The extensor pollicis longus is one of the muscles of the deep layer of the posterior compartment of the forearm.

The posterior compartment of the forearm consists of a superficial layer (7 muscles) and a deep layer (5 muscles).

There are 3 "pollicis" muscles in the deep layer which act on the thumb: abductor pollicis longus, extensor pollicis longus and extensor pollicis brevis.

All the muscles of the posterior compartment are innervated by the radial nerve.

The muscles of the deep layer include:

Origin
  • Posterior surface of the ulna and adjacent interosseus membrane (distal to abductor pollicis longus)
Insertion
  • Dorsal surface of base of distal phalanx of the thumb

The extensor pollicis longus muscle emerges between the extensor digitorum and the extensor carpi radialis brevis muscles, but is separated from them due to its course around the dorsal tubercle (Lister’s tubercle) of the distal radius. The other two tendons run in parallel within a groove on the dorsalateral aspect of the distal radius.

The tendon forms the medial margin of the anatomical snuff box.

Innervation
  • Posterior interosseus nerve
Blood Supply
  • Superficial surface – branches from the posterior interosseus artery
  • Deep surface – branches from the anterior interosseus artery
Action
  • Extension of the thumb at all joints: interphalangeal, metacarpophalangeal, and carpometacarpal joints.