Glenohumeral, Scapular, and Thoracic Angles at Maximum Shoulder External Rotation in Throwing

Abstract

Background Because a majority of throwing injuries occur near the maximum external rotation (MER) of the throwing shoulder, sports medicine practitioners and researchers have paid special attention to the shoulder kinematics at the MER in throwing. However, little is known about the individual kinematics of the glenohumeral, scapulothoracic, and thoracic joints at the MER. Purpose To demonstrate the glenohumeral, scapulothoracic, and thoracic joint movements and their contribution to the MER in throwing for baseball players. Study Design Descriptive laboratory study. Methods We collected throwing motion data for 20 collegiate baseball players by using 3 high-speed cameras and established 3-dimensional coordinates of each landmark with a direct linear translation method. We then obtained the MER of the shoulder complex, the external rotation angle of the glenohumeral joint, the posterior tipping angle of the scapula, and the thoracic extension angles at the MER in throwing. Results The mean (± standard deviation) value of the MER was 144.2° ± 11.0°. The mean (± standard deviation) values of the glenohumeral external rotation, the scapular posterior tipping, and the thoracic extension angles at the MER were 105.7° ± 15.5°, 23.5° ± 13.9°, and 8.9° ± 7.3°, respectively. Conclusion Our results indicate that not only the glenohumeral joint movement but also the scapular and thoracic movements make major contributions to the MER angle. Clinical Relevance To better understand the pathomechanics of the shoulder complex in throwing, we need to take into account the individual contributions of the glenohumeral, scapulothoracic, and thoracic extension movements to the MER.