The rotator cuff opposes superior translation of the humeral head.

To determine the influence of rotator cuff muscle activity on humeral head migration relative to the glenoid during active arm elevation we studied five fresh cadaveric shoulders. The shoulder girdles were mounted in an apparatus that simulated contraction of the deltoid and rotator cuff muscles while maintaining the normal scapulothoracic relationship. The arms were abducted using four different configurations of simulated muscle activity: deltoid alone; deltoid and supraspinatus; deltoid, infraspinatus, teres minor, and subscapularis; and deltoid, supraspinatus, infraspinatus, teres minor, and subscapularis. For each simulated muscle configuration the vertical position of the humeral head in relation to the glenoid was determined at 30 degrees, 60 degrees, 90 degrees, and 120 degrees of abduction using digitized anteroposterior radiographs. Both muscle activity and abduction angle significantly influenced the glenohumeral relationship. With simulated activity of the entire rotator cuff, the geometric center of the humeral head was centered in the glenoid at 30 degrees but had moved 1.5 mm superiorly by 120 degrees. Abduction without the subscapularis, infraspinatus, and teres minor muscles caused significant superiorly directed shifts in humeral head position as did abduction using only the deltoid muscle. These results support the possible use of selective strengthening exercises for the infraspinatus, teres minor, and subscapularis muscles in treatment of the impingement syndrome.