Biceps brachii muscle
The biceps brachii muscle is one of the chief muscles of the arm, it acts on both the shoulder joint and the elbow joint. It derives its name from its two heads which insert on the coracoid apex and the supraglenoid tubercle. In this article, we're going to discuss the attachments, innervation, and functions of the biceps muscle.
Origins and Insertions
The biceps brachii muscle (biceps) is a large, thick muscle of the upper arm consisting of two heads.
- long head: originates at the supraglenoid tubercle above the socket of the scapula. It lies within the intracapsular space but it still remains extrasynovial. The long biceps tendon makes a sharp turn at the humeral head and continues its course in the bicipital groove (intertubercular sulcus). This turning point is secured by ligaments at the capsular area (=biceps pulley).
- short head: arises at the coracoid process of the scapula where it partly blends with the origin tendon of the coracobrachialis.
Both heads unite to one large muscle at the anterior side of the humerus and attach to the radial tuberosity. A fibrous membrane emerging from the distal part of the muscle (bicipital aponeurosis, also called lacertus fibrosus) inserts at the deep fascia of forearm.
In terms of innervation, this muscle is supplied by the musculocutaneous nerve (C5-C6), a branch of the brachial plexus.
The surface anatomy of the anterior side of the upper arm is essentially formed by the biceps. While both its origin tendons are covered by the deltoid its insertion tendon can be easily seen and palpated at the crook of the arm. The space between the biceps and triceps forms two grooves (medial and lateral bicipital grooves). Within the medial bicipital groove course the brachial artery and both the ulnar and median nerves. The lateral bicipital groove contains the radial nerve.
The biceps is a two-joint muscle. In the shoulder joint both muscle heads partially enforce opposite movements. The long head pulls the arm away from the trunk (abduction) and turns it inwards (inward rotation) whereas the short head pulls the arm back towards the trunk (adduction). When both heads contract simultaneously it leads to an arm bend (flexion).
In the elbow joint the muscle bends the forearm (flexion) and rotates it outwards (supination). The supination is most powerful in a flexed elbow. In addition to the movement functions, the biceps has the important task to support the humeral head within the shoulder joint.
In physical examination, the biceps plays an important role. It provides an orientation for the palpation of the brachial artery. Hereby the artery is pushed with the fingertips against the humerus in the medial bicipital groove. Furthermore it serves as a reference muscle for the nerve roots C5 and C6. In this exam the biceps reflex is tested by striking the insertion tendon with a hammer and thus activating a contraction of the muscle.
Due to the close relationship between the long biceps tendon and the rotator cuff, inflammatory and degenerative processes often affect each other. Common consequences are a pulley lesion or biceps tendinitis. A pulley lesion is characterized by damage to the biceps pulley complex through which the long biceps tendon is no longer secured in the shoulder joint and thus slips out of the intertubercular sulcus. A biceps tendinitis is an inflammation of the long biceps tendon often caused by a bursitis or other tendinitides involving the rotator cuff, more rarely through overuse. In severe cases the tendon can even tear apart completely (biceps tendon rupture).