The extensor carpi radialis longus is a muscle that helps move the hand. It also facilitates movement at the wrist. It is involved in extending and abducting the hand at the wrist joint. The muscle also plays a role in flexion of the elbow. It is part of a muscle group with the extensor carpi radialis brevis, which is located adjacent to it. The extensor carpi radialis longus is longer and thinner than its brevis counterpart.

The muscle originates along the lateral supracondylar ridge of the humerus. It inserts at the posterior base of the second metacarpal. Within the hand, this is the bone connected to the phalanges of the index finger. The radial nerve controls the muscle's movement. The radial artery and radial vein support the circulation of blood to this muscle group. One of the most common injuries associated with the extensor carpi radialis longus is known as lateral epicondylitis, or 'tennis elbow.' This painful condition occurs when tendons in the elbow have been overworked. This is often due to repetitive movements of the forearm or hand.