Extensor pollicis brevis arises from the lower third of the extensor surface of the radius and adjacent part of the interosseous membrane. It passes distally over brachioradialis and extensor carpi radialis longus and brevis. At the wrist, it forms the radial side of the anatomical snuff box. It inserts into the posterior surface of the base of the proximal phalanx of the thumb.
Its action is to extend the metacarpophalangeal joint of the thumb. It is supplied by the posterior interosseous branch of the radial nerve (C7, C8).