The abductor 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:
It usual splits into 2 slips distally:
The abductor pollicis longus emerges distally between the extensor digitorum and the extensor carpi radialis brevis muscles to form its tendon which runs in a groove within the lateral side of the distal radius, accompanied by the tendon of the extensor pollicis brevis.