In the normal situation, the ball is kept in the center of the socket by the ligaments and rotator cuff tendons. There are four rotator cuff tendons: one in the front of the ball (the subscapularis muscle and tendon), one larger one in the back of the shoulder (the infraspinatus muscle and tendon), one smaller one in the back of the shoulder (the teres minor muscle and tendon) and one on the top of the shoulder (the supraspinatus muscle and tendon). The rotator cuff muscle in the front (the subscapularis) mostly moves your arm toward the front, as when you scratch your stomach. The ones in the back (infraspinatus and teres minor) move your arm to the side, as when you reach out to the side for a glass or cup. The one on the top moves your arm up, as when you reach into a cabinet. The one that is most commonly torn is the one that is on the top of the shoulder (namely, the supraspinatus). Sometimes that tendon is torn such that it cannot be repaired.