Muscles of the Posterior Compartment of the Forearm

Overview

The muscles in the posterior compartment of the forearm are commonly known as the extensor muscles, generally functioning to extend the wrist and fingers. They are all innervated by the radial nerve. The posterior compartment can be divided into a deep and superficial layer, separated by a layer of fascia.

Superficial Muscles

The superficial layer of the posterior forearm contains seven muscles. Four of these muscles – extensor carpi radialis brevis, extensor digitorum, extensor carpi ulnaris and extensor digiti minimi share a common tendinous origin at the lateral epicondyle.

Brachioradialis

The brachioradialis paradoxically flexes at the elbow although its origin and innervation are characteristic of an extensor muscle.

• Origin: Proximal aspect of the lateral supracondylar ridge of the humerus
• Insertion: Attaches to the distal end of the radius, just before the radial styloid process
• Actions: Flexes at the elbow
• Innervation: Radial nerve (C5 – T1)
• Arterial Supply: Recurrent interosseous artery, a branch of the common interosseous artery (arising from ulnar artery)
• Relations:
  • In the distal forearm, the radial artery and radial nerve course between the brachioradialis and the deep flexor muscles.

Extensor Carpi Radialis Longus and Brevis

The extensor carpi radialis muscles are situated on the lateral aspect of the posterior forearm. Due to their position, they are able to produce abduction as well as extension at the wrist.

• Origin:
  • Extensor carpi radialis longus – supracondylar ridge of the humerus
  • Extensor carpi radialis brevis – lateral epicondyle of the humerus
• Insertion: Metacarpal bones II and III.
• Actions: Extends and abducts the wrist.
• Innervation: Radial nerve (C5 – T1)

Extensor Digitorum (Communis)

The extensor digitorum is the main extensor of the fingers.

• Origin: Lateral epicondyle of the humerus
• Insertion: Splits into four flat tendons which inserts into the extensor hood of the medial four fingers
• Actions: Extends medial four fingers at the metacarpal and interphalangeal joints.
• Innervation: Deep branch of radial nerve (C5 – T1)

Extensor Digiti Minimi

• Origin: Lateral epicondyle of the humerus
• Insertion: Extensor digitorum tendon, into the extensor hood of the little finger.
• Actions: Extends the little finger, and contributes to extension at the wrist.
• Innervation: Deep branch of radial nerve (C5 – T1)

Extensor Carpi Ulnaris

The extensor carpi ulnaris is located on the medial aspect of the posterior forearm.

• Origin: Lateral epicondyle of the humerus
• Insertion: Base of metacarpal V
• Actions: Extension and adduction of wrist
• Innervation: Deep branch of radial nerve (C5 – T1)

Anconeus

The anconeus is situated medially and superiorly in the extensor compartment of the forearm, it’s fibres are blended with those of the triceps brachii.

• Origin: Lateral epicondyle of the humerus
• Insertion: Attaches to the posterior and lateral part of the olecranon.
• Actions: Extends and stablises the elbow joint. Abducts the ulna during pronation of the forearm.
• Innervation: Radial nerve

Deep Muscles

There are five muscles in the deep compartment of the posterior forearm which act on the thumb and index finger (except the supinator).

Supinator

• Origin:
  • One head: Lateral epicondyle of the humerus
  • The other head: Posterior surface of the ulna.
• Insertion: Posterior surface of the radius.
• Actions: Supinates the forearm.
• Innervation: Deep branch of radial nerve (C5 – T1)
• Relations: The supinator forms the floor of the cubital fossa.

Abductor Pollicis Longus

The abductor pollicis longus is situated immediately distal to the supinator muscle.

• Origin: Interosseous membrane and the adjacent posterior surfaces of the radius and ulna
• Insertion: Lateral side of the base of the first metacarpal
• Actions: Abducts the thumb
• Innervation: Radial nerve (posterior interosseous branch)
• Arterial Supply: Posterior interosseous artery, a branch of the common interosseous artery (arising from ulnar artery)
• Relations: In the hand, its tendon contributes to the lateral border of the anatomical snuffbox

Extensor Pollicis Brevis

The extensor pollicis brevis can be found medially and deep to the abductor pollicis longus.

• Origin: Interosseous membrane and the posterior surface of the radius
• Insertion: Base of the proximal phalanx of the thumb
• Actions: Extends at the metacarpophalangeal and carpometacarpal joints of the thumb.
• Innervation: Radial nerve (posterior interosseous branch)
• Arterial Supply: Posterior interosseous artery, a branch of the common interosseous artery (arising from ulnar artery)
• Relations: In the hand, its tendon contributes to the lateral border of the anatomical snuffbox

Extensor Pollicis Longus

The extensor pollicis longus muscle has a larger muscle belly than the extensor pollicis brevis.

• Origin: Interosseous membrane and the posterior surface of the radius
• Insertion: Distal phalanx of the thumb.
• Actions: Extends all joints of the thumb: carpometacarpal, metacarpophalangeal and interphalangeal.
• Innervation: Radial nerve (posterior interosseous branch)
• Arterial Supply: Posterior interosseous artery, a branch of the common interosseous artery (arising from ulnar artery)
• Relations: In the hand, its tendon forms the medial border of the anatomical snuffbox

Extensor Indicis Proprius

• Origin: Interosseous membrane and the posterior surface of the radius (distal to the extensor pollicis longus)
• Insertion: Extensor hood of the index finger
• Actions: Extends the index finger
• Innervation: Radial nerve (posterior interosseous branch)
• Arterial Supply: Posterior interosseous artery, a branch of the common interosseous artery (arising from ulnar artery)

Antomical Variants

• Fused extendor digitorum (communis) and extensor digitorum minimis