Glenohumeral Internal Rotation Deficit on Pitching Biomechanics and Muscle Activity

To characterize the scapular pitching biomechanics in symptomatic GIRD pitchers (SG) compared to asymptomatic GIRD (ASG) and healthy pitchers. The scapular kinematics and associated muscle activities during pitching were recorded in 33 high school pitchers. Compared to healthy, GIRD pitchers had less scapular posterior tilt in each pitching event (average difference, AD=14.4 degrees, p<0.01) and ASG demonstrated less scapular upward rotation at ball release (AD=12.8 degrees, p<0.01) and greater muscle activity in the triceps brachii in the early-cocking phase (AD=9.9%, p=0.015) and in the serratus anterior in the late-cocking phase (AD=30.8%, p<0.01). Additionally, SG had less muscular activity on triceps brachii in the acceleration phase and serratus anterior in the cocking phase (AD=37.8%, p=0.016; AD=15.5%, p<0.01, respectively) compared to ASG. GIRD pitchers exhibited less scapular posterior tilt during pitching, which may cause impingement. Since tightness of the anterior shoulder is a common cause of inadequacy of posterior tilt during arm elevation, stretching exercise of the anterior shoulder is recommended. Given the inadequate recruitment during pitching in the GIRD pitchers, symptoms may develop following potential impingement.