Deep extensors of the forearm

Author: Achudhan Karunaharamoorthy • Reviewer: Dimitrios Mytilinaios MD, PhD • Last reviewed: March 27, 2020

The deep extensors of the forearm consist of five muscles located at the posterior side of the forearm. These muscles are: the supinator muscle, abductor pollicis longus, extensor pollicis brevis, extensor pollicis longus and extensor indicis muscle.

They act on interphalangeal joints causing certain fingers to extend, but also act on the wrist joint causing its dorsal flexion. They are all innervated by the radial nerve, which is the branch of the brachial plexus. Their muscle bellies and tendons form the surface of the distal forearm and the wrist where they can be easily palpated.

This article will discuss the anatomy of the deep extensors of the forearm.

Key facts
Deep extensors of the forearm	Supinator Abductor pollicis longus Extensor pollicis brevis Extensor pollicis longus Extensor indicis
Innervation	All are innervated by the posterior interosseus nerve, a branch of the radial nerve.
Functions	Except for the supinator muscle that supinates the forearm, all the other muscles move the joints of the hand and fingers.

Contents

Anatomy and supply
Functions
Clinical note
Related diagrams and images

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Anatomy and supply

Extensor muscles of the forearm (diagram)

The following muscles are known to be the deep extensors.

Supinator muscle

The supinator muscle has a complex origin tendon arising from the lateral epicondyle of the humerus (superficial part) and the supinator crest of the proximal ulna (deep part). Furthermore, it also attaches to the radial collateral ligament and radial anular ligament.

From there, the muscle curls dorsally around the neck of the radius and inserts at the posterior, lateral and anterior sides of the ulna, between the radial tuberosity and the insertion of the pronator teres.

Key facts about the supinator muscle
Origins	Lateral epicondyle of humerus Radial collateral ligament Anular ligament Supinator crest of ulna
Insertions	Lateral, posterior, and anterior surfaces of proximal third of radius
Innervation	Posterior interosseous nerve (C6, C7)
Function	Forearm supination at the proximal radioulnar joint

Abductor pollicis longus muscle

This muscle originates at the posterior surface of the proximal half of the radius, ulna and the interosseous membrane. It inserts at the base of the metacarpal bone of the thumb. Often the insertion tendon splits into two and additionally attaches to the trapezium.

Key facts about the abductor pollicis longus muscle
Origins	Posterior surface of the proximal halves of the radius and ulna; interosseous membrane
Insertions	Base of the first metacarpal
Innervation	Posterior interosseous nerve (C6, C7)
Function	Thumb abduction and extension at the carpometacarpal joint; Abduction (radial deviation) at the wrist joint

Extensor pollicis longus muscle

The EPL arises at the posterior surface of the middle third of the ulna and the interosseous membrane and inserts at the base of the distal phalanx of the thumb. There, it forms the extensor expansion/dorsal aponeurosis covering the thumb medially, laterally and dorsally. This sheet-like tendinous expansion serves mainly as an insertion point for muscles and ligaments (similar to the dorsal aponeurosis of the extensor digitorum).

Key facts about the extensor pollicis longus muscle
Origins	Posterior surface of the middle third of the ulna; interosseous membrane
Insertions	Posterior surface of the base of the distal phalanx of the thumb
Innervation	Posterior interosseous nerve (C6, C7)
Function	Extension of distal phalanx of the thumb at the interphalangeal joint; extension of the carpometacarpal and carpophalangeal joints

Extensor pollicis brevis muscle

The EPB begins more distally at the posterior surface of the distal third of the radius and the interosseous membrane and courses to the posterior aspect of the proximal phalanx of the thumb.

Key facts about the extensor pollicis brevis muscle
Origins	Posterior surface of the distal third of the radius; interosseous membrane
Insertions	Posterior aspect of the base of the proximal phalanx of the thumb
Innervation	Posterior interosseous nerve (C6, C7)
Function	Extension of proximal phalanx of the thumb at the metacarpophalangeal joint; extension of the carpometacarpal joint

Extensor indicis muscle

The extensor indicis originates at the posterior surface of the distal third of the ulna and the interosseous membrane. It inserts at the extensor expansion/dorsal aponeurosis of the index finger.

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Key facts about the extensor indicis muscle
Origins	Posterior side of the distal third of the ulnar shaft; interosseous membrane
Insertions	Extensor expansion of the 2nd digit
Innervation	Posterior interosseous nerve (C6, C7)
Function	Extension of the 2nd digit at metacarpophalangeal and interphangeal joints

Innervation of the deep extensors

Like all extensors of the forearm, these five muscles are innervated by the radial nerve (C6-C8).

The radial nerve divides into a superficial branch and deep branch at the level of the radial head. While the superficial branch runs along the brachioradialis, the deep branch continues between the two layers of the supinator. Thereupon, it penetrates the supinator muscle and branches off as the posterior interosseous nerve (C7-C8), which is responsible for the innervation of almost all deep extensors. The supinator is the only deep extensor that receives supply directly from motor branches of the radial nerve.Learn more about the deep extensors of the forearm with our study material.

Extensors of the forearm Explore study unit

The anatomical 'snuffbox'

A triangular depression lies at the radial side of the dorsum of the hand, which becomes even more prominent during hand extension. It is commonly referred to as the anatomical snuff box (tabatière anatomique) or radial fossa. The depression is framed by the radius (proximal) and both the tendons of the extensor pollicis longus (dorsal), and of the extensor pollicis brevis (palmar); its base is formed by the trapezium and scaphoid bone. The radial artery courses through the anatomical snuff box and, thus, can be easily felt here.

Functions

The main function of the deep extensors is to move the joints of the hand and fingers (except for the supinator muscle).

The abductor pollicis longus pulls the thumb forward (abduction) at the 'saddle-like' caprometacarpal joint of the thumb. It also contributes to a lateral movement of the radius at the wrist joint (radial abduction).
The extensor pollicis longus extends the thumb at its caprometacarpal, metacarpophalangeal and interphalangeal joints. Its may also be involved in weak extension of the hand at the wrist joint.
The extensor pollicis brevis also extends the thumb but only at the caprometacarpal and metacarpophalangeal joints of the thumb.
The extensor indicis pulls the index finger straight by fulfilling extension at the metacarpophalangeal, proximal interphalangeal and distal interphalangeal joints. This also leads to a weak extension of the hand at the wrist joint.
In contrast, the function of the supinator is to pull the radius, causing a lateral rotation at the radioulnar joints (supination). This returns the hand to the anatomical position of palms facing forward.

Extensors of the forearm Explore study unit

Neurovasculature of the elbow and forearm Explore study unit

Clinical note

The narrow course of the deep branch of the radial nerve between the two layers of the supinator muscle carries a high risk of entrapment, this is a condition referred to as supinator syndrome.

A predilection site for this condition is a tendinous arch located at the superficial part of the supinator - arcade of Frohse, whereas rare causes include muscle hypertrophy or space-occupying lesions.

The guiding symptom is pain at the proximal forearm, which increases during turning movements and can radiate to the wrist. Since the deep branch of the radial nerve supplies the extensors of the forearm, the affected patients additionally exhibit a weakened ability to extend the hand, fingers and thumb.

Deep extensors of the forearm