male participants volunteered to participate in this study and were finan­
cially compensated. They were all college students at the Universite de Montreal. Their
experience in different manual handling jobs was limited and varied between 3 and 14 months
(one to three summertime jobs). Their mean age was 21.1 years (range: 19-23 years), their mass
was 76.9 kg (range: 65.9-88.2 kg), and their height was 1.78 m (range: 1.65-1.86 m). The written
consent of all participants was obtained after they were properly informed; the experimental
protocol had been written according to the guidelines of the Ethics Review Board of the
Universit6 de Montreal and was accepted by the Board.
The tasks were designed in order to compare different types of deposit as a result of
different combinations of hand position and box tilts while handling. Therefore, the tasks were
all executed with the same feet position (57 cm apart); participants maintained their feet fixed
throughout the tasks and were asked to keep their knees flexed. They were asked to move a
12-kg box (32 cm X 32 cm X 46 cm) from a 16-cm shelf in front of them to a shelf of the same
height at their left. The initial and final positions formed a 90° angle. There were three tasks
with the hands on diagonally opposed corners: the right hand on the furthest upper right corner
and the left hand on the lower left corner of the box (3/7, Figure 1). These three tasks were
distinguished by the tilt of the box during handling resulting in three different types of deposit:
First, there was not any tilt (flat technique, Figure 2A), the box was kept parallel to the ground
and the deposit was flat on the bottom of the box; second, the box was tilted backward
(backward tilt technique, Figure 2B) or toward the participant’s body resulting in a deposit on
the lower front edge of the box; third, the box was tilted laterally to the right (right tilt
Figure 2. Illustration of th e tasks w hile transferring th e box from th e initial shelf (participant's
right side) to th e final shelf (participant's left side) for: (A) th e flat tilt technique, (B) th e backward
tilt technique, (C) th e right tilt technique, and (D) th e left tilt technique.
112
A. DELISLE, M. GAGNON, AND P. DESJARDINS
technique, Figure 2C) resulting in a deposit on the lower right edge of the box. The fourth task
involved another type of grip resulting in another type of tilt and deposit: The box was first
tilted on its lower left edge, the right hand was then placed on the middle of the lower right
edge and the left hand on the middle of the upper left edge of the box (8/2, Figure 1). After
having tilted the box to the left (left tilt technique, Figure 2D), and after having transferred it,
the deposit was made on the lower left edge of the box.
The data were collected using two AMTI (Advanced Mechanical Technology Inc., Newton,
MA) force platforms, for the measurement of the three components of the resulting force
under each foot as well as the-point of application of the resulting force and the couple about
the vertical axis of the platform. A Peak motion measurement system (Peak Performance
Technologies, Englewood, CO) with four video cameras (Panasonic WVD-5100) were used to
collect the positions of the 29 anatomical markers on the participants. The force and film data
were electronically synchronized by an electrical pulse and were sampled at 60 Hz. The
three-dimensional (3-D) locations of the markers as well as the locations of the markers for
the orientation of the force platforms were obtained by Direct Linear Transformation (DLT)
procedures (Marzan, 1976). The calibration was realized by filming 24 points distributed in a
volume of 3.9 m3. The accuracy and precision of the 3-D reconstruction were assessed by
filming another set of 24 control points covering the same volume. Because in tridimensional
analysis of handling tasks the markers are not always seen by all four cameras, the accuracy
and precision of the 3-D reconstruction were assessed for each combination of two, three, and
four cameras. The DLT reconstruction error, evaluated as the root mean square (RMS),
averaged 3 mm along the x, y, and z axes. The maximal difference between real and recon­
structed coordinates of any point along any axis was 10 mm.
The 3-D dynamic segment model included 14 segments: feet, shanks, thighs, pelvis, and the
trunk with a lower part (from L5/S1 to T12/L1) and an upper part (from T12/L1 to C7/T1),
head-neck, arms, and forearms plus hands. The positions of the markers on each segment were
filtered with a fourth-order zero-phase Butterworth filter (Winter, 1990). The cutoff frequen­
cies were selected automatically from residual analyses (Cappozzo, Leo, & Pedotti, 1975) and
ranged from 0.1 to 3.2 Hz. Finite-difference techniques were applied to calculate linear
velocities and accelerations.
Each body segment was assumed to be a rigid body and had a local coordinate system made
of the orthopaedic axes, corresponding to longitudinal, sagittal, and transverse axes. An inverse
dynamic analysis was performed on each segment to provide the net moment and net forces
at the joints. The inertial properties included the mass, the position vector of the centre of mass
in the local coordinate system, and the moments of inertia about the axes of the local system
(Zatsiorsky & Seluyanov, 1983,1985). It was assumed that the axes of the local coordinate
system corresponded with the principal axes about which the moments of inertia were defined.
For the current analysis, the net muscular moments were reported about the three orthogonal
orthopaedic axes on the trunk at L5/S1 and on the shanks at the knees to represent moments
in axial twisting, lateral bending, and flexion/extension; the trunk was considered as one
segment (from L5/S1 to C7/T1). More information about the development of the 3-D segmen­
tal model can be found in Gagnon and Gagnon (1992).
For the evaluation of the participants’ stability during the tasks, the minimal horizontal
force applied at the centre of gravity that could move the centre of pressure out of the base of
support was estimated. The method for this estimation is described elsewhere (Delisle,
Gagnon, & Desjardins, 1996a). This estimation is based on the assumption that when the centre
of pressure passes outside of the base of support, the system is no longer in equilibrium,
because there is no support against which the reaction force can be applied. It was hypothe­
sized that the static friction force under the feet is always large enough to compensate for any
horizontal force applied to the system. Therefore, a single horizontal force applied at the centre
of gravity that can bring the centre of pressure out of the base of support was calculated to
characterize the level of stability of the participants, and was called the destabilizing force. The
larger the destabilizing force the more stable the participant was.
To better comprehend specific trunk positions relative to the pelvis, the approach described
HANDGRIPS AND BOX TILTS IN HANDLING
113
by Grood and Suntay (1983) was modified and applied to the trunk. This approach is described
elsewhere (Plamondon, Gagnon, & Gravel, 1995). Grood and Suntay’s approach was also
applied at the knees to characterize their orientation. Flexion/extension was defined about the
transverse axis of the thigh, internal/external torsion was defined about the longitudinal axis
of the shank, and abduction/adduction motion was defined about a floating axis normal to the
two preceding axes.
Another tool was incorporated to supplement the kinematics of the tasks, more specifically
to characterize the asymmetry of the whole body. Five ergonomic angles were computed to
evaluate the asymmetry of the body during the tasks. These angles were computed from the
projections of four vectors on the horizontal plane. These vectors characterized the orientation
of the grip, shoulders, pelvis, and feet. The grip was characterized by a vector between the right
and left hand markers, the shoulders by a vector between the two markers at C7/T1 level (14
cm apart), and the pelvis by a vector between the right and left lateral markers at L5/S1. Finally,
the orientation of the feet was defined by a vector normal to the mean longitudinal vector of
both feet. The five ergonomic angles computed were: (a) the grip relative to the shoulders and
(b) to the pelvis, (c) the shoulders relative to the pelvis and (d) to the feet orientation, and (e)
the pelvis relative to the feet orientation. Finally, the orientation of the feet relative to the
initial position of the box was also computed.
Analyses of variance with repeated measures were applied to test the differences, and
multiple comparisons were used to identify the differences. A probability level of .05 was
chosen to identify the major differences.
3. RESULTS
The handgrip/box tilting strategies affected specific trunk efforts (extension and lateral bend­
ing) and specific trunk orientations (torsion and lateral bending), resulting at deposit in larger
coupling of extension and lateral bending efforts with the backward tilt technique and in some
degree of trunk asymmetry for all tasks (Table 1). The trunk resultant and extension moments
were smaller for the backward tilt technique, whereas the right lateral bending moment was
larger. These lateral efforts combined with the trunk extension efforts imply important cou­
pling of efforts for this backward tilt technique. Note that the right tilt technique, as opposed
to the other techniques, resulted in a left lateral bending moment at deposit. At the same time,
the trunk was similarly and deeply flexed for all tasks (about -40°). Larger left torsion was
reached with the right tilt technique(-18°), smaller with the backward tilt technique (-4°),
whereas small lateral bending was present for all tasks, except for the right tilt technique.
For the left knee, all efforts were smaller with the left tilt technique, although its orientation
was similar to all tasks (Table 2). A flexion moment (31-46 Nm), an external torsion moment
(22-27 Nm), and an external lateral bending moment (35-52 Nm) were the predominant
combined efforts at deposit. The right knee characteristics were not studied, because most of
the participants’ weight was supported by their left lower limb at that time.
The ergonomic angles revealed that at deposit the left tilt technique offered the smallest
asymmetry of the grip relative to the pelvis and shoulders, and that the backward tilt technique
offered the smallest asymmetry between the shoulders and pelvis (Table 3). For all tasks, the
pelvis relative to the feet angle was similar (about 40°) and accounted for about 60% of the
asymmetry between the shoulders and feet. The shoulders relative to the pelvis angle was
smaller for the backward tilt technique (16°) and larger for the right tilt technique (30°). As to
the grip orientation, it was more parallel to the pelvis and shoulders for the left tilt technique
than all other tasks, and it was more parallel to the pelvis with the backward tilt technique than
with the flat and right tilt techniques. Finally, the stability was poorer for the left tilt technique
given the smaller minimal destabilizing force encountered at deposit with this task. However,
the orientation of the minimal destabilizing force was similar for all tasks, that is, the greater
risk of a loss of balance was toward the left and slightly toward the back of the participants.
114
A. DELISLE, M. GAGNON, AND P. DESJARDINS
TABLE 1.
Means and Standard Deviations of the Trunk Characteristics at Deposit, and the
Probability Levels ( N = 14, p < .
0
5
)
_____________________________________
Significance Levels
Type of Tilt of Box
Right
(R)
Backward
(B)
Variables
Left
(L)
Flat
(F)
Trunk Efforts
Resultant
m om ent (Nm)
207
(21)
208
(20)
192
(24)
206
(25)
ns
ns
.00
.00
ns
.00
Extension
m om ent (Nm)
203
(21)
203
(23)
184
(23)
203
(25)
ns
ns
.00
.00
ns
.00
Lateral bending
m om ent (Nm)a
28
(27)
33
(24)
51
(14)
-2 2
(19)
.00
ns
.00
.02
.00
.00
-3 9
(8)
-1 3
(6)
6
(7)
-4 2
(8)
-9
(7)
8
(6)
-4 0
(8)
-4
(8)
9
(4)
-4 0
(7)
-1 8
(6)
0
(6)
ns
ns
ns
ns
ns
ns
.01
.04
.00
.00
.00
.00
.00
ns
.02
ns
.00
.00
Trunk Orientation
Flexion angle (°)b
Torsion angle (")b
Lateral bending
angle Ob
Lvs. R L vs. F Lvs. B Fvs. B Fvs. R B vs. R
a Negative sign means an effort in left lateral bending.
b Trunk orientation relative to pelvis. Negative signs mean flexion, left torsion, and right bending.
TABLE 2.
Means and Standard Deviations of the Left Knee Characteristics at deposit, and
the Probability Levels (N = 1 4 , p ^ .05)
_____ _________________________________________
Type of Tilt of Box
Variables
Backward
(B)
Significance Levels
Right
(R)
Left
(L)
Flat
(F)
55
(15)
31
(18)
22
(5)
35
(19)
66
(15)
44
(18)
26
(6)
44
(17)
71
(18)
38
(19)
26
(5)
52
(19)
72
(13)
46
(24)
27
(5)
46
(17)
.00
.01
.00
ns
ns
ns
.01
.00
.03
ns
ns
ns
.00
.01
.00
ns
ns
ns
.01
.03
.00
.03
ns
ns
49
(14)
-1 9
(13)
-4
(8)
53
(14)
-2 6
(13)
-7
(10)
48
(16)
-2 2
(13)
-8
(8)
49
(14)
-2 0
(11)
-8
(6)
ns
ns
ns
ns
ns
ns
ns
.01
ns
ns
.05
ns
.02
ns
.00
ns
ns
ns
Lvs. R L vs. F L vs. B Fvs. B Fvs. R B vs. R
Left Knee Efforts
Resultant
m om ent (Nm)
Flexion
m om ent (Nm)
External torsion
m om ent (Nm)
External bending
m om ent (Nm)
Left Knee Orientation
Flexion angle (°)a
Torsion angle (°)a
Lateral bending
angle (°)a
a Knee orientation w ith Grood and Suntay (1983) approach; negative signs mean extension (0° is fu ll ex­
tension), internal torsion, and adduction of the tibia.
HANDGRIPS AND BOX TILTS IN HANDLING
115
TABLE 3. Means and Standard Deviations of th e Ergonomic Angles and Stability Variables,
and th e Probability Levels (/V = 14, p < .05)
Type of T ilt o f Box
Variables
Ergonom ic Angles
Mean feet
orientation (°)a
Shoulders/feet
at deposit (°)
Pelvis/feet
at deposit (°)
Shoulders/
pelvis
at deposit O
G rip/pelvis
at deposit O
G rip/shoulders
at deposit (°)
S ta b ility
M inim al
destabilizing
force at deposit
(N)
M inim al
destabilizing
force/feet O b
Significance Levels
Left
(L)
Flat
(F)
21
(3)
66
(7)
41
(5)
24
(7)
22
(4)
66
(6)
41
(6)
26
(6)
21
(4)
57
(4)
40
(4)
16
(4)
21
(3)
70
(6)
40
(6)
30
(6)
28
(6)
3
(7)
48
(8)
23
(8)
33
(4)
17
(5)
55
(19)
68
(19)
30
(36)
31
(37)
Backward
(B)
Right
(R)
L vs. R L vs. F L vs. B F vs. B F vs. R B vs. I
ns
ns
ns
ns
ns
ns
.00
ns
.00
.00
.00
.00
ns
ns
ns
ns
ns
ns
.00
ns
.00
.00
.00
.00
49
(6)
19
(7)
.00
.00
.01
.00
ns
.00
.00
.00
.00
.00
.01
ns
64
(23)
71
(15)
.02
.03
ns
ns
ns
ns
45
(13)
35
(47)
ns
ns
ns
ns
ns
ns
a Relative to the initial box position.
b Relative to the base o f support.
4. DISCUSSION
A tridimensional dynamic rigid body model was used to analyze the effect of different types
of deposit, as induced by different grips and tilts of the box, on trunk and knee efforts as well
as on the asymmetry of the body and the stability of the participants in asymmetrical handling
of low-lying loads. The trunk and knee postures, the moment magnitudes about the three
orthopaedic axes at L5/S1 and at both knees, five ergonomic angles, as well as the level of
stability of the participants were used to describe differences in handling techniques occurring
as a function of handgrip and tilt techniques. It was believed that the use of different strategies
of deposit, as induced by different handgrips and tilts of the box, may help minimize the
asymmetric efforts on the body. Furthermore, these strategies may influence the worker’s
stability.
The comparison of the angle between the shoulders and pelvis and the trunk orientation
angles in torsion and lateral bending (modified Grood and Suntay’s approach) shows good
agreement between both methods. This gives evidence of the validity of the new method.
However, correspondence cannot be perfect, because the angles to describe the whole body
asymmetry were computed from the projections of different vectors on the horizontal plane.
This implies that the asymmetry occurring in the frontal plane cannot be described by these
angles. For example, in the tasks studied, the trunk was deeply flexed (about 40°), and the true
torsion of the trunk about its longitudinal axis was not completely considered with the angles
116
A. DELISLE, M. GAGNON, AND P. DESJARDINS
computed between the shoulders and pelvis vectors’ projections. Despite this, the results
obtained from both methods for assessing trunk asymmetry showed good agreement; for
example, the right tilt technique showed larger trunk asymmetry with both methods.
This new tool provided valuable information. Different angles were computed to calculate
the level of asymmetry taken by the lower limbs as compared to the level of asymmetry oc­
curring in the trunk. For the tasks studied, the lower limbs accounted for about A of the
asymmetry observed between the shoulders and the base of support, and the trunk took the
other part. Moreover, the results obtained with the grip relative to the pelvis angle revealed,
as anticipated, that the left tilt and the backward tilt techniques were the handgrips most
parallel to the pelvis. One has to remember that the handgrip used with the left tilt technique
imposed the hands to be on the middle of the edges of the box, whereas the backward tilt
of the box was such that the hands were brought at the same distance antero-posteriorly,
which reduced the asymmetry of the grip relative to the pelvis.
The results of this study showed that the use of different handgrip/box tilting strategies
influences the net muscular moments encountered at deposit, especially at the L5/S1 joint.
However, the types of deposit executed in this study did not succeed in minimizing trunk
asymmetrical efforts. Asymmetrical hand positions have been shown to offer greater vertical
and horizontal stability of the box (Coury & Drury, 1982) and, combined with the tilting of the
box, were perceived as a potential alternative to better distribute the weight of the box in a
symmetrical manner on both hands and to reduce asymmetrical efforts at the trunk (Authier
et al., 1996). However, the box tilts, as executed in this study, resulted in important asymmet­
rical counteracting efforts that can be explained. The right tilt technique, which transferred
most of the weight of the box toward the right hand, resulted in a predominant counteracting
left lateral bending moment. The backward tilt technique transferred most of the weight of the
box toward the left hand, which resulted in a predominant right lateral bending moment. This
technique further shifted the centre of gravity of the box closer to the L5/S1 joint, which
resulted in reduced trunk extension efforts. The left tilt technique also shifted most of the
weight of the box toward the left hand, and a right lateral bending moment was observed at
deposit. This was also the case for the flat technique. Therefore, the flat technique, associated
to novices, was not worst nor better than any other tilting techniques associated with expert
handlers. An important fact that probably limited this study is that all tilts were made in a
single direction (left, right, backward). The combination of different tilts could be advanta­
geous. For example, a lateral tilt combined with a backward tilt of the box could result in a
better distribution of the weight of the box on both hands, and a better balance of trunk lateral
bending efforts. In fact, Authier et al. (1996) observed that experts more frequently laid down
the box on a corner than novices, which implies a combination of tilts in several directions. This
seems an interesting avenue to reduce asymmetric efforts and deserves further research.
Based on the results of this study, the handgrip/box tilting strategies proved to involve
important trunk asymmetric efforts. Gagnon, Plamondon, and Gravel (1993) studied the
effects of different starting postures on trunk triaxial net muscular moments in asymmetrical
tasks. With the trunk flexed forward and twisted about the longitudinal axis of the trunk, they
observed maximum lateral bending moments of 60 Nm. On the other hand, Plamondon et al.
(1995), studying different starting positions of the load (90°, 45°, 0° from the sagittal plane),
observed maximum lateral bending moments of 37 Nm, and these efforts were similar for all
starting positions of the box. In this study, left lateral bending moments at the trunk reached
51 Nm at deposit with the backward tilt technique. Although box dimensions and the tasks in
the aforementioned studies differed from this study, feet position was also fixed. Therefore, it
appears that tilting the box can have an effect on trunk lateral bending moments as important
as the initial trunk orientation or the initial box placement. Moreover, these box tilting
strategies also had some impact on knee efforts, but to a lesser extent. Tilting the box to the
left probably shifted the line of gravity closer to the left knee and reduced the efforts encoun­
tered at deposit for this joint, which was, however, linked to the imposed feet position.
However, efforts at deposit were relatively important for the left knee with 22 to 27 Nm in
internal torsion and 35 to 52 Nm in external bending. The values in torsion are close to the
HANDGRIPS AND BOX TILTS IN HANDLING
117
maximum torsional strength for cadaver knees (35-80 Nm; Piziali, Nagel, Koogle, & Whalen,
1982) and could involve some risks for ligament injury. However, the contraction of the
muscles crossing the knee probably reduces the potential for injury (Louie & Mote, 1987).
Efforts in lateral bending are less critical, because they are much smaller than the maximum
strength of cadaver knees in lateral bending (125-210 Nm; Piziali et al., 1982).
Interestingly, the handgrip/box tilting strategies affected the level of stability of the partici­
pants. The left tilt technique was significantly less stable, probably because this technique shifted
the global centre of gravity closer to the left limit of the base of support, which was for all tasks
the side of the base most at risk for a loss of balance. Furthermore, the impact of the
handgrip/box tilting strategy on stability observed in this study was as important as the impact of
the width of the base of support or the flexion of the knees reported before (Delisle et al., 1996b).
In this study, feet position was imposed and the feet remained fixed throughout the tasks,
and only two handgrips were used to analyze the effect of four tilts of the box. However, in
free handling tasks, up to 40 different handgrips were used by expert handling workers, and
the type of tilt of the box as well as the position of the feet were among handling parameters
reported to be affected by the height of grasp and weight of the box (Authier, Lortie, &
Gagnon, 1995,1996), which confirms the wide diversity of handling strategies and their com­
plexity. The choice of a handling strategy is context dependent, and there are many possibilities
of combination of handgrips, box tilts, and feet positions that may represent safe or risky
handling strategies. For the first time, this study revealed the effect of tilting the box on trunk
and knee loadings, body posture, as well as on the participant’s stability. This effect was as
important as the effect of the width of the base of support and flexion of the knees addressed
before (Delisle et al., 1996b). One important aspect that has received little attention and that
could also be determinent is the strategy of feet displacement. Although these handling
parameters are studied separately, they are probably interdependent, which means that one
parameter could modify the effect of another. However, by studying them separately, it is
possible to determinent the potential of each of these parameters to affect body joints’
muscular efforts, body posture and stability. Further research will be required to study the
effect of the interaction of these parameters and will necessitate the analysis of strategies
closer to real handling task situations.
In conclusion, the deposits performed on edges of the box (resulting from different tilts),
and associated with expert handlers, did not reduce trunk asymmetric efforts as compared to
deposit performed with the box kept flat and associated with novices. However, the box tilts
were executed in a single direction, and it may be possible that combined tilts of the box could
help reduce trunk asymmetric efforts. The handgrip/box tilting strategies had little effect on
knee loadings, although one strategy showed smaller resultant left knee moment at deposit.
One strategy, consisting of tilting the box in the direction of the movement (the left tilt
strategy), resulted in poorer stability, because this strategy shifted the centre of gravity toward
the left, which was in the direction most at risk for a loss of balance. This study showed the
importance of considering the handgrip/box tilting strategy when assessing the risks encoun­
tered in asymmetrical handling.
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