The Effects of String Type and Tension on impact

INTERNATIONAL JOURNAL O F SPORT BIOMECHANICS, 1987, 3, 40-46
The Effects of String Type and Tension
on impact in Midsized and Oversized
Tennis Racquets
10
Jack L. Groppel, In-Sik Shin,
Ann Thomas, and Gregory j. Welk
This study explored the effects of string type and tension on various factors
involved with tennis ball-racquet impact in midsized and oversized racquets.
String and racquet material, racquet flexibility, and grip firmness were held
constant for each test condition. The dependent variables included postimpact and preimpact ball velocity ratio, racquet head displacement, maximum
ball compression, and ball contact duration. It was found that racquet size
and string material have varying effects on impact. Although changes in string
tension do affect the impact, it is not in a linear fashion. Various string tensions change a racquet's flexibility, thus affecting ball velocity and other factors
associated with impact. The results of this study demonstrate the complexity
of string and frame interaction.
It is well accepted that highly skilled tennis requires an optimum combination of stroke velocity and control. Many factors play a role in the outcome of
a competitor's shot, but one of the most important factors is the racquet itself.
With the many technological advances in racquet design, much emphasis has been
placed on the effects of size and shape of racquet frame, racquet frame flexibility, string type, and string tension.
Regarding the effects of increased string tensions, Larson (1979) and
Plagenhoef (1979) noted that these resulted in increased ball velocity after impact. These results were refuted by Baker and Wilson (1978) and Elliott, Blanksby, and Ellis (1980). Baker and Wilson found that increasing the string tension
beyond 50 lbs did not result in increased ball velocity. Elliott et al. determined
Jack Groppel is with the Dept. of Physical Education at the University of Illinois
at Urbana-Champaign. In-Sik Shin is an assistant professor at Seoul National University
in South Korea. Jo Ann Thomas was a master's student at the University of Illinois at
the time of this writing. Gregory Welk is a master's student at the University of Iowa.
Direct all correspondence to Jack L. Groppel, 117 Freer Hall,906 S. Goodwin,
University of Illinois, Urbana, IL 61801.
STRING TYPE AND TENNIS RACQUETS
41
that 55 1bs of string tension produced the highest postimpact ball velocity. Knuttgen (1959) and Ellis, Elliott, and Blanksby (1978) compared the velocity ratios
produced by gut with those from nylon and found that gut gave higher velocities.
But this may not hold true at different tensions. The size of the racquet frame
and its flexibility also may affect the performance of gut and nylon.
This study set out to determine the effect of string type and tension on
ball velocity ratios (post- to preimpact velocity), racquet head displacement, ball
compression, and ball contact duration in a condition with string material, racquet shape and size, flexibility of the frame, and grip firmness held constant.
Methods
Four Davis graphite racquets were studied (two midsized and two oversized).
Within each group, racquets were strung alternately with Victor gut and nylon
(both string types were 15 gauge) at five different tensions: 40, 50, 60, 70, and
80 lbs. All stringing was done by one individual on the same calibrated stringing
machine (machine calibration was checked with standard kilogram weights). All
tensions were rechecked after stringing and just prior to testing with a Poly Control Tension Tester. Prior to filming, each racquet was clamped horizontally using a large C clamp securing the racquet at the grip. Although new Penn tennis
balls were used, rebound heights from a fixed position were measured and only
those with similar rebound characteristics were employed. An Apollo Wizard
Ball Machine was placed 90 cm from the racquet and was used to fire the tennis
balls at the clamped racquet. The mean preimpact ball velocity was 23.1 rnls.
To fully examine the phenomenon of tennis impact, we conducted two separate
experiments.
In Experiment 1, 10 central impacts were made at each string tension for
all racquet sues and string types. Each ball-racquet impact was filmed using a
16mm Locam camera set at 200 fps and placed 3.25 meters from the center of
the racquet. In Experiment 2, one ball impact per string type at each tension was
filmed using a 16mm Hycam camera set at 4000 fps and placed 3.0 meters from
the racquet center. Frame rates in both cameras were calibrated by using internal
timing light generators, set at 100 Hz in the Locam and 1000 Hz in the Hycam.
Five tungsten lamps illuminated the filming area. A mirror was placed at a 45'
angle behind the racquet to allow us to determine where the ball struck the racquet relative to the geometric center of the frame, as well as to quantify ball compression. Only impacts in the geometric center of the racquet face were considered
successful trials. The racquet frame was marked at various points with adhesive
tape to allow for calculation of the geometric center of the racquet.
The data were obtained from the film using a Vanguard Transport System
and a Numonics digitizer. To ensure reliability of data collection, two experimenters reduced the data at different times. Two samples of each data point
were measured by each individual. A Pearson product-moment correlation coefficient of 0.95 was determined for the replicated measures. Ball and racquet
coordinates were determined so the resultant pre- and postimpact ball velocities
could be calculated. Preimpact velocity was determined by recording ball displacement data prior to impact, smoothing that data using a Butterworth digital
filter with cutoff frequency set at 6, and then differentiating the data. Postimpact
velocity was calculated in a similar manner following ball contact. Ball velocity
GROPPEL, SHIN, THOMAS, AND WELK
42
ratios (postimpact and preimpact velocity) were calculated for all trials at each
of the five string tensions for both gut and nylon strings.
Racquet head displacement was measured on the Hycam films by digitizing
the tip of the frame in successive frames during one complete oscillation-from
rebound to maximum backward deflection, to forward maximum deflection and
back to its original position. Displacement data were again smoothed. Ball compression data were obtained by digitizing the two widest points of the ball before, throughout, and just after impact. In addition, ball contact duration at each
string tension was determined by counting the frames that showed the ball in contact with the racquet and multiplying by the frame rate.
For the parameters measured, means and standard deviations were determined for gut and nylon during the 10 trials at each of the five string tensions
for both rnidsized and oversized racquets. ANOVA was run on the ball velocity
ratios to determine whether there was a significant difference between the mean
values obtained for each of five string tensions and between string types.
Results and Discussion
Our primary goal in Experiment 1 (the Locam filming) was to study how ball
velocity ratios were affected by string type, string tension, and racquet size. As
shown in Figures 1 and 2, the oversized racquet produced significantly higher
ratios for both gut and nylon strings than the midsized racquet (F = 9.45, p 4
.05). In general, the resultant ball velocities are lower at higher tensions, which
substantiates the results of Baker and Wilson (1978) and Elliott et al. (1980).
Our study, however, indicates that this trend is not linear and may depend on
the combination of string material, string tension, and racquet size for subtle variations.
In the oversized racquets, significant differences were found between gut
and nylon (F = 8.60, p < .05); the results indicate that nylon produces a higher
-----
-nylon
gut
RACQUET TENSION (lbs)
- - - - - nylon
-
RACQUET TENSION (Ibs)
Figures 1 and 2 - Effects of racquet string type and tension on resultantball velocity in oversized racquets (left) and midsized racquets (right).
43
STRING TYPE AND TENNIS RACQUETS
ball velocity. This finding differs with the work of Ellis et d. (1978) and Larson
(1979), who found that gut produced a greater ball velocity after impact for both
jumbo and regular-sized racquets. It is possible that the resiliency of the gut material is negated by the largeness of the oversized racquet's frame and the amount
of string involved at impact.
The results from the midsized racquets are less concrete. Although gut produced a higher ball velocity at 60-lb tension in the oversized racquet, greater fluctuations were seen in the midsized frame. At certain tensions gut produces higher
ratios, but at others nylon provides higher results. It is possible that gut has better playing characteristics in the midsized racquet. Furthermore, the majority of
data shows the racquet with gut had less displacement (Figures 3 and 4), which
may indicate that less vibration was occurring in the racquet at these tensions.
This could translate to improved control and a better "feel" of the racquet.
OVERSIZED
40
- - - - - nylon
-gut
50
w
m
.
RACQUET TENSION
80
(Ibs)
C
,
,
40
50
- - - - - nylon
-gut
W
70
80
RACQUET TENSION (Ibd
Figures 3 and 4 - Effects of racquet string type and tension on racquet head displacements in midsized racquets (left) and oversized racquets (right).
At specific tensions there is a large difference between the behavior of
gut and nylon as they affect ball velocity. At 60 lbs, gut produces a higher ball
velocity ratio in both midsized and oversized racquets (Figures 1 and 2). Nylon
produces no such peak at 60 lbs with either racquet size, but displays an increase
in the ball velocity ratio at 80 lbs. One explanation could be that instead of observing the string characteristics inside the midsized racquet frame at 80 lbs, we
began observing the string and the racquet as one unit.
The data on racquet displacement from Experiment 2 (Figures 3 and 4)
correlate with the ball velocity ratios, but to a stronger degree with midsized racquets. A smaller displacement of the racquet head corresponded with a larger
velocity ratio. Presumably, the energy used to displace the racquet is not returned
to the ball (Groppel, 1984). The amount of racquet displacement depends on the
type of string and the string tension. For example, with the midsized racquet,
nylon produced less racquet displacement at 50 Ibs but more at 60 Ibs, when com-
GROPPEL, SHIN, THOMAS, AND WELK
44
pared with the gut strings at the same tensions. The differences in this interaction
effect are dramatic and may account for the different ball velocity ratios between
gut and nylon at the various tensions. At 50 lbs, nylon outperformed gut (.403
> .380), but at 60 lbs the situation was reversed with gut outperforming nylon
(.428 > .380).
This effect was not as evident with the oversized racquet. The nylon strings
produced higher ball velocity ratios at four of the five tensions. At 60 lbs, however, gut produced a slightly higher ball velocity. The smaller racquet displacement for gut at 60 lbs may play a role, but it appears there is a stronger relationship
between ball compression (Figure 5) and ball velocity for the oversized racquet.
The ball compression data in Figure 5 seem to play a role in determining
the velocity ratios for oversized racquets, but not for midsized racquets (Figure
6). In the oversized racquets higher velocity ratios resulted from smaller compressions. In four out of five tensions, nylon had less compression and therefore
higher ratios. It is possible that a smaller compression produces a lesser dissipation of kinetic energy and therefore a higher ball velocity (Brody, 1979). The
expected decrease in compression as tension increased was not observed. At 60
lbs tension, the compression from the nylon strings increased while that from
the gut strings decreased. This, along with the smaller racquet displacement, helps
to account for the higher velocity ratio of gut over nylon at 60 lbs.
OVERSIZED
--
L
CI
E
0
Y
0.7-
-.
Z
0.6-
(I)
0.5..
0
--
z
0
(I)
..
W
TT
V)
W
20!
0.4::
0
0.3-
a
0
2
0
".'
0
-1
1
4
m
0.1
-----
40
nylon
-gut
50
6U
70
..
80
RACQUET TENSION (Ibs)
,
40
- - - - - nylon
-gut
5'0
60
70
80
RACQUET TENSION ( I b d
Figures 5 and 6 - Effects of racquet string type and tension on ball compression
in oversized racquets (lefk) and midsized racquets (right).
In the midsized racquet, the ball compression data do not appear to correlate with the ball velocity data. It could be that the contribution of ball compression to velocity is overshadowed by a more dominating racquet displacement effect. Ball velocity ratios from oversized racquets, on the other hand, appear to
be affected by a combination of racquet displacement and ball compression.
Experiment 2 also yielded information on how contact duration is affected
by string type and racquet size (Figures 7 and 8). First, it was observed that ball
STRING TYPE AND TENNIS RACQUETS
OVERSIZED
'.OT
t ::
I
40
- - - - - nylon
-gut
50
60
70
80
RACQUET TENSION (Ibs)
-
'
..-nylon
g
u
t
46
5'0
66
70
80
RACQUET TENSION (Ibs)
Figures 7 and 8 - Effects of racquet string type and tension on ball contact duration in oversized racquets (left) and midsiied racquets (right).
contact was longer for all string tensions in the oversized racquet than for the
midsized racquet. Although the duration of ball contact fluctuated somewhat, it
generally decreased in the oversized racquet as tension increased. The contact
duration has possible implications concerning a player's ability to control a shot.
At higher tensions ball compression generally seemed to increase (although this
is not a definite trend), which may permit an embedding of the strings into the
ball. It follows that a greater embedding may facilitate more control over the shot
(Groppel, 1984). Also, with a smaller duration of contact, the racquet has less
time to rotate in reaction to an off-center impact and send the ball off in an errant
direction (Groppel, 1984). If this is correct, as contact duration decreases, control improves.
The oversized racquet produces a longer duration of ball contact than the
midsized racquet for all tensions. As tension increased (Figure 7), the difference
in contact duration between the two sizes usually decreased. This may imply that
as the strings become tighter, the characteristics of the racquet and strings have
less effect on the outcome of the shot.
The variation in contact duration as tension changes is different for midsized
and oversized racquets. The midsized racquets (Figure 8) generated nearly sinusoidal curves for contact duration versus tension. The duration was greater at 60
lbs than at 40 lbs for midsized racquets with gut and nylon. The oversized racquet (Figure 7) followed a more linear decrease in duration as string tension was
increased. The contact duration at lower tensions in the oversized racquet was
so great that it produced a trampoline effect. It was this excessive string movement in the oversized racquets that prompted some manufacturers to suggest tensions of at least 72 lbs for oversized racquets (Prince Manufacturing Co., Princeton, NJ). The peak at 60 lbs for the midsized racquets is difficult to explain, but
could be caused by the same interaction effect that caused the difference between
ball velocity ratios at 50 and 60 lbs in the midsized racquet. One possible expla-
46
GROPPEL, SHIN, THOMAS, AND WELK
nation is that at 60 lbs tension the string and racquet vibrations are very similar.
This increases the vibration of the whole system and possibly keeps the ball on
the strings longer. At other tensions the two vibrations may be dissimilar and
cancel each other out. More research is needed on this interaction effect at various tensions and how string type affects its manifestations.
Significant differences were observed in the behavior of string types in the
midsized and oversized racquets. Contact duration for nylon strings was less than
or equal to that produced by gut strings for all string tensions in the midsized
racquet. In the oversized racquet, however, gut had smaller or equal contact durations.
Conclusion
We discovered that changes in string tension do not affect the behavior of the
racquet in a linear progression. Different string tensions change the flexibility
characteristics of the racquet, which then produce variations in ball velocity and
contact duration. These effects are expressed differently depending on string type
and racquet size. In addition, the general behavior of gut and nylon varies with
the racquet's frame size. With regard to postimpact ball velocity, gut seemed
to outperform nylon in the midsized racquet, whereas nylon performed better
in the oversized racquet. The results demonstrate the complexity of the interaction of strings and racquet size and suggest that more research be conducted
to better quantify and explain this interaction.
References
Baker, J., & Wilson, B. (1978). The effect of tennis racket stiffness and string tension
on ball velocity after impact. Research Quarterly, 49(3):255-259.
Brody, H. (1979). Physics of the tennis racket. American Journal of Physics, 47(6):482-487.
Elliott, B.C., Blanksby, B.A., & Ellis, R. (1980). Vibration and rebound velocity characteristics of conventional and over-sized tennis rackets. Research Quarterly, 51(4):
608-615.
Ellis, R., Elliott, B., & Blanksby, B. (1978). The effect of string type and tension in
jumbo and regular sized racquets. Sports Coach, 2(4):32-34.
Groppel, J.L. (1984). Tennis for advanced players: And those who would like to be.
Champaign, IL: Human Kinetics.
Knuttgen, H. (1959). 7be effects of varying tennis racket dimensions on stroke pegormance. Unpublished doctoral dissertation, The Ohio State University.
Larson, C.L. (1979). 7be effect of selected tennis racket and string variables on ball
velocity and the force of ball-racquet impact. Unpublished doctoral dissertation,
Indiana University.
Plagenhoef, S. (1979). Tennis racquet testing related to tennis elbow. In J. Groppel
(Ed.). Proceedings of the National Symposium on the Racquet Sports (pp. 291-310).
Champaign, IL: University of Illinois Conferences and Institutes.