Biceps brachii

Musculus biceps brachii

Also known as: biceps, bicipital aponeurosis, lacertus fibrosus

A two-headed muscle of the front of the arm. It crosses the shoulder and elbow and is the most powerful supinator of the forearm and a strong elbow flexor.

Overview

The long head arises from the supraglenoid tubercle and the glenoid labrum. Its tendon runs inside the shoulder joint capsule, wrapped in synovial membrane, and then descends in the intertubercular sulcus under the transverse humeral ligament. The short head arises from the tip of the coracoid process with coracobrachialis. The two bellies unite in the lower arm and insert on the radial tuberosity by a strong tendon. A flat bicipital aponeurosis (lacertus fibrosus) fans medially into the deep fascia of the forearm and protects the brachial artery and median nerve beneath it.

Because it inserts on the radius, biceps is the most powerful supinator of the forearm, especially when the elbow is flexed (turning a screwdriver, opening a door handle). It is also a strong elbow flexor, most effective when the forearm is supinated. Across the shoulder it weakly flexes the arm, and the long head helps hold the humeral head in place. The biceps reflex (C5–C6) is elicited by tapping its tendon in the cubital fossa.

Location

Anterior compartment of the arm, superficial to brachialis.

Parts & subdivisions

Long head

Supraglenoid tubercle → intra-articular tendon in the intertubercular sulcus.

Short head

Tip of the coracoid process.

Bicipital aponeurosis

Medial expansion into the forearm fascia, superficial to the brachial artery and median nerve in the cubital fossa. The median cubital vein lies superficial to it.

Anatomy

Origin

Short head: tip of the coracoid process. Long head: supraglenoid tubercle of the scapula.

Insertion

Radial tuberosity and, via the bicipital aponeurosis, the deep fascia of the medial forearm.

Action

Supination of the forearm; flexion of the elbow (strongest in supination); weak flexion of the arm at the shoulder.

Innervation

Musculocutaneous nerve (lateral cord).

Nerve roots

C5–C6

Arterial supply

Muscular branches of the brachial artery.

Relations

Lies on brachialis. The musculocutaneous nerve runs between them. The cephalic vein runs along its lateral side and the basilic vein along its medial side.

Clinical significance

Rupture of the long head tendon

Typically in older people with degenerative tendinopathy. The belly bunches up in the lower arm ("Popeye" sign). Function is largely preserved by the short head and brachialis, so treatment is usually non-operative.

Distal biceps tendon rupture

Sudden eccentric loading of the flexed elbow in middle-aged men causes a palpable gap in the cubital fossa (positive hook test) and marked weakness of supination. Early surgical repair is recommended.

Biceps reflex

Tests C5–C6 and the musculocutaneous nerve.

Test yourself

Which muscle is the most powerful supinator of the forearm when the elbow is flexed?

Supinator
Brachioradialis
Biceps brachii
Pronator teres

Answer: Biceps brachii

Biceps brachii is the most powerful supinator when the elbow is flexed, which is how a right-handed person drives a screw. Supinator acts alone in slow, unresisted supination and when the elbow is extended.