190 Anatomy & Physiology Essentials Copyright Goodheart-Willcox Co., Inc. flexion of the arm. The latissimus originates on the lower 6 thoracic vertebrae, all of the lumbar vertebrae, ribs 10–12, the sacrum, and the iliac crest, enabling extension, adduction, and medial rotation of the arm. The deltoid originates on the scapular spine, acromion, and clavicle, making it a prime mover for arm abduction. Four muscles attaching the humerus to the scapula also contribute to glenohumeral joint stability and produce rotational movements of the humerus. Because these muscles and their tendons form a cuff where they insert around the head of the humerus, they are collectively known as the rotator cuff. Figure 6.21 shows these muscles, and Figure 6.22 describes their locations and functions. The muscles of the rotator cuff are nicknamed the “SITS muscles,” after the first letter of each of their Muscles Acting at the Shoulder Because the shoulder is a ball-and-socket joint and the most freely movable joint in the human body, the movement capabilities of the upper limb are impressive. This large range of motion is achieved because the bone structure of the glenohumeral joint provides little to no stability. Although this gives the shoulder a large range of motion, it also makes the shoulder susceptible to dislocation. Therefore, it is up to the large, powerful muscles surrounding the shoulder to maintain the stability and integrity of the joint. Sizable muscles inserting on the humerus include the pectoralis major on the anterior side of the joint, the latissimus dorsi on the posterior side of the joint, and the deltoid covering the lateral aspect of the joint. The pectoralis originates on the sternum, clavicle, and ribs 1–7, positioning it to contribute to adduction and Supraspinatus Rotator cuff (SITS) muscles Infraspinatus Teres minor Subscapularis Acromion Coracoid process Glenoid fossa Inferior angle Clavicle © Body Scientific International Figure 6.21 Lateral view of the rotator cuff muscles. Rotator Cuff Muscles Muscle Origin Insertion Primary Functions Supraspinatus supraspinous fossa of scapula greater tubercle of humerus assists arm abduction, stabilizes humeral head against downward slippage Infraspinatus infraspinous fossa of scapula greater tubercle of humerus lateral rotation of humerus, stabilizes humeral head against upward slippage Teres minor lateral border and posterior scapula greater tubercle of humerus lateral rotation of humerus, stabilizes humeral head against upward slippage Subscapularis subscapular fossa of scapula lesser tubercle of humerus medial rotation of humerus, stabilizes humeral head against upward slippage Figure 6.22 Goodheart-Willcox Publisher