What is the Flexor Pollicis Longus?

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  • Written By: Alex Paul
  • Edited By: Jacob Harkins
  • Last Modified Date: 24 September 2019
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The flexor pollicis longus is a long muscle located in the forearm. It is most commonly used to flex and control the thumb and is only found in humans — no other primates have the muscle. Its secondary action is to help flex the wrist when the thumb is in a flexed state. The name comes from Latin and literally means “long thumb bender." There are a number of injuries the muscle can suffer including tendinitis.

The origin of the flexor pollicis longus is on the radius. From there it extends down the arm and eventually becomes a flat tendon. This passes through the carpal tunnel and inserts onto part of the thumb. The innervation of the muscle is via a nerve known as the anterior interosseous and the blood supply is from the anterior interosseous artery.

Visually, the muscle is located on the inner side of the forearm. Although the muscle is present in nearly all humans there are some variations in its presentation. For example, the muscle may attach to an additional tendon in some cases. There may also be slips that connect with other muscles such as the pronator teres.


The action of the muscle is to flex the thumb at two different joints. These are known as the primary and weak joints of the muscle. The primary is the interphalangeal joint and this is where the muscle is most powerful. It also helps to flex the thumb at the metacarpophalngeal joint although without the same force as the primary joint.

The most common daily use of the flexor pollicis longus is to grip an object with the hand such as a glass or can. Any movement that involves flexing the thumb, however, will involve using the muscle. For this reason an injury to the flexor pollicis longus can be extremely frustrating and adversely change a person’s quality of life until it heals.

Stretching the flexor pollicis longus using a wrist flexion stretch is important in order to increase muscle flexibility and reduce the chance of injury. To perform a wrist flexion stretch the stretcher should stand in front of a wall with arm straight out in front and palm upwards. Slowly the stretcher should flex the wrist downwards so that the palm is facing towards the wall. This should induce a stretch in the wrist flexor muscles. As with all stretches this should be held for around 30 seconds before being repeated several times.


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