Biceps brachii

The biceps brachii serves as the primary supinator of the forearm—rotating the palm upward—and a powerful flexor of the elbow joint. It also assists in shoulder flexion, and its long head contributes to stability of the glenohumeral (shoulder) joint by running within the bicipital groove of the humerus.

The Popeye deformity is a visible bulge in the upper arm caused by proximal biceps tendon rupture. When the long head of the biceps tendon tears—most commonly at its attachment to the supraglenoid tubercle of the scapula—the muscle belly retracts distally, creating a characteristic rounded bulge resembling the cartoon character's arm.

The biceps reflex tests the integrity of the C5 and C6 nerve roots. A clinician strikes the biceps tendon at the elbow with a reflex hammer; the expected response is elbow flexion. An absent or diminished reflex may indicate pathology at the C5 or C6 spinal levels, while an exaggerated reflex may suggest an upper motor neuron lesion.

Bicipital tendinitis is inflammation of the long head of the biceps tendon, usually where it runs through the bicipital groove of the humerus. It presents as anterior shoulder pain that worsens with overhead activities, lifting, or resisted forearm supination. It often accompanies rotator cuff pathology.

The biceps brachii crosses both the shoulder and elbow joints. Proximally, it attaches via two heads—the long head to the supraglenoid tubercle and the short head to the coracoid process of the scapula—and distally to the radial tuberosity and bicipital aponeurosis. This dual-joint architecture allows it to act on both joints simultaneously.