Developing of evaluation method of motion repeatability by motion capturing

Motion repeatability is one of the required skills for any sports including tennis. One of the reasons why expert tennis players can accurately serve the ball to another side is their motion repeatability. On the other hand, however the novice players can luckily serve the ball, they are required to practice again and again to repeat the motion. The reliable criterion by the objective numerical indicator is needed for training and evaluation of motion repeatability. In this study, the method of numerical evaluation of motion repeatability would be developed by a motion capture system. The correlation coefficients of each segment angular fluctuation on the serving motions were validated as the criterion for motion repeatability. 17 sensors measure three-dimensional 23 segments and 22 joints angle fluctuations during the two seconds around the ball impact. Three experienced tennis players and three novice players tried to serve ten balls by right arms. The results of the experiments indicated that the repeatability of upper body tends to be stable comparing arms and legs for all the subjects. The correlation coefficients of the fluctuations at the measurement joints that are far from the upper body tend to be low. Comparing the experts and the novices on the motion of the arms and legs, the novices have much variation of correlation coefficients of each motion than the experts. In addition, most of the motion fluctuations of novices are larger than those of the experts. This study revealed that the joint angular fluctuation would be able to be applied as a criterion of motion repeatability.