15.14: Muscles of the Shoulder
The muscles surrounding the shoulder girdle, including the clavicle and scapula, primarily stabilize the scapula. This stable base allows other muscles to move the humerus effectively. Scapular movements often mirror those of the humerus and extend its range of motion. For instance, raising the arm above the head would not be feasible without simultaneous upward rotation of the scapula.
Anterior Thoracic Muscles
The anterior thoracic muscles include the serratus anterior, subclavius, and pectoralis minor. The serratus anterior, often known as the boxer’s muscle, starts from the first to eighth or ninth ribs. It inserts along the superior and inferior angle of the scapula as well as its medial border. This muscle is crucial for abducting and rotating the scapula upward. When the scapula is fixed, it helps elevate the ribs, assisting in punching movements.
The pectoralis major joins the pectoral girdle to the thorax. It originates from the anterior surface of the superior margins of the second or third to fourth or fifth ribs and inserts on the coracoid process of the scapula. It tilts the scapula anteriorly and pulls it inferiorly. It also elevates the thorax, aiding in respiration.
The subclavius muscle 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.
Posterior Thoracic Muscles
The posterior thoracic muscles include the trapezius, levator scapulae, and the rhomboid minor and major muscles. The trapezius muscle, one of the broadest and most distinct muscles, has its roots in the bony structure of the cervical and thoracic regions. The muscle originates from the external protuberance of the occipital bone, ligamentum nuchae, and the spinous process of the seventh cervical and all thoracic vertebra. It inserts at the clavicle along its lateral third in addition to the acromion and the spine of the scapula. It is categorized into three parts — the upper, middle, and lower fibers, each performing unique actions. The upper fibers elevate the scapula and extend the neck, the middle fibers adduct the scapula, and the lower fibers depress the scapula.
The levator scapulae originate from the transverse process of the cervical vertebrae, C1-C4, and insert into the scapulae at the superior part of its medial border. It elevates the scapula and causes its downward rotation. It also flexes the neck laterally.
Originating from the nuchal ligament and spinous processes of C7 and T1, the rhomboid minor inserts into the vertebral border of the scapula, superior to the spine. On the other hand, the rhomboid major originates from the spinous processes of the T2 to the vertebral border of the scapula, inferior to the spine. These muscles help retract the scapula and rotate it medially.
