STRESS STUDIES IN TENNIS – DIFFERENCES BETWEEN TENNIS RACKETS OF DIFFERENT LENGTHS
Keywords: tennis rackets, tennis stroke, impact points on rackets, EMG, acceleration, ball velocities
AbstractINTRODUCTION: Recently several manufacturers of tennis rackets have been offering elongated rackets. The extension of these rackets (27 to 32 inches) ranges between 1 and 5 inches. Considering the biomechanical characteristics of handling and efficiency of shot of these "Long-Bodies" leads to the following presuppositions: 1. With equal body-segment-velocities the longer lever results in an increased velocity of the racket head. 2. For the realization of both equal bodysegment -velocity and an increased velocity of the racket head, the player requires more power within one shot cycle than is the case with standard racket lengths. 3. In case the muscle power per time unit which has to be used with standard racket lengths cannot be further increased, the timing must consequently be adapted so that the same body-segment-velocities can be realized. It can be assumed that not just positive results will appear when using "Long- Bodies". Regarding these presuppositions, in this study the following parameters in the use of tennis racket with different lengths will be investigated: first impact points on the tennis rackets, second impact points in the field, muscle actions during the strokes, accelerations of the tennis racket handles during the impact, velocities of the balls. METHODS: A better quality player had to play 30 shots with each racket length. The first impact points were recorded with a newly developed measuring device, the Treffpunkt-Analyzer. The EMG recordings were realized with a 25 Channel EMG recorder. To record the accelerations of the racket handles we used a threedimensional accelerometer. The determination of ball velocities was realized using a kinematographic procedure. RESULTS AND CONCLUSIONS: The investigation shows different results within the use of different racket lengths. The acceleration of the racket handle is more dependent on the stiffness of the racket than on the length. We measured 70g maximum acceleration along the vertical axis using the 32" racket and 102g by using the 27" racket. A clear increase in accelerations dependent on racket-length could not be found. The velocities of the balls are significantly different (Sig. 2- tailed: < .0005). The longer rackets produce higher ball velocities (27" -> 101.6 km/h, 29" -> 107.6 km/h, 32" -> 107 km/h). The first impact points are significantly different (Sig. 2-tailed: .005). When using the 27" racket the first impact points are located closer (ca. 10 mm) to the racket head. Differences in the precision regarding the second impact points in the field could not be found. The EMG evaluation shows that players using longer rackets do not need more muscle activity (integral of the EMG signals) than when using the 27" racket. When using 27" rackets, significantly higher muscle activity could be found for several muscles.
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