The main muscles that flex the shoulder joint are those of the rotator cuff. The rotator cuff consists of four muscles: the supraspinatus, the infraspinatus, the subscapularis, and the teres minor.
The muscles of the rotator cuff all connect to the shoulder blade or scapula and end at the top of the upper arm bone, the humerus bone.
The shoulder is prone to injury. The most common shoulder injury is a dislocation of the shoulder. This can occur due to sudden movements of the arm, particularly for small children. Surgery may be required if the injury is serious. The rotator cuff muscles can also be damaged due to overuse or sudden stress.