The muscular connections of the shoulder are present in multiple layers. In the deep layer, four muscles (three originating in the outer surface of the scapula, one originating on the inner surface of the scapula) the Supraspinatus, the Infraspinatus, the Teres Major and Subscapularis connect the scapula to the humerus to form the "rotator cuff" (Figure 9).

The supraspinatus draws the humoral head toward the glenoid fossa to stabilize the shoulder joint, while the infraspinatus and teres minor contribute to the external rotation of the shoulder. The fourth muscle, the Subscapularis, (originating on the inner surface of the scapula) plays an internal rotation function for the shoulder. All four of these muscles pass from the scapula to attach to the head of the humerus in a cuff of musculo-tendinous attachments that encircle approximately 75% of the diameter of the humoral head. This nearly circumferential tendinous insertion is called the "rotator cuff" and provides the muscular attachments required to achieve rotational movements of the glenohumeral joint and keep the humeral head up against the glenoid fossa. Also in this deep layer of shoulder muscles is the Serratus anterior (Figure 10A), which attaches to the inside edge of the scapula, passes under the scapula (between the scapula and the rear chest cage wall), under the arm pit and attaches to the ribs on the side of the chest cage (Figure 10B). This muscle pulls the scapula outward when it contracts, to elevate the glenohumoral joint. On the front of the chest, the next layer of muscles includes the Pectoralis minor muscles which attach the coracoid process to the anterior ribs of the chest, stabilizing the scapula, plus the short tendon of the biceps muscle, which also attaches to the coracoid process to further stabilize the scapula.

On the back, the second layer of shoulder muscles is composed of the Rhomboideus major and minor muscles (Figure 11) which attach to the inner edge of the scapula and pass to the thoracic spine, and the Levator scapulae muscle which attaches the upper, inner edge of the scapula to the cervical spine. A fourth muscle, the Trapezius, attaches to the boney ridge that runs diagnonally across the back of the scapula and extends to the spine throughout the entire cervical and thoracic regions. A fifth muscle, the Deltoid, attaches to the opposite side of the boney scapular ridge and extends across the glenohumoral joint to attach to the humerus. The Deltoid forms the most superficial muscular attachment of the shoulder joint and contributes to lifting of the arm at the shoulder.