Application of the extrapolation method to motion analysis of the flat power serve in tennis.

The purposes of the present study were to examine the distortions of three-dimensional kinematic data using existing three extrapolation methods and to determine the most effective extrapolation method for removing the influence of the collision between the racket and the ball during a flat power serve in tennis. For a flat power serve, it is natural that the velocity of the racket head toward the net would be maximal at impact, i.e., the acceleration would be zero. The present study shows that the criterion acceleration data toward the net was approximately zero. Since REF and LEX have characteristics that produce zero acceleration at impact, the differentiated data toward the net in REF and LEX were similar to the criterion data in analysis at 1000 Hz. However, in analysis at a lower sampling time, at which the practical analysis of tennis movement was commonly analyzed, LEX yielded high acceleration from just before impact, and the difference between the criterion velocity data and the velocity in LEX at impact became large in proportion to the shortening of sampling time. In contrast, REF invariably produced the zero acceleration at impact, and the velocity in REF at impact was similar to the criterion velocity data for each sampling time. Thus, in the practical analysis of the tennis serve, REF is the most suitable method for correcting the distortion by the collision between racket and ball for the axis toward the net, where the acceleration would be zero at ball impact.