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Effect of String Tension and Impact Location on Ball Rebound Accuracy in Static Tennis Impacts

Duane Knudson

The rebound accuracy of tennis impacts was studied by measuring the vertical angles of approach and rebound of tennis balls projected in a vertical plane at a clamped racket. Three identical oversized tennis rackets were strung with nylon at 200, 267, and 334 N of tension. Ten impacts were filmed at 200 Hz for each string tension with the ball impacting the strings centrally and 8 cm off-center. A two-way ANOVA revealed significant (p < .01) main effects for string tension, impact location, and the interaction of string tension and impact location. Treatment-Contrast Interactions demonstrated one significant (p < .01) difference: The decrease in rebound accuracy from a string tension of 200 to 334 N was significantly different for central versus off-center impacts.

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Intrasubject Variability of Upper Extremity Angular Kinematics in the Tennis Forehand Drive

Duane V. Knudson

The intra subject variability of the angular kinematics of the wrist and elbow joints in the tennis forehand drive were studied. Two varsity tennis players were filmed as they performed flat forehand drives. The DLT method of 3-D reconstruction was used to measure the angular motion of the upper extremity for eight strokes to assess the intra subject variability of selected kinematic variables. Curves were synchronized to impact and averaged. Wrist and elbow angular position data were quite consistent, with curve coefficients of variation (CV) less than 5.9%. The consistent angular positions during the forward stroke did not result from highly consistent patterns of angular velocities or accelerations. For both the wrist and elbow joints, intra subject variability increased for the angular velocity (CV = 90.6%) and angular acceleration (CV = 129.5%) curves. Biomechanical studies comparing derivatives or kinetic variables across subjects may have to be interpreted with reference to intra subject variability.

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Forces on the Hand in the Tennis One-Handed Backhand

Duane V. Knudson

This study examined the pattern of forces and peak loads on the hands of six advanced and six intermediate level male tennis players as they performed one-handed backhand drives. Two miniature load cells were mounted on a midsized graphite racket. The force on the thenar and hypothenar eminences of the hand were sampled at 1000 Hz. Forces on the thenar eminence in preparation for impact were significantly larger and less variable for the advanced subjects. Postimpact peak forces did not differ across skill level and were smaller than the loads reported for forehand drives. The significantly lower thenar forces the intermediate subjects used in preparation for impact may provide less resistance to the acceleration of the racket created by ball impact. A large impact acceleration may be related to a rapid stretch of the wrist extensors, which has been hypothesized to be the cause of tennis elbow.

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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.