Muscles of the Shoulder

The anterior shoulder muscles include the subclavius, pectoralis minor, and serratus anterior. The subclavius stabilizes the clavicle during shoulder movements. The pectoralis minor tilts the scapula anteriorly, pulls it inferiorly, and aids in respiration by elevating the thorax. The serratus anterior, known as the boxer's muscle, protracts the scapula and assists in punching movements.

The serratus anterior originates from the first to eighth or ninth ribs and inserts along the superior and inferior angles and medial border of the scapula. This fan-shaped muscle protracts and upwardly rotates the scapula, enabling abduction of the arm. When the scapula is fixed, it elevates the ribs, assisting in punching movements and other dynamic shoulder actions.

The posterior shoulder muscles include the trapezius, levator scapulae, and rhomboid major and minor. The trapezius is a large triangular muscle that elevates, retracts, and rotates the scapula while extending the neck. The levator scapulae elevates and adducts the scapula. The rhomboid muscles retract the scapula and rotate it medially.

The trapezius originates from the occipital bone, ligamentum nuchae, and cervical and thoracic vertebrae, inserting on the clavicle and scapula. Its upper fibers elevate the scapula and extend the neck, middle fibers adduct the scapula, and lower fibers depress the scapula. This three-part arrangement allows diverse scapular and neck movements.

The shoulder girdle muscles primarily stabilize the scapula, creating a stable base that allows other muscles to move the humerus effectively. Scapular movements mirror those of the humerus and extend its range of motion. Without simultaneous upward rotation of the scapula, raising the arm above the head would not be feasible.

The subclavius originates on the first rib and inserts into the lower surface of the lateral clavicle. Its primary function is to stabilize the clavicle during shoulder and arm movements. It also depresses the clavicle and moves it anteriorly, contributing to overall shoulder girdle stability.

The rhomboid minor and major originate from the cervical and thoracic vertebrae and insert along the vertebral border of the scapula. These muscles retract the scapula, pulling it medially toward the spine, and rotate it medially. This action is essential for movements requiring scapular adduction and internal rotation.