Effect of finger–ball friction on upper limb movement during fastball pitching in baseball

This study investigated the effect of the friction between the ball and fingertips (finger–ball friction) on upper limb movement during four-seam fastball pitching in terms of the centrifugal force acting on the ball. Eight skilled pitchers threw four-seam fastballs at approximately 130 km/h toward a target behind the home base. Water was applied as a low-friction condition and rosin powder was applied as a high-friction condition between the fingertips and the ball. Hand velocity and pitching radius (i.e., radius of the motion trajectory of the hand) were calculated from motion capture data. Centrifugal force evaluation index was calculated as the square of hand velocity divided by the pitching radius. Statistical parametric mapping was performed to compare the time-series of each variable between foot contact and ball release. Although no significant differences were observed, a tendency for hand velocity to decrease under low-friction conditions during the acceleration phase was observed. Additionally, the pitching radius was significantly greater under low-friction conditions around the maximum shoulder rotation timing, and consequently, the centrifugal force index significantly decreased during the first half of the acceleration phase. These findings deepen our understanding of how pitchers adjust their throwing motion under different finger–ball friction conditions.