172


PART III THE UPPER EXTREMITY


traverses over the humeral head, and descends through the intertubercular groove. When the biceps is activated, the tendon pulls the humeral head inferiorly. The long head of the triceps that attaches to the infraglenoid tuber- cle contributes to humeral head stability during humeral motion.


The subscapularis muscle provides medial humeral rotation and anterior stability to the shoulder. The infraspinatus and teres minor muscles laterally rotate the humerus, clearing the greater tuberosity from the cora- coacromial arch during shoulder elevation. By prevent- ing excessive superior translation of the humerus toward the acromial arch and producing lateral rotation of the humeral head, these muscles avoid possible impinge- ment of structures in the subacromial space. Rotator cuff muscle malfunction resulting from overuse or injury can lead to subacromial impingement, pain, and shoulder dysfunction.


Primary Movers


During normal shoulder movement, the dynamic stabi- lizers provide the stability and arthrokinematic motion necessary for the primary movers to accomplish shoul- der motion through large arcs of motion. These muscles include the deltoid, latissimus dorsi, teres major, pecto- ralis major, and coracobrachialis.


The deltoid muscle has proximal attachments to the clavicle and acromion process of the scapula and con- verges to attach to the deltoid tuberosity of the humerus. The muscle has three functional units, each determined by the orientation of its fi bers. The anterior portion pro- duces GH fl exion and horizontal adduction and con- tributes to medial rotation. Conversely, contraction of the posterior fi bers can result in GH extension, hori- zontal abduction, or lateral rotation. Because of their attachments proximal to the humerus, the long head of the biceps and coracobrachialis assist the deltoid during shoulder fl exion (Fig. 8.30). The middle deltoid fi bers contribute to shoulder abduction once the supraspinatus muscle initiates the motion. In a resting position, the deltoid muscle line of pull initially elevates the humerus toward the coracoacro- mial arch but has little ability to produce humerus rotary movement and abduction. The supraspinatus initiates the abduction, while the other rotators counter the superior movement of the humerus produced by the deltoid. As the shoulder moves toward 50° of abduction, the del- toid’s moment arm to produce motion increases, and the deltoid becomes the primary mover for the remaining range of abduction.


The major adductor and extensor muscles of the GH joint are the posterior deltoid, latissimus dorsi, teres major, long head of the triceps, and the sternocostal


Figure 8.30 The deltoid muscle produces shoulder flexion, abduction, or extension depending on which portion of the muscle fibers are activated. The coracobrachialis can assist the deltoid muscle with shoulder flexion and may contribute to shoulder stability.


Coracobrachialis


Deltoid


portion of the pectoralis major. The action of the broad latissimus dorsi muscle on the scapula was discussed earlier in the chapter. As the muscle traverses anteriorly from its posterior attachments, it forms the posterior border of the axilla before attaching to the lesser tubercle of the humerus. Its attachment is proximal to the teres major, which originates on the lateral scapula border distal to the teres minor. The latissimus dorsi is activated during GH extension or adduction and is considered a medial rotator of the joint. During upper extremity activities, such as pulling, where the arm is extending and adducting against resistance, the teres major assists the latissimus dorsi muscle. The long head of the triceps, with its infraglenoid attachments, can also contribute to shoulder extension (Fig. 8.31).


The pectoralis major muscle is located anteriorly as it originates on the clavicle, ribs, and sternum and attaches to the crest of the greater tubercle. The muscle is divided into two portions: the clavicular head and the sternocos- tal head. The arrangement of muscle fi bers and sites of attachment allow these two portions to function indepen- dently of each other. The muscle as one unit performs GH adduction, medial rotation, and horizontal adduc- tion. The clavicular portion of the muscle assists with shoulder fl exion, and the sternocostal portion extends the GH joint from a fully fl exed position (Fig. 8.32).


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