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 scapulohume ral muscle groups during Four specified glenohumeral motions were calc ulated. Moment arm values were derived from a plot of tendon excursion versus glenohumeral joint rotation angle during horizontal flexion al ong the 90 degrees elevation plane and elevation in the sagittal, scap ular, and coronal planes. The deltoid had the largest moment arm in el evation. 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 infraspi natus have a smaller potential elevation torque in the scapular plane than has been previously reported. Furthermore the subscapularis may p otentially be a more important elevator in the scapular plane than eit her the supraspinatus or infraspinatus, especially in the latter phase s of motion. The pectoralis major has the largest horizontal Flexion m oment arm with the humerus elevated 90 degrees whereas the posterior d eltoid and infraspinatus have the largest horizontal extension moment arms in this plane.