Flexor Pollicis Longus
Forearm · Upper Limb
The flexor pollicis longus is the deep flexor dedicated to the thumb. It arises from the anterior surface of the radius and the adjacent interosseous membrane and runs distally to insert on the base of the distal phalanx of the thumb. It flexes the phalanges of the thumb, bending it across the palm, and is the only muscle able to flex the thumb's interphalangeal joint. It is supplied by the anterior interosseous branch of the median nerve (C8, T1).
Origin, Insertion, Action & Nerve
Origin
Anterior surface of radius and interosseous membrane
Insertion
Base of distal phalanx of thumb
Action
Flexion of the phalanges of the thumb
Nerve
Anterior interosseous branch of median nerve (C8, T1)
Attachments explained
The Flexor Pollicis Longus is defined first by where it attaches. Its origin is the more fixed anchor, usually the proximal or more stable end that stays put during contraction, while its insertion is the more mobile point that is pulled toward the origin when the muscle shortens.
Origin
Anterior surface of radius and interosseous membrane
Insertion
Base of distal phalanx of thumb
Action & function
When the Flexor Pollicis Longus contracts, it produces the following movement: Flexion of the phalanges of the thumb.
As part of the Forearm group of the Upper Limb, it works alongside neighbouring muscles to generate smooth, coordinated movement and to stabilise the structures it acts on.
Nerve supply & clinical relevance
The Flexor Pollicis Longus receives its nerve supply from the Anterior interosseous branch of median nerve (C8, T1). This nerve carries fibres from spinal segment(s) C8, T1.
Because a muscle can only contract when its nerve is intact, injury to the Anterior interosseous branch of median nerve or to its spinal roots (C8, T1) can weaken or paralyse the Flexor Pollicis Longus, impairing the movements it normally produces (flexion of the phalanges of the thumb). This is why knowing the innervation is central to localising nerve lesions in clinical practice.
How to study the Flexor Pollicis Longus (the OIANS method)
Learn flexor pollicis longus as the thumb's counterpart to flexor digitorum profundus: it is the sole muscle that flexes the thumb's interphalangeal joint, just as the profundus is the sole flexor of the finger DIP joints. Its origin on the anterior radius and insertion on the distal phalanx of the thumb give it that reach.
Group its nerve with the deep flexors under the anterior interosseous branch of the median nerve. A neat clinical test of that nerve is the OK sign: if the thumb cannot bend at its tip to meet the index finger, flexor pollicis longus and its nerve are implicated.
Frequently asked questions
Where is the Flexor Pollicis Longus located?
The Flexor Pollicis Longus is a muscle of the Forearm group, located in the Upper Limb.
What is the origin of the Flexor Pollicis Longus?
Anterior surface of radius and interosseous membrane
What is the insertion of the Flexor Pollicis Longus?
Base of distal phalanx of thumb
What movements does the Flexor Pollicis Longus produce?
Flexion of the phalanges of the thumb
What nerve supplies the Flexor Pollicis Longus?
Anterior interosseous branch of median nerve (C8, T1)
Is the Flexor Pollicis Longus free to study in OIANS?
Yes. The Flexor Pollicis Longus is free in OIANS: its full origin, insertion, action and nerve details are open to everyone in the Muscle Directory, and because it belongs to the Upper Limb it is also covered by the free Quiz and Flashcard decks. The other regions, the "All Muscles" deck, and the Progress and Explore tools are unlocked with a one-time Lifetime purchase.
Related muscles
Study the Flexor Pollicis Longus
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