The relevance of the moment arm of shoulder muscles with respect to axial rotation of the glenohumeral joint in four positions

Objective. This study was undertaken to determine the efficiency of the shoulder girdle muscles during axial humeral rotation based on measurements of the moment arms. Design. The instantaneous muscle moment arms of 10 shoulder muscles, including the three portions of the deltoid, the rotator cuff muscles, teres major, and the thoracohumeral muscle group, were measured during four specified glenohumeral rotations. Background. Axial humeral rotation is a commonly performed movement during activities of daily living and is a targeted motion of shoulder rehabilitation, particularly in those protocols emphasizing rotator cuff strengthening. An understanding of the function of the movers and stabilizers of the shoulder requires such basic information of muscle moment arms. Methods. The instantaneous moment arm values of the muscles were derived from the slope of the plot of tendon excursion versus glenohumeral joint rotation angle. Motion studied included axial rotation with the humerus elevated 90 degrees in the coronal, scapular, and sagittal planes, as well as in the neutral position with the arm at the side. Results. Based on the findings, with the humerus in both neutral and elevated positions, the infraspinatus is potentially the most powerful external rotator, followed by teres minor and posterior deltoid. Subscapularis and possibly pectoralis major are the most effective internal rotators in this position. Conclusions. The moment arm in providing axial humeral rotation of 10 shoulder muscles in four planes were obtained. Ln general, the teres minor and infraspinatus had the largest moment arms in external relation, and the subscapularis had the largest moment arm in internal rotation. The muscle function for axial humeral rotation was found to be modified by the plane of arm elevation.