The flexor pollicis longus (FPL) is a long muscle located at the deep layer with flexor digitorum profundus and pronator quadratus in the anterior compartment of the forearm. Though it is situated at the forearm, it is classified as part of the extrinsic muscles of the hand as it's function is seen in thumb movement.

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The [[anterior interosseous nerve]] is a purely motor branch of the [[median nerve]] that arises in the upper forearm. Anterior interosseous neuropathy (AIN) produces no sensory loss and weakness of the 3 muscles supplied by the nerve:

1. [[flexor digitorum profundus]] (FDP) I & II: flexion of distal phalanx of digits 2 & 3

2. [[flexor pollicis longus]] (FPL): flexion of distal phalanx of thumb

3. [[pronator quadratus muscle]] (in the distal forearm): difficult to isolate clinically