The purpose of this study was to investigate the relationship between motion and ball spin in tennis serves. Ten male university tennis players participated. The three-dimensional coordinates of the players performing flat, kick and slice serves were collected using a motion capture system with 8 cameras (250 Hz). Similarly, the three-dimensional coordinates of reflective markers on the ball were also collected (500 Hz). The primary variables computed were: racquet face velocity and direction at impact, velocity and angular velocity of the ball after impact, hitting point, angles of the upper limb joints, and segment angles of the upper trunk. The differences in racquet face velocity among flat, kick, and slice serves were divided into the following terms: 1) DeltaVposture: A difference in velocity resulting from a change in upper trunk posture, 2) DeltaVswing: A difference in velocity resulting from a change in arm swing (kinematics of the upper limb), 3) DeltaVutrk: A difference in velocity resulting from a change in upper trunk translational and rotational motion. Repeated measures ANOVA (p<0.05) with Bonferroni multiple comparison was used to evaluate the effects of changes in form (with differences in ball spin) on each parameter. The findings are summarized as follows. 1) The impact point and swing direction were mainly controlled not by a change in arm swing motion, but by a change in upper body posture. 2) To generate ball spin, it is necessary to avoid a head-on collision between the ball and the racquet (a normal vector of the racquet face is parallel to the racquet face velocity vector). Therefore, players decreased the amount of upper trunk leftward rotation in kick and slice serves at the point of impact so as to swing the racquet more laterally. 3) It is necessary to swing the racquet more vertically in order to lean the rotation axis of the ball. Therefore, players controlled the upper trunk leftward-rightward and forward-backward leaning in a kick serve at the point of impact. 4) Changes in upper body posture cause changes in the direction the racquet faces. Therefore, players mainly controlled their elbow pronation-supination angle in order to maintain a racquet face direction that satisfies a legal serve.
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