EFFECT OF GRIP MODELS ON REBOUND ACCURACY OF OFF-CENTER TENNIS IMPACTS
AbstractINTRODUCTION AND PURPOSE The accuracy of strokes is of prime importance in winning tennis play. Most studies of ball rebound accuracy have used firmly clamped rackets for experimental control. Recent modeling and experimental data suggest that hand-held rackets may behave mechanically at impact as if they were unrestrained rather than clamped (Brody, 1987; Casolo & Ruggieri, 1991; Hatze, 1992, 1993; Knudson, 1997). The purpose of this study was to compare ball rebound accuracy of three models of off-center tennis impacts, free standing (FS), hand held (H), and firmly clamped. METHODS The effect of the three grip models on the rebound accuracy impacts was documented by measuring lateral deviation of ball rebound in the transverse plane. An oversized tennis racket was strung with nylon at 311 N (70 lbs) of tension. The racket stood in a vertical position on an immobilized bench, standardized by markings and a carpenters square. Six new tennis balls were projected by a ball machine at 24.1 m/s at a 14 degree angle to the racket face. Impact locations on the racket face were monitored by 15 (five for each model) measurements of ball imprints made on paper taped to the strings. Mean impact location was 20 f 5 mm off-center. Rebound accuracy was defined as the lateral displacement of the rebounding ball measured from ball imprints made by carbon paper on poster board taped to a wall 1.52 m from the racket. Twelve impacts were measured for each grip model. Displacement data were analyzed with a one-way ANOVA and Tukey-Kramer HSD post hoc tests with statistical significance accepted at the p < 0.05 level. RESULTS AND CONCLUSIONS ANOVA analysis revealed a significant (F2.33 = 11.4, p = 0.0002) effect of grip model on rebound accuracy. Forty-one percent of the variance (h2 = 0.409) of rebound accuracy was related to grip model. Tukey-Kramer post hoc tests demonstrated that the C model was significantly more accurate than the FS and H models. The rebound accuracy of the FS and H models were not significantly different. The mean angles of rebound (measured normal to the racket face) that correspond to the lateral displacements observed in the C, H, and FS models were 14.6, 16.3, and 17.0 degrees. Since most impacts occur off-center on the racket during tennis play, studies of rebound accuracy in tennis should use H or FS models rather than C models. Previous studies of tennis impact mechanics using clamped racket models should be interpreted with caution. REFERENCES Brody, H. (1987b). Models of tennis racket impacts. International Journal of Sport Biomechanics, 3, 293-296. Casolo, F., & Ruggieri, G. (1991). Dynamic analysis of the ball-racket impact in the game of tennis. Meccanica, 26.67-73. Hatze, H. (1992). Objective biomechanical determination of tennis racket properties. International Journal of Sport Biomechanics, 8,275-287. Hatze, H. (1993). The relationship between the coefficient of restitution and energy losses in tennis rackets. Journal of Applied Biomechanics, 9, 124-142.