Muscle · Forearm

Extensor pollicis longus

Musculus extensor pollicis longus

The muscle that straightens the thumb all the way to its tip. It arises from the back of the middle of the ulna and the sheet beside it, and its tendon takes a sharp turn on the way out: it hooks around a small ridge on the back of the radius, which acts as a pulley and swings the line of pull across the wrist toward the thumb. Beyond that corner it runs down the back border of the anatomical snuffbox — the opposite edge from abductor pollicis longus and extensor pollicis brevis — and inserts into the base of the thumb's last bone. Because its tendon changes direction around bone under load, it is the thumb tendon whose course, rather than whose muscle, does most of the work.