ff muscles is isometric control of the humeral head; they actually compress the bony head into the glenoid fossa. The four muscles achieve this through the formation of force couples. The most important of these couples consists of the subscapularis counterbalanced against both the infraspinatus and the teres minor muscles (16:339). Yet another couple is formed by the anterior deltoid and the infraspinatus and teres minor muscles. These later two muscles additionally provide passive restraint posteriorly (11:205). By holding the humeral head fast onto the glenoid fossa, the muscles restrain gross movements initiated by the larger muscles and minimize the humerus' translation (16:339).
Besides stabilizing the glenohumeral joint, the rotator cuff muscles can also act as movers. Both the deltoid and the supraspinatus are responsible for glenohumeral abduction as well as torque production. With the arm hanging vertical, their direction of force is nearly straight up.
In addition to exerting a compressive force, the infraspinatus,
...