Movements in the shoulder joint are produced by 11 muscles. Two of these muscles (the biceps brachii and triceps brachii) have only assisting actions in the shoulder joint because their major action is at the elbow joint. The main muscles are the deltoid, supraspinatus, pectoralis major, coracobrachialis, latissimus dorsi, teres major, infraspinatus, teres minor, and subscapularis.
The musculature surrounding the shoulder joint is arranged so that it produces large stabilizing components especially by the four rotator cuff muscles (supraspiatus, teres minor, infraspinatus and subscapularis). Regardless of the position of the arm, the anterior, posterior, and middle deltoid also have large stabilizing components because of their small angle of pull.
Further stability is provided by the long heads of the biceps brachii on the anterior side of the shoulder and the triceps on the posterior side. As with the deltoid muscle, the upward pull of these muscles is counteracted by the downward pull of the rotator cuff muscles (except for the supraspinatus).
Most of the other muscles surrounding the shoulder joint also exert a stabilizing force but their main function is to move the arm. In addition, as the arm goes into motion, the muscles involved change their angles of pull considerably. Thus they may not always be major stabilizers (or movers) of the shoulder joint.
The muscles that serve as the primary movers of the arm at the shoulder joint are the deltoid, coracobrachialis, pectoralis major, latissimus dorsi and teres major, the long and short heads of the biceps and the long head of the triceps on the posterior side. The muscles located on the front of the chest and shoulder are involved mainly in flexion and horizontal adduction while those on the posterior side are involved mainly in extension and horizontal abduction.
