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Flexor pollicis longus muscle
Front of the left forearm. Deep muscles. (Flexor pollicis longus is shown in blue)
Latin musculus flexor pollicis longus
Gray's subject #125 449
Origin The middle 2/4 of the volar surface of the radius and the adjacent interosseus membrane. (Also occasionally a small origin slightly on the medial epicondyle of the ulna.)
Insertion    The base of the distal phalanx of the thumb
Artery Anterior interosseus artery
Nerve Anterior interosseous nerve (branch of median nerve) (C8, T1)
Actions Flexion of the thumb.
Antagonist Extensor pollicis longus muscle, Extensor pollicis brevis muscle

The flexor pollicis longus (FPL, Latin flexor, bender; pollicis, thumb; longus, long) is a muscle in the forearm and hand that flexes the thumb. It lies in the same plane as the flexor digitorum profundus.

This muscle is unique to humans, being "either rudimentary or absent" in other primates.[1]


Human anatomy


Origin and insertion

It arises from the grooved anterior (side of palm) surface of the body of the radius, extending from immediately below the radial tuberosity and oblique line to within a short distance of the pronator quadratus muscle.[2]

It arises also from the adjacent part of the interosseous membrane of the forearm, and generally by a fleshy slip from the medial border of the coronoid process of the ulna.[2] In 40 per cent of cases, it is also inserted from the medial epicondyle of the humerus, and in those cases a tendinous connection with the humeral head of the flexor digitorum superficialis is present.[3]

The fibers end in a flattened tendon, which passes beneath the flexor retinaculum of the hand through the carpal tunnel. It is then lodged between the lateral head of the flexor pollicis brevis and the oblique part of the adductor pollicis, and, entering an osseoaponeurotic canal similar to those for the flexor tendons of the fingers, is inserted into the base of the distal phalanx of the thumb.[2]


The anterior interosseous nerve (a branch of the median nerve) and the anterior interosseous artery and vein pass downward on the front of the interosseous membrane between the flexor pollicis longus and flexor digitorum profundus.[2]

Injuries to tendons are particularly difficult to recover from due to the limited blood supply they receive.


The flexor pollicis longus is a flexor of the phalanges of the thumb; when the thumb is fixed, it assists in flexing the wrist.[2]


Slips may connect with flexor digitorum superficialis, flexor digitorum profundus, or the pronator teres. An additional tendon to the index finger is sometimes found.[2]

Evolutionary variation

Modern humans are unique among hominids in having a flexor pollicis longus (FPL) muscle belly that is separate from that of the flexor digitorum profundus (FDP). While the FPL is not a separate muscle belly in extant great apes, a distinct tendon from the FDP belly might be present. In some individuals this tendon tend to act more like a ligament which restricts extension of the interphalangeal joint of the thumb. In orangutans there is a tendon similar in insertion and function to the FPL in humans, but which has an intrinsic origin on the oblique head of the adductor pollicis. [4]

Lesser apes (i.e. gibbons) and Old World monkies (e.g. baboons) share an extrinsic FPL muscle tendon with humans. In most lesser apes the FPL belly is separate from the FDP belly, but in baboons the FPL tendon bifurcates from the FDP tendon at the wrist within the carpal tunnel and, because of the lack of differentiation in both the FDP and FPL musculature, it is unlikely that baboons can control individual digits independently. [4]

Additional images

Bones of left forearm. Anterior aspect.  
Bones of the left hand. Volar surface.  
Cross-section through the middle of the forearm.  
Transverse section across distal ends of radius and ulna.  
Transverse section across the wrist and digits.  
The mucous sheaths of the tendons on the front of the wrist and digits.  
The muscles of the left hand. Palmar surface.  
Ulnar and radial arteries. Deep view.  
Nerves of the left upper extremity.  


  1. ^ Straus, W. L. (1942). Rudimentary digits in primates. Q. Rev. Biol. 17, 228–243.
  2. ^ a b c d e f Gray 1918, Flexor Pollicis Longus, paras 20, 25
  3. ^ Platzer 2004, p 162
  4. ^ a b Tocheri et al. 2008, pp 556-7


This article was originally based on an entry from a public domain edition of Gray's Anatomy. As such, some of the information contained within it may be outdated.

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