extensor

An extensor is a muscle or tendon that increases the angle of a joint or moves a part of the body away from the midline, functioning as the counterpart to flexors that bend joints. In humans, extensor muscles are mainly found in the posterior compartment of the forearm and act to straighten the wrist, fingers, and thumb.

The forearm contains several well-known extensor muscles. Major examples include the extensor carpi radialis longus and extensor carpi radialis brevis, extensor carpi ulnaris, extensor digitorum, extensor indicis, extensor digiti minimi, and the deep extensors such as the extensor pollicis longus and extensor pollicis brevis. These muscles are primarily innervated by the radial nerve or its deep branch, the posterior interosseous nerve, after passing through the supinator.

The extensor mechanism of the fingers involves tendons on the back of the hand that extend the finger joints, with the extensor digitorum playing a central role. Extensors also contribute to wrist extension in coordination with wrist flexors. Tendons can interact with the extensor hood or expansion system, enabling coordinated movement of the fingers.

Clinical relevance includes extensor tendon injuries, which can occur from cuts or sports-related trauma to the dorsum of the hand and may cause difficulties in extending the fingers. Lateral elbow pain associated with overuse of the forearm extensors is known as tennis elbow (lateral epicondylitis), reflecting pathology at the origin of these muscles. Extensor tendinopathies and related conditions are common in activities requiring repetitive wrist and finger extension.