For that reason, and because of the dexterity of the shoulder joint itself, the musculature of the shoulder is complex, ranging from massive prime mover muscles to finer stabilizer and fixator muscles. The shoulder (or humeroscapular) joint is formed by the articulation of the head of the humerus with the scapula.
On the anterior side of the shoulder, the coracobrachialis, serratus anterior, pectoralis major, and pectoralis minor muscles work as a group to flex and adduct the scapula and humerus anteriorly toward the sternum. The latissimus dorsi and teres major on the posterior side extend and adduct the arm towards the vertebrae of the back.
The shoulder has about eight muscles that attach to the scapula, humerus, and clavicle. These muscles form the outer shape of the shoulder and underarm. The muscles in the shoulder aid in a wide range of movement and help protect and maintain the main shoulder joint, known as the glenohumeral joint.