Baseball is one of the most popular sports in Japan and in U.S.A. However, many players, especially pitchers, have shoulder injuries incurred during pitching motions because the shoulder suffers repeated stress in the late cocking phase. In that phase the shoulder is externally rotated far beyond the normal range of motion by the inertial force applied on the forearm due to the rapid forward movement of the shoulder. Many researchers have analyzed pitching motion. However no one could calculate the inter-joint force applied between the glenoid fossa and the humeral bone. They calculated the "joint force", which makes the center of gravity of the upper arm accelerate or decelerate. This force is different from inter-joint force. The purpose of this study was to calculate the joint moment, the muscle force, and the inter-joint force on the shoulder during pitching. The subjects were five elite baseball pitchers. Twenty-five reflective markers were attached to the subjects. Four 60-Hz CCD TV cameras (Expert Vision; Motion Analysis) were used to calculate 3D locations of these markers. Shoulder joint angles were calculated from the orientation of the markers. Eulerian angles were calculated to represent the spatial orientation of the upper arm and the fore arm. Angular velocity around the local axes fixed on the upper arm and the fore arm were calculated from Eulerian angles. Joint moments were calculated from the angular velocity and angular acceleration. As the first step of this study, joint moment and joint force, which were calculated by the ordinal method, were analyzed. The ball speeds of these trials were 71 to 87% of each subject's average speed during real baseball matches, so the trials were suitable for analyzed. The average time of the late cocking period and acceleration period were 73ms and 54ms respectively. These were comparable to the 60ms and 50ms of Pappas's results. Maximum external rotation of the shoulder was about 55 degrees, which was smaller than Feltner's data. The angular velocity, joint force, and joint moment were smaller than the Feltner's results. The reason why our results were smaller than others' was partly due to the difference in physical stature between Japanese and American players. Maximum external and horizontal adduction moments were found in the late cocking phase, not in the acceleration phase. All joint moments decreased when the ball was released. Approximately 54ms prior to the ball release, the shoulder was rapidly rotated in the external direction, and it was believed to cause the over-stress on the shoulder joint. At the same time, however, the shoulder was horizontally adducted to reduce the stress.
