THE ENTIRE ROTATOR CUFF CONTRIBUTES TO ELEVATION OF THE ARM

The function of the infraspinatus, teres minor, and subscapularis duri ng elevation of the arm remains poorly defined. These muscles may gene rate moments that contribute to abduction of the arm, although they fr equently are classified as humeral depressors. The purposes of this st udy were to measure the contributions to abduction made by the more in feriorly positioned rotator cuff muscles relative to the contributions of the supraspinatus and to determine the range of motion at which th e muscles are most effective. Five fresh cadaveric shoulder girdles we re mounted in an apparatus designed to simulate contraction of the del toid and rotator cuff while maintaining the normal relationship betwee n glenohumeral and scapulothoracic motions. The deltoid force required for elevation was measured without simulated contraction of the rotat or cuff and with simulated contraction of the entire rotator cuff, of the supraspinatus only, and of the infraspinatus-teres minor and subsc apularis only. A significant reduction in deltoid force when other mus cle activity was added indicated that the additions contributed signif icantly to abduction. The deltoid force required with concurrent contr action of the entire rotator cuff averaged 41% less than with the delt oid alone but was not significantly different than with the deltoid an d supraspinatus or with the deltoid, infraspinatus-teres minor, and su bscapularis. Concurrent application of forces to the supraspinatus os the infraspinatus-teres minor and subscapularis significantly reduced the required deltoid force over the range of motion studied by an aver age of 28 and 36%, respectively. The contributions of the rotator cuff muscles to abduction of the arm were greatest at low abduction angles (30 and 60 degrees) and were insignificant by 120 degrees. The infras pinatus-teres minor and subscapularis contribute significantly to abdu ction: their contibution was equal to that of the supraspinatus and, l ike the supraspinatus, they are most effective during the first 90 deg rees of abduction.