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. The hand velocity and pitching radius (i.e., radius of the motion trajectory of the hand) were calculated from motion capture data. The centrifugal force evaluation index was calculated as the square of hand velocity divided by the pitching radius. This was calculated at the time of maximum external rotation (MER) of the shoulder and ball release (BR), then compared between the two conditions. At the time of MER, the pitching radius was larger under low friction, whereas the centrifugal force was similar between under both conditions. At the time of BR, hand velocity was decreased and pitching radius was increased under low friction, reducing the centrifugal force on the ball compared to the high friction condition. These findings deepen our understanding of how pitchers adjust their throwing motion under different finger–ball friction conditions.