15.15: Muscles that Move the Arm
Nine muscles are involved in arm movements. Two of these, the pectoralis major and latissimus dorsi, originate from the axial skeleton and are called axial muscles. The other seven originate from the scapula and are called the scapular muscles.
The pectoralis major has two origins. Its clavicular head originates on the medial half of the clavicle. In contrast, the sternocostal head originates on the costal cartilages of ribs 1-6, the sternum, and the aponeurosis of the external oblique of the anterior abdominal wall. Both these heads merge and insert on the intertubercular sulcus of the humerus. Its clavicular head flexes the humerus, and the sternocostal head extends a flexed arm back to the anatomical position. When the head works together, they adduct and medially rotate the humerus.
The latissimus dorsi originates from the spinous processes of the thoracic vertebrae T7 to T12, crests of sacrum and ilium, and ribs 9 to 12. It inserts anteriorly on the intertubercular sulcus of the humerus. Acting with the scapular muscle, teres major, it extends, adducts, and medially rotates the humerus. The origin of the scapular muscle, teres major, is at the inferior angle of the scapula. It inserts into the humerus at the medial lip of its intertubercular sulcus. Its actions are the same as that of latissimus dorsi.
The scapular deltoid is the most superficial muscle of the shoulder joint and is responsible for its rounded shape. Its anterior fibers originate on the lateral of the clavicle — the lateral fibers originate from the acromion of the scapula, and the posterior fibers originate from the spine of the scapula. All the heads merge and insert on the deltoid tuberosity of the humerus. While the lateral fibers abduct the arm, the anterior fibers flex and medially rotate it, and the posterior fibers extend and laterally rotate it.
The scapular muscles — supraspinatus, infraspinatus, teres minor, subscapularis, and their corresponding tendons, combine to create the rotator cuff. The origin of the supraspinatus muscle lies on the supraspinous fossa of the scapula. Conversely, the infraspinatus muscle originates from its infraspinous fossa. Both these muscles are inserted at the greater tubercle of the humerus, aiding in arm abduction and lateral rotation, respectively.
Another muscle, the teres minor, also inserts at the greater tubercle of the humerus but originates from the inferior angle of the scapula. It acts along with the infraspinatus muscle to laterally rotate and extend the arm. The origin of the subscapularis muscle is at the subscapular fossa of the scapula. It inserts on the lesser tubercle of the humerus and aids in the medial rotation of the arm. Finally, the last scapular muscle involved in arm movements is the coracobrachialis. The origin of this muscle is at the coracoid process of the scapula, and the insertion is on the middle surface of the medial shaft of the humerus. It helps with arm flexion and adduction.
The rotator cuff muscles are prone to injuries, especially in people active in sports. Due to its positioning between the head of the humerus and the acromion of the scapula, the supraspinatus muscle is particularly prone to strain and degenerative changes.
