Flexor Pollicis Longus

Forearm · Upper Limb

Flexor Pollicis Longus belongs to the Forearm group of the Upper Limb. It arises from Anterior surface of radius and interosseous membrane and attaches to Base of distal phalanx of thumb. Functionally, it flexion of the phalanges of the thumb. Innervation is provided by the Anterior interosseous branch of 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)

OIANS stands for Origin, Insertion, Action and Nerve, the four facts that uniquely define every skeletal muscle. To learn the Flexor Pollicis Longus, work through them in order: picture its origin, trace the muscle to its insertion, reason out the action that shortening between those two points must create, then add the nerve that drives it.

Most students remember the Flexor Pollicis Longus fastest by linking its action back to its attachments rather than memorising each fact in isolation. Once the origin and insertion make sense, the action usually follows logically.