Dynamics of the racket-arm joints during the soft-tennis smash

comparison between standing smash and jumping smash

Abstract

In various soft-tennis smash techniques, the standing smash (SS) and jumping smash (JS) are considered to be fundamental techniques, but have quite different kinematics. In the present study, SS and JS of eight skilled volleyers were videotaped and the racket-arm dynamics was analyzed using the procedures of 3-D motion analysis. The joint forces/torques along the anatomical axes, the joint powers and the mechanical works at each joint were compared between SS and JS.

All of the joint forces of SS and JS showed considerably similar changes at the all joints; however, the peak value of the anterior force at the elbow joint and the proximal force at the elbow and wrist joints of JS were significantly larger than those of SS. This will be caused by the difference in the movement direction of the body; Since center of the gravity of the whole body moves backward in JS (SS: forward), larger forces act on the joints not to distract the racket arm, which moves forward both in SS and JS. SS and JS also showed similar changes in joint torque; however, an elbow extension torque peak in the early phase of the forward swing was observed only in SS. This torque may do a part to control the arm position toward the impact point, and also to be a trigger that starts extending the elbow joint. The horizontal adduction torque near impact of JS was marginally significantly smaller than that of SS; on the contrary, the adduction torque of JS was not significantly but quite larger in the mean value than that of SS. There are substantial differences in how to utilize the shoulder torque between SS and JS. In contrast with the joint force and joint torque, the joint force powers showed quite different changes between SS and JS, particularly near impact. In JS, additional peaks before impact were found at the shoulder and elbow joints compared with SS. Moreover, the mechanical works done by these joint forces (time integral of joint force power) of JS were smaller than those of SS at the all joints. This seems to meet the result that the kinetic energy of the hand-racket segment at impact was also smaller in JS. In spite of the small energy, the enough racket head velocity in JS may be obtained by the larger rotation of the hand-racket segment. Changes of the joint torque powers of SS and JS showed similar tendencies as a whole. However, a slight peak of the torque power at the elbow joint was observed only in SS. This indicates that the energy generation by muscles actually occurs when the elbow extension torque acts in SS. The positive peak value of the torque power at the wrist joint was a marginally significantly large in JS. Consequently, it will be proper to emphasize the larger wrist motion in the situation of JS coaching. As for the mechanical work done by the elbow joint torque (time integral of joint torque power), both of SS and JS were negative (energy absorption), and the absolute value of JS was marginally significantly larger than that of SS. In conclusion, because changes of the joint force and joint torque are quite similar between SS and JS, players may feel little difference in how to use the racket-arm as a whole. However, investigating more details, delicate differences were clarified at any joint, and these results will reflect the subtle characteristics of each technique.