RELATIONSHIPS BETWEEN SHOULDER JOINT KINEMATICS AND KINETICS IN PROFESSIONAL BASEBALL PITCHING

Abstract

10 The upper extremity is vulnerable to injury during throwing due to the repetitious nature of the motion, the extremes in range of motion, and the high angular velocities and torques generated at the shoulder and elbow. The purpose of this study was to investigate the relationships between kinetics and kinematics of the throwing shoulder, and to identify the specific aspects of the throwing motion which place the overhead athlete most at risk. Video data were collected on 40 Major League Baseball pitchers during Spring Training in the 1998 Cactus League. High-speed (120 Hz) cameras were necessary to sufficiently capture the pitching motion for scientific analysis. Twenty body landmarks and the ball were digitized, frame by frame, throughout the pitching motion from two camera views for each athlete. Data from the two cameras were then synchronized in time and mathematically combined to produce a three-dimensional representation of the pitch for one fast ball for each pitcher. Over 50 kinematic variables were computed, and the resultant joint forces and torques at the throwing arm elbow and shoulder were calculated. Average ball speed was 85 mph for the 40 pitchers. Maximum external rotation at the shoulder joint averaged 184 ± 13 deg. Internal rotation velocities in excess of 8500 deg/sec were documented. A significant compressive force (952 ± 162 N), acting to resist distraction, occurred at the shoulder joint near the instant of ball release. A regression equation relating the kinematic parameters of pitching mechanics to shoulder distraction was obtained. Assessment of major league pitching mechanics furnish information regarding a highly dynamic and potentially harmful throwing motion. Parameters of pitching mechanics were identified to reduce joint stress.