Abstract
Although joint torque about internal-external rotation axis at throwing-ball-side shoulder joint shows large values in magnitude prior to ball release during baseball pitching motion, the dynamic contribution of the axial torque shows small value to the generation of ball speed throughout forward swing motion. The purpose of this study was to make clear this phenomenon through quantification of dynamic contributions of shoulder joint torques to the generation of shoulder joint angular motions which would play crucial roles in generating launched ball speed. Ten male collegiate right-handed baseball pitchers were instructed to pitch a ball as fast as possible to the target. Three-dimensional coordinate data of the pitching motion and ground reaction forces of each leg were captured using a 20-camera motion capture system with two force platforms. Dynamics contributions of joint torque, gravity, motion-dependent term(MDT), and modeling error term to the generation of ball speed were quantified using the equation of whole-body motion for the target system. Furthermore, the generating factor of the MDT was considered using a recurrent formula derived from the equation. The results in this study indicate that 1) the internal rotation torque at the shoulder joint induces angular velocity about not only internal rotation axis but also horizontal abduction axis at the shoulder joint, and 2) the dynamic contribution of the internal rotation torque to the generation of ball speed would become small values because the ball speeds generated via these two angular velocities cancel with each other.
