Abstract
The purpose of this study was to clarify the relationship between kinematics of the pitching motion and the direction of the spin axis of a pitched baseball. Eight male pitchers were recruited to pitch fastballs. The pitching motion was captured with a three-dimensional motion analysis system. The orientations of whole body segments in a global coordinate system were calculated from the captured data. The directions of spin axis were also calculated on the basis of positional changes of reflective markers that attached to the ball. The spin axis directions were significantly correlated with the upper arm (r=-0.831, p<0.05) and the forearm (r=-0.767, p<0.05) orientations. When a subject whose wrist pronated at the moment of ball release was excluded, the spin axis directions were correlated with the orientations of the hands (r=0.774, p<0.05 / r=0.925, p<0.01) and trunk (r=0.936, p<0.01) as well as the forearms (r= -0.805, p<0.05) and the upper arms (r=0.822, p<0.05 / r=0.763, p<0.05 / r=-0.852, p<0.05). These results suggested that most subjects controlled the spin axis directions with their angular displacement of the upper limb and trunk, but the subject whose wrist pronated at the moment of ball release controlled the spin axis directions with his fingers the only segment that contacts to the ball.
