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.
Medically reviewed by Healthline’s Medical Network on March 23, 2015. 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. The largest of these shoulder muscles is the deltoid. This large triangular muscle covers the glenohumeral joint and gives the shoulder its rounded-off shape.