INTERNATIONAL JOURNAL OF SPORT BIOMECHANICS, 1989, 5, 324-331
Forces on the Hand
in the Tennis Forehand Drive:
Application of Force Sensing Resistors
Duane V. Knudson and Scott C. White
Two force sensing resistor force transducers were utilized to measure the
forces on the hand of seven skilled tennis players performing the tennis forehand drive. Repeatable gripping force patterns were recorded for the subjects given the experimental protocol used for the study. The magnitude of
the peak postimpact force on the hand was highly variable, ranging from 4
to 309 N, and was found to be related to high-frequency vibrations of the
racket. There was less variability in the magnitude of preimpact gripping
forces, indicating that the subjects utilized a consistent gripping pattern in
preparation for impact. The large within- and between-subject variability of
postimpact forces warrant further study in order to establish the range of loadings in tennis play that may be related to overuse injuries.
Although the repeated loading of the body in tennis play has been hypothesized to contribute to various upper extremity injuries (tennis elbow), no studies
have adequately documented the variability of forces on the hand in the forehand
drive. To understand the mechanical interaction of the soft tissue of the hand with
the racket, and to establish a relationship between loading and potential injury,
the measurement of forces on the hand is critical. Plagenhoef (1979) reported
the only study measuring the force transmitted to the hand in forehand drives
in an attempt to study the effect of various tennis rackets on the forces transmitted to the hand. Representative force curves were reported, but comparisons across
rackets were not possible due to the variability of impact forces, which he attributed
to impact location.
The measurement of fdrces acting on the body has been a considerable problem in biomechanics. Force transducers have typically been developed for use
with clean, nearly rigid, and regular surfaces. Research in biomedical engineering and robotics has begun to develop force transducing applications of conductive elastomers. These transducers are inexpensive and are useful for measuring
forces on soft tissue (Mokshagundarn, 1988). Force sensing resistors (Interlink
Duane V. Knudson is with the Department of HPER, Baylor University, BU Box
7313, Waco, TX 76798-7313. Scott C. White is with the School of Health Related Professions, SUNY at Buffalo, 435 Stockton Kimball Tower, Buffalo, NY 14214.
THE TENNIS FOREHAND DRIVE
325
Electronics, Inc., Santa Barbara, CA) are commercially available conductive
elastomers that have recently been used to measure the pressure under the foot
(Maalej et al., 1987). The purpose of this investigation was to utilize a new conductive elastomer sensor to measure gripping and impact related forces on the
hand and to establish their intrasubject variability in a typical tennis forehand
drive condition.
Method
Two force sensing resistors (FSR) were mounted on the handle of a tennis racket
to measure forces on two key areas of the hand during the tennis forehand drive.
An FSR is constructed by sandwiching a conductive rubber polymer and a silver
ink conductive array, both of which are screen printed on a thin Mylar film
(Figure 1). As the sensor is compressed, the contact resistance decreases in a
nonlinear fashion. The advantages of FSR sensors are that they are small, non-
1.5 VOLTS
OUTPUT
Mylar
Sheets
Figure 1
- Interlink force sensing resistor and voltage divider circuit.
326
KNUDSON AND WHITE
invasive, inexpensive, resilient, give reproducible results with soft tissue, and
can be made to fit irregular surfaces. Their disadvantages are their nonlinearity
and hysteresis, both of which must be accounted for by calibration.
A midsized Pro-Kennex wood, graphite, and boron composite tennis racket
was instrumented with two 1.6-cm diameter FSRs and was strung with nylon
at 245 N (55 lbs) of tension. The wooden handle of the racket was sanded smooth
and the sensors were glued 12.5 and 5 cm up from the bottom of the handle.
To achieve the greatest reproducibility (1 % within-sensor), the force over the
sensor must compress the whole sensing area. Small sensors and felt were used
in conjunction with the soft tissue of the hand to keep the sensor area compressed.
By controlling the placement of the hand on the racket in an eastern forehand
grip, the top sensor was oriented to record impact forces at the base of the index
finger (Plagenhoef, 1979) while the bottom sensor was placed to record the primary
gripping force created by the last three fingers of the hand (hypothenar eminence).
FSR sensors require a stable, low current input voltage to prevent an increase in resistance due to heating. Two C batteries provided the input voltage
to the circuit (Figure 1). Twenty-eight gage wires that carried the signal from
the batteries were connected to the sensors by conductive epoxy adhesive (E-Solder
#3021, New Haven, CT). The batteries were secured to a belt worn by the subjects that also provided a base for wire leads from the sensors to an AID converter.
Each sensor was then covered with a synthetic grip material commonly used by
tennis players (Gamma Grip), which was wrapped around the racket handle. This
provided a typical gripping surface and made the force sensors imperceptible to
the subjects.
The FSRs were calibrated by applying force over the exact area of each
sensor using a 100-lb load cell. Repeated trials of FSR and load cell data were
AID converted at 60 Hz and averaged to create a voltagelforce calibration file
for each sensor. Five least-squares best fitting polynomials (2 linear, 1 quadratic,
and 2 cubic) were fitted to each sensor's calibration file (r2=.988). One calibration file and the corresponding polynomial function curves are illustrated in
Figure 2. The effect of hysteresis was considered negligible because it was small
(<5 %), and the unloading forces analyzed were not in the range of the hysteresis.
An Endevco triaxial accelerometer (#7267A, 50 grams) was mounted at
the center of gravity (CG) of the racket to measure racket vibration, which has
been shown to contribute to increasing the forces of impact on the racket (Brody,
1987a; Hatze, 1976; Ohmichi, Miyashita, & Mizumo, 1979). Vibrations measured at the hand have only recently been reported (Brody, 1987b). Each acceleration channel was calibrated with gravitational acceleration and amplified
(bandpass 0-2000 Hz).
Three varsity tennis players and four tennis professionals stroked flat forehand drives down the center of the court from balls projected by a ball machine
at 20 mls. Subjects used their natural eastern forehand grip. Typical racketlball
closing velocities at impact, preimpact, and postimpact ball velocities were 26,
12, and 30 m*s-', respectively. Grip force and acceleration data were 12 bit AID
converted at 2900 Hz using an IBM PCIAT and stored on disk for 10 to 32 strokes.
Several force variables were analyzed to document gripping and impact forces
in the tennis forehand drive. The force at impact and the postimpact peak force
were measured for the top sensor, while the force at impact and average force
50 ms prior to impact were measured for the gripping force on the hypothenar
eminence of the hand.
THE TENNIS FOREHAND DRIVE
0
0.2
327
0.4
0.6
0.8
1
1.2
VOLTS
Figure 2 - Sensor calibration file and the five polynomial cuwes defing sensor
output. Polynomial order: A-linear, B-quadratic, C--cubic.
Results and Discussion
Grip force data demonstrated considerable variability in magnitude, but consistent grip force patterns were recorded across all subjects within 100 ms of impact.
An out-of-phase pattern of the two force curves was observed (Figure 3). The
gripping force measured at the hypothenar sensor increased in preparation for
impact, decreased due to the moment created by the force of impact, and later
increased, probably due to the subject attempting to retain control of the racket.
The force at the top sensor decreased in preparation for impact, which may be
related to a combination of increased hypothenar force evident as large gripping
forces measured by the lower sensor (Figure 3) and the small accelerations of
the racket near impact. The force of impact displaced the racket backward relative to the forearm, creating a sharp increase in the force recorded by the sensor
at the top of the hand, which quickly decreased. The forces observed at the top
of the hand were consistent with the shape and magnitude of the force data presented by Plagenhoef (1979).
The variability of the peak forces recorded on the top of the hand and
hypothenar gripping forces at impact can be seen in Tables 1 and 2. The gripping
forces measured on the hypothenar eminence at impact were consistent (mean
CV =27 %), ranging from 5 to 7 1 N, while the postimpact peak forces at the base
of the index finger were highly variable (mean CV=69%), ranging from 4 to
309 N. Plagenhoef (1979) reported that the large variability of peak forces after
impact were related to impact location, although no measure of location was made.
The accelerometer records from this study suggest that much of the variability of the forces on the hand created by impact were related to high-frequency
KNUDSON AND WHITE
328
Figure 3 - Forces on the hand in the forehand drive. Top force recorded at the base
of the index figer; bottom force at the hypothenar eminence.
Table 1
Postimpact Peak Forces at the Base of the Index Finger
Subject
n
Force (N)
Range (N)
CV (%)
Note. Mean force is calculated for each subject for the number of stroking impacts (n), and
CV is the standard deviation divided by the mean.
THE TENNIS FOREHAND DRIVE
Table 2
Forces at Impact on the Hypothenar Eminence
Subject
n
Force (N)
Range (N)
CV (%)
Note. Mean force is calculated for each subject for the number of stroking impacts (n), and
CV is the standard deviation divided by the mean.
vibrational forces. This is illustrated in Figure 4, where a 2-ms delay is observed
between the linear acceleration perpendicular to the racket CG as measured by
the accelerometer and the force recorded at the bottom of the hand. High-frequency
vibration peaks were also present in the force transmitted to the top of the hand,
contributing to the large variability of the peak force after impact. Previous
research has shown a linear relationship between the distance an impact occurs
off center and the magnitude of the resulting racket vibration (Elliott, Blanksby,
& Ellis, 1980), and increased impulse on the racket created by off-center impacts
in simulated tennis strokes (Elliott, 1982).
The large variability of postimpact forces cannot be attributed to vibrational
forces alone. It is likely that balllracket impact velocity, impact location, gripping conditions, and racket vibration interact to determine the force transmitted
to the hand. Similar complex interactions have been reported for various racket
size and stringing conditions (Groppel, Shin, Thomas, & Welk, 1987). Present
results and previous research suggest that impact location and racket vibration
may have a significant influence on the variability of forces transmitted to the hand.
Reproducible force measurements were recorded using the Interlink FSR
in the forehand drives studied. Gripping forces at the bottom of the hand were
consistent in preparation for impact. Postimpact forces were more variable and
may be related to the forces created by ball impact conditions and the subsequent
vibration of the racket. Future research should document the range and variability
of forces on the hand for the variety of strokes and stroking conditions encountered in tennis play in order to shed light on the force loading in tennis. A larger
sample of tennis players is also needed because the highly skilled players in the
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THE TENNIS FOREHAND DRIVE
331
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Acknowledgment
This research was supported in part by a grant from the United States Tennis Association Center for Education and Research.