Proportion of joint angles in shoulder complex to shoulder maximum external rotation angle in throwing

Abstract

The purpose of this study was to identify the angles of the shoulder complex which consist of glenohumeral joint, scapulothracic joint, and thoracic joint at the maximum external rotation (MER) of the shoulder complex during throwing in baseball players, and to analyze the correlation of each angle. The subjects were 19 collegiate baseball players. Throwing motion data was collected by three high-speed cameras and the three-dimensional (3D) coordinates of the shoulder complex were established by direct leaner translation method for the MER calculation. A 3D analysis was performed to obtain the external rotation (ER) angle of the glenohumeral joint, the posterior tilt angle of the scapula, and the extension angles of thoracic at MER of shoulder complex. The mean (±SD) value of the MER was 145.5±10.3°. The mean (±SD) values of the glenohumeral ER, the scapula posterior tilt angle and the thoracic extension angle at MER were 105.3 ±16.0°, 24.3±15.0°, and 9.1 ±7.2°respectively. Multiple linear regression analysis was used to relate the MER angle to each joint angle. The final linear regression model included the posterior tilting angle of scapula (r=0.56, p<0.05), and external rotation of the glenohumeral joint (r= 0.40, p<0.05). Significant negative correlation was observed between the posterior tilting angle of the scapula and external rotation of the glenohumeral joint (r=-0.52, p<0.05). This finding suggested that scapula motion could be very important for the prevention of throwing injuries.