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- Posterior ulna to the thumb.
- The proximal attachment is onto the middle 1/3 of the posterior ulna and interosseus membrane.
- The distal attachment is via its dorsal digital expansion onto the base of the distal phalanx of the thumb.
- Extends the thumb at the carpometacarpal, metacarpophalangeal, and interphalangeal joints.
- Laterally rotates the thumb at the carpometacarpal joint.
- Extends and radially deviates the hand at the wrist joint.
- Supinates the forearm at the radioulnar joints.
- The belly of the extensor pollicis longus lies deep in the posterior forearm, but its distal tendon become superficial farther distally.
- The distal tendon of the extensor pollicis longus forms the dorsal digital expansion of the thumb.
- The distal tendon of the abductor pollicis longus forms the medial border of the anatomic snuffbox.