Abstract
This study examined the effect of the site of contact of the stepping foot on shoulder loading during baseball pitching. Measurements were performed in 10 right-handed pitchers who belonged to a university baseball club, on an indoor flat floor. Pitching movements were classified into 7 categories:（1） [normal],（2）Stepping stride with a [narrow], [medium], or [wide] and（3）Stepping with an [inside]（in the right direction for the pitcher）, [center], and an [outside] direction. VICON was used to make the measurements. The velocity of the ball and motion-dynamic data of the right shoulder joint were calculated based on the 3-D coordinate values obtained. As a result, the velocity of the ball and the anterior, inferior shearing forces in the humeral head were significantly higher in the [normal] compared with [narrow] and [outside] conditions. There were no significant differences in the internal rotation and posterior shearing force under the different conditions. This suggests that, when the velocity of the ball is low, the pitcher has fewer injuries in relation to the anterior and inferior shearing forces. However, even when a step condition that slows down the ball velocity is selected, the shoulder loading may be relatively great, depending on the direction of the shearing force.
