Flexor Pollicis Longus Muscle Beneath the Skin, Anterior View

Description

Running along the volar (anterior) forearm, the flexor pollicis longus (FPL) belly occupies the deep flexor compartment, arising from the anterior surface of the radius and adjacent interosseous membrane. Medial to it lie the flexor digitorum profundus bellies, while the more superficial flexor digitorum superficialis overlies the deep plane proximally as the forearm approaches the elbow. Distally, the FPL tendon narrows and courses toward the wrist, passing deep to the flexor retinaculum in the carpal tunnel and continuing to the base of the distal phalanx of the thumb, where it flexes the interphalangeal joint. Skin is retained as a superficial envelope so the muscle’s length and trajectory can be appreciated in situ. An anterior presentation of FPL matters because it is the key motor for thumb pinch and is commonly implicated in anterior interosseous nerve palsy, where weakness of FPL (often with flexor digitorum profundus to the index finger) produces the classic inability to form a tight “OK” sign. It also clarifies why volar wrist lacerations and carpal tunnel pathology can compromise thumb flexion: the tendon shares the confined space beneath the flexor retinaculum with the median nerve and the digital flexor tendons. Small space, big consequences. Use this artwork for upper limb anatomy teaching (deep anterior forearm, carpal tunnel, and thumb biomechanics), for hand surgery and orthopaedic references discussing FPL tendon repair or AIN syndrome, and for rehabilitation materials focused on thumb flexion and pinch mechanics in male patients. Anatomical accuracy verified by SciePro's Medical Advisory Board.