Posterior Compartment of the Forearm

The posterior compartment of the forearm is also called the extensor-supinator compartment. It contains muscles that primarily extend the wrist and fingers and supinate the forearm. These muscles are innervated by the radial nerve (deep branch or posterior interosseous nerve) and receive blood from the posterior interosseous artery.

Boundaries

Structure	Description
Anterior	Radius, ulna, interosseous membrane
Posterior	Antebrachial fascia and skin
Medial & Lateral	Intermuscular septa separating it from anterior compartment

Muscle Layers

It consists of a superficial layer and a deep layer.

Superficial Layer (7 Muscles)

Muscle	Origin	Insertion	Innervation	Function
Brachioradialis	Lateral supracondylar ridge	Styloid process of radius	Radial nerve	Flexes elbow (despite being in extensor group)
Extensor carpi radialis longus	Lateral supracondylar ridge	Base of 2nd metacarpal	Radial nerve	Wrist extension, radial deviation
Extensor carpi radialis brevis	Lateral epicondyle of humerus	Base of 3rd metacarpal	Deep branch of radial nerve	Wrist extension, radial deviation
Extensor digitorum	Lateral epicondyle of humerus	Extensor expansions of digits 2–5	Posterior interosseous nerve	Extension of fingers at MCP and IP joints
Extensor digiti minimi	Lateral epicondyle of humerus	Extensor expansion of digit 5	Posterior interosseous nerve	Extension of little finger
Extensor carpi ulnaris	Lateral epicondyle and ulna	Base of 5th metacarpal	Posterior interosseous nerve	Wrist extension and ulnar deviation
Anconeus	Lateral epicondyle	Lateral olecranon and posterior ulna	Radial nerve	Assists in elbow extension and stabilization

Deep Layer (5 Muscles)

Muscle	Origin	Insertion	Innervation	Function
Supinator	Lateral epicondyle, ulna	Proximal radius	Deep branch of radial nerve	Forearm supination
Abductor pollicis longus	Ulna, radius, interosseous membrane	Base of 1st metacarpal	Posterior interosseous nerve	Abduction of thumb at CMC joint
Extensor pollicis brevis	Radius and interosseous membrane	Base of proximal phalanx of thumb	Posterior interosseous nerve	Extension of thumb at MCP joint
Extensor pollicis longus	Ulna and interosseous membrane	Base of distal phalanx of thumb	Posterior interosseous nerve	Extension of thumb at IP joint
Extensor indicis	Ulna and interosseous membrane	Extensor expansion of index finger	Posterior interosseous nerve	Extension of index finger

Function

Wrist Extension: ECRL, ECRB, ECU
Finger Extension: Extensor digitorum, EDM, EI
Thumb Extension: EPL, EPB; Thumb Abduction: APL
Forearm Supination: Supinator

Surface Anatomy

The tendons of extensor muscles (especially EPL and APL) form the borders of the anatomical snuffbox. The extensor tendons become visible on the dorsum of the hand and wrist during finger extension.

Clinical Significance

Lateral Epicondylitis (Tennis Elbow): Overuse injury involving the common extensor origin
Posterior Interosseous Nerve Syndrome: Motor neuropathy without sensory loss; caused by entrapment in the supinator (arcade of Frohse)
Extensor Tendon Injuries: May result from trauma or lacerations, leading to loss of finger extension
Wrist Drop: Radial nerve palsy causing inability to extend wrist or fingers

Comparative Anatomy

In quadrupeds, extensors are stronger for weight-bearing. In humans, they have adapted for coordinated wrist and finger movements essential for activities like typing, writing, and playing instruments.

Published on May 12, 2025
Last updated on May 12, 2025