野球の打撃におけるローリングの速さを決定する力学的要因

抄録

 A wide range of topspin rotation of a bat around the long-axis, referred to as “rolling”, has been observed in baseball batting, but the mechanical reasons for the large variability among individual batters has not been examined. The purpose of this study was to investigate the factors determining this variability in rolling velocity among professional baseball players. Twenty-nine professional batters each performed eight “free-batting” trials. An electromagnetic tracking device was used to measure the three-dimensional rotational motion of the bat in each trial. The rolling velocity was 678±376°/s, comprised a negative contribution attributable to the batter's effort of exerting torque (Mechanism 1; −1144±488°/s) and a positive contribution attributable to the gyroscopic effect (Mechanism 2; 1808±339°/s). A significant positive correlation (r=0.67, p<0.05) was found between the rolling velocity and the negative contribution of Mechanism 1. These results indicate that (a) the torque exerted by the batter resists the rolling and that (b) a higher rolling velocity is attained by batters who exert a smaller resistive torque on the bat than those who exert a larger torque. On the other hand, no correlation (r=0.09) was found between the rolling velocity and positive contribution of mechanism 2. These findings suggest that the batter makes an active effort to resist rolling, and that the amount of resistive torque exerted by the batter is the primary reason for the inter-individual difference in rolling velocity attained at the instant of ball impact. As the resistive torque is likely to be exerted by the nobside hand in the form of pronation torque (Ae et al. 2015) and the pronation causes lowering of the bat-head (increasing nutation angle), a reduction of the pronation torque should decrease the resistive torque acting on the bat, helping to attain a high rolling velocity. In fact, we observed a greater deceleration of nutation velocity in the fast-rolling group than in the slowrolling group (p<0.05). To attain the high rolling velocity, therefore, we suggest that batters should aim to build up the nutation velocity of the bat until about 50 ms before ball impact, and then vigorously decelerate it immediately before ball impact.