Muscles flexing the arm at the shoulder joint

Pectoralis major

Deltoid – anterior fibres

Biceps brachii – long head

Coracobrachialis

Pectoralis major

Pectoralis major is found on the upper half of the anterior surface of the thoracic wall. It is a thick triangular muscle with clavicular and sternocostal parts, which may be separated by a groove, although they are usually continuous with each other. On their way to the humerus, the twisting fibres of pectoralis major from the rounded anterior fold of the axilla.

The smaller, clavicular attachment of pectoralis major is from the medial half of the anterior surface of the clavicle; the larger, sternocostal attachment comes from the anterior surface on the manubrium and body of the sternum, the anterior aspects of the upper six costal cartilages, the anterior part of the sixth rib as well as the aponeurosis of the external oblique muscle of the abdomen.

From this large central attachment, the muscle narrows and inserts via a laminated tendon into the lateral hip of the intertubercular groove of the humerus. The anterior lamina, which comprises the clavicular part of the muscle, runs to the lower part of the humeral attachment. Fibres from the sternocostal part of the chest wall form the posterior lamina, which passes upwards behind the anterior lamina to the upper part of the attachment of the muscle to the humerus. In this way the tendon comes to resemble a U in cross-section. The posterior part blends with the shoulder joint capsule, while the anterior, clavicular fibres blend with the attachment of deltoid.

As the most superficial of the muscles of the anterior thoracic wall, pectoralis major lies on top of pectoralis minor, the ribs and serratus anterior. In the female, the muscle is covered by the breast; indeed the fibrous septa of the breast are attached to the deep fascia overlying pectoralis major. Pectoralis major is separated from deltoid by the deltopectoral groove(the infraclavicular fossa) in which lie the cephalic vein and branches from the thoracoacromial artery.

Nerve supply

Pectoralis major is supplied by the medial(C8, T1) and the lateral(C5, 6, 7) pectoral nerves; the clavicular part by roots C5 and 6, and the sternocostal part by C7, 8 and T1. The skin over pectoralis major is supplied by roots T2 to T6.

Action

Pectoralis major as a whole is a powerful adductor and medial rotator of the humerus at the shoulder joint. In addition to the clavicular part can flex the humerus to the horizontal, while the sternocostal fibres, because of their direction, can extend the flexed humerus, particularly against resistance to the anatomical position. With the humerus fixed, as in gripping a bed table or chair back, pectoralis major pulls on the upper ribs to assist inspiration during respiratory distress.

Functional activity

Pectoralis major is one of the major climbing muscles, so that if the arms are fixed above the head, the massive power of the muscle can be used to pull the trunk upwards. It is assisted in this activity by latissimus dorsi. In pushing, punching and throwing movements, pectoralis major acts to move the humerus forwards forcefully, whilst serratus anterior and pectoralis minor simultaneously protract the pectoral girdle.

In exercises, such as the “press-up”, pectoralis major contracts concentrically on the upward movement and eccentrically on the downward pressure. The sternocostal part is best palpated if this same position is maintained against an upward pressure. The integrity of the muscle can be tested by adduction of the arm against resistance.

Biceps brachii

Biceps brachii is a prominent fusiform muscle on the anterior aspect of the arm. It arises by two tendinous heads as its upper end, and attaches by one tendinous heads at its upper end, and attaches by one tendinous insertion at its lower end. The upper end is covered by deltoid and pectoralis major, but the main part of the muscle is only covered by skin and subcutaneous fat.

The short head of biceps brachii arises by a flat tendon, shared with coracobrachialis, from the apex of the coracoid process of the scapula. The long head arises from the supraglenoid tubercle of the scapula and the adjacent glenoid labrum of the shoulder joint. The tendon of the long head runs within the shoulder joint wrapped in a synovial sleeve and leaves to enter the intertubercular groove by passing deep to the transverse humeral ligament. The two fleshy bellies continue towards the elbow with the two heads fusing to form with the two heads fusing to form a single muscle just below the middle of the arm. At the elbow, a single flattened tendon is formed which twists through 90° before attaching to the posterior part of the radial tuberosity. A bursa separates the tendon from the remainder of the radial tuberosity. The prominent bicipital aponeurosis, a strong membranous band arising from the lateral side of the main tendon, runs downwards and medially across the cubital fossa, in front of the brachial artery and median nerve, to attach to the deep fascia on the ulnar side of the forearm.

Nerve supply

Biceps brachii is supplied by the musculocutaneous nerve, root value C5, and 6. The skin over biceps is supplied by the roots C5, 6, T2 and T3.

Action

Biceps brachii is not only an important flexor of the elbow joint, but also a powerful supinator of the forearm. Often these two actions are performed together with any unwanted actions being cancelled by antagonists. Maximum power is achieved for both flexion and supination with the elbow at 90°. When the elbow is fully extended the supinating action of biceps is lost. Biceps is also a flexor of the shoulder joint, and the fact that the long head crosses the superior part of the joint means that it has an important stabilizing role.

Functional activity

Biceps may use its supinatory and flexing actions sequentially in an activity, as, for example, in inserting a corkscrew and pulling out the cork. During this activity the head of the ulna may move medially due to the force of the biceps contraction transmitted to its posterior border via the bicipital aponeurosis.

When deltoid is paralysed, the long head of biceps can be re-educated to abduct the shoulder. This is accomplished by laterally rotating the humerus at the shoulder joint in order to put the long head into a more appropriate position.

Palpation

With the elbow flexed to 90° and the forearm pronated, the muscle can be felt contracting in the middle of the arm when supination against resistance is attempted.

The lower part of the muscle is easily palpated through the skin. Proximally, each tendon may be palpated but with some degree of difficulty. The tendon of the long head lies between the two tubercles. Having determined these, firm deep pressure between them is needed to locate the tendon. This is the point at which deep transverse frictions or electrical treatments are applied when the tendon becomes inflamed.

The short head can be found by first palpating the apex of the coracoid process, and then placing the fingers just below it. As the elbow is flexed, the tendon can be felt to stand out.

At the elbow, the tendon of insertion is best palpated with the elbow flexed to 20°. In this position it can be easily gripped between the index finger and thumb. If, in this same position, the subject is asked to resist a strong downward pressure on the forearm, the upper border of the bicipital aponeurosis can be seen and felt as a crescentic border running downwards and medially from the main tendon.

The tendon of biceps is the point at which the biceps reflex is tested, often by the examiner placing his or her thumb over the tendon and then tapping it with a patella hammer. The resultant reflex contraction can be felt below the thumb and biceps may be seen contracting if the reflex is brisk enough.

Coracobrachialis

Coracobrachialis is the only true representative in the arm of the adductor group of muscles found on the leg. It arises via a rounded tendon, in conjunction with the short head of biceps brachii, from the apex of the coracoid process of the scapula and attaches by a flat tendon to the medial side of the shaft of the humerus at about its midpoint, between triceps and brachialis. Some fibres may continue into the medial intermuscular septum of the arm.

Nerve supply

Coracobrachialis is usually supplied by the musculocutaneous nerve as it pierces the muscle, root value C6, 7. However, the nerve to coracobrachialis may arise directly from the lateral cord of the brachial plexus. The skin over the muscle is supplied by roots T1 and T2.

Action

Coracobrachialis is an adductor and weak flexor of the arm at the shoulder joint.

Palpation

Coracobrachialis can be seen and felt as a rounded muscular ridge on the medial side of the arm when it is fully abducted and then adducted against resistance.