Abductor pollicis longus abducts and extends the thumb at the carpometacarpal joint.
The abductor pollicis longus (APL) is a deep muscle in the posterior compartment of the forearm. It is primarily responsible for abduction of the thumb at the carpometacarpal joint and plays a key role in thumb positioning for gripping and precision movements. It often shares a close anatomical relationship with the extensor pollicis brevis, with which it passes through the first dorsal compartment of the wrist.
The APL is located in the deep layer of the posterior forearm, lying lateral to the extensor pollicis longus and often partially overlapping the extensor pollicis brevis. Its tendon travels obliquely from the forearm to the base of the first metacarpal, forming part of the lateral border of the anatomical snuffbox.
The muscle has a long, fleshy belly that gives rise to a strong tendon. It frequently divides into multiple slips before inserting.
The abductor pollicis longus contributes to:
Innervated by the posterior interosseous nerve (C7–C8), a branch of the radial nerve.
The APL lies deep to the extensor digitorum and superficial to the radius. Its tendon is lateral to that of the extensor pollicis brevis at the wrist, and both together define the lateral boundary of the anatomical snuffbox. The radial artery passes deep within this space.
The APL tendon becomes visible on the dorsolateral wrist during thumb abduction or resistance. It contributes to the prominence on the lateral side of the anatomical snuffbox.
The APL arises from the dorsal muscle mass of the forearm during embryonic development. Like other extensors and abductors of the thumb, it is innervated by the radial nerve derivatives.
In many quadrupeds, the APL functions as an abductor of the first digit or pollex but lacks the refined role seen in humans. In humans, it has evolved to support the high degree of thumb mobility necessary for opposable grip and tool use.