BALL IMPACT LOCATION ON A TENNIS RACKET HEAD AND ITS INFLUENCE ON BALL SPEED, ARM SHOCK AND VIBRATION

Abstract

INTRODUCTION: Brody (1988) defined 3 different ‘sweet spots’ on the strings of a tennis racket. When a ball hits the racket at its point of maximum restitution (CoR), the rebound velocity of the ball will be highest. For ball hits at the node of the racket, vibrations are minimal. Ball contacts at the center of percussion (CoP) cause minimal shocks to the arm. The definitions of these points are based on the application of the laws of physics to a simple mechanical body - the tennis racket only. In a real game situation, however, tennis rackets are not simple and well-defined mechanical bodies. The player’s muscle actions continuously modify the grip forces at the racket handle. Therefore, mechanical coupling of the racket handle with the body is changed with each modification of grip force, resulting in a complex mechanical behavior of the racket-arm system. Using a game-like situation, this study investigated the influence of ball impact location on the racket head on ball velocity, arm shock and vibration. METHODS: Each of 19 expert tennis players performed 30 fore- and backhand strokes and 30 straight serves. A “Kuebler Intertial Light” tennis racket was instrumented to determine the ball contact and its movement across the racket head (Hennig & Schnabel, 1998). For the measurement of shock and vibration from the racket to the arm an accelerometer was fastened to the wrist (Proc. styloideus ulnae). Ball velocity was measured by a photocell arrangement. To determine relationships between the variables, simple and multiple regression analyses were performed. RESULTS AND CONCLUSIONS: Although all subjects belonged to a group of expert players with a similar level of performance, the patterns of ball contact were quite different between players. However, independent of each player’s individual technique, minima of arm shock and vibration were identified at impact locations, closer to the racket handle than those proposed by Brody (1988) as CoP and node. Using individual analyses across the 30 repetitive trials of each player, high multiple regression coefficients were found between ball contact points and roll distance on the strings, loads on the arm, ball velocity and contact time. REFERENCES: Brody, H. (1988). Tennis science for tennis players, 4th ed. Philadelphia: Univ. of Pennsylvania Press. Hennig, E., & Schnabel, G. (1998). A method to determine ball impact location and its movement across the strings of a tennis racket. In ISBS Proceedings, Konstanz, Germany.