Factors of individual differences between the speeds of the hand and ball during baseball pitching: Assessment of middle finger function based on the hierarchical linear model

This study aimed to reveal individual differences between the speeds of the hand and ball and clarify the factors affecting those differences. Motion data from 20 highly skilled baseball pitchers, captured using a 16-camera motion analysis system with a sampling frequency of 1,000 hz, were used for correlation and hierarchical linear model analyses. For the hierarchical linear models, the speed of the metacarpophalangeal joint was the independent variable in the 1^st level. The independent variables chosen in the 2^nd level were the variables regarding the finger length, mean peak angular velocities, and mean peak accelerations of the middle finger joint. Intra-subject correlation analysis showed high positive correlation coefficients between the metacarpophalangeal joint and ball speeds, ranging from 0.803 to 0.992. The difference between the hand and ball speeds ranged from 8.4 m∙s^-1 to 12.0 m∙s^-1. The hierarchical linear model analyses indicated that the individual differences between the speeds of the hand and ball were partly accounted for by the mean peak accelerations of the proximal and distal interphalangeal joints, suggesting that the force exerted by proximal interphalangeal joint flexion plays an important role in ball speed increment.