Shoulder muscle moment arms during horizontal flexion and elevation

The instantaneous muscle moment arms of 10 shoulder muscles including the three portions of the deltoid and the rotator cuff and scapulohumeral muscle groups during four specified glenohumeral motions were calculated. Moment arm values were derived from a plot of tendon excursion versus glenohumeral joint rotation angle during horizontal flexion along the 90° elevation plane and elevation in the sagittal, scapular, and coronal planes. The deltoid had the largest moment arm in elevation. The anterior deltoid has a larger moment arm in the anterior planes, whereas the midportion is greater in the scapular and coronal planes. The muscles with the largest depressor (adductor) moment arms are the pectoralis major, latissimus dorsi, and teres major. Contrary to the findings of other investigators, the supraspinatus and infraspinatus have a smaller potential elevation torque in the scapular plane than has been previously reported. Furthermore the subscapularis may potentially be a more important elevator in the scapular plane than either the supraspinatus or infraspinatus, especially in the latter phases of motion. The pectoralis major has the largest horizontal flexion moment arm with the humerus elevated 90°, whereas the posterior deltoid and infraspinatus have the largest horizontal extension moment arms in this plane. (J Shoulder Elbow Surg 1997;6:429-39.)