Muscles of the Forearm

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The cubital fossa | Main Anatomy Index | The hand

Last updated 30 March 2006

Muscles of the Forearm

These muscles act on the elbow and wrist joints and on those of the digits.
In the proximal part of the forearm, the muscles form fleshy masses inferior to the medial and lateral epicondyles of the humerus.
The tendons of these muscles pass through the distal part of the forearm and continue into the hand.

These muscles can be divided into flexor-pronator and extensor-supinator groups.
The flexor-pronator group arises by a common flexor tendon from the medial epicondyle of the humerus; this is referred to as the common flexor attachment or origin.
The extensor supinator group arises by a common extensor tendon from the lateral epicondyle of the humerus; this is referred to as the common extensor attachment or origin.

Front of Forearm

The anterior forearm muscles are divided into 3 muscular layers; a deep layer, intermediate layer and superficial layer.
A septum of deep fascia separates the deep layer of flexor muscles from the superficial and intermediate layers.

Most of the flexor tendons are held in place by the flexor retinaculum, a thickening of the deep fascia of the forearm.
This retinaculum prevents bow stringing of the tendons when the flexor muscles contract and also help improve the effective of the muscles by changing the direction of force of the tendons.

Deepest Layer

The Supinator Muscle (p. 559)

This muscle lies deep in the cubital fossa and, along with the brachialis, forms its floor.
The humeral and ulnar heads of attachment of the supinator muscle envelop the neck and proximal part of the body of the radius.

Proximal attachments are: lateral epicondyle of humerus, radial collateral and anular ligaments, supinator fossa and crest of ulna.
Distal attachments are: lateral, posterior, and anterior surfaces of proximal third of radius.
Innervation: deep branch of radial nerve.

The supinator muscle supinates the forearm by rotating the radius.
It is the prime mover in supination of the forearm and hand.
The biceps brachii assists the supinator in rapid and forceful supination, particularly when resistance is required and the forearm is flexed.

The Pronator Quadratus (p. 571)

This small muscle is quadrangular and pronates the forearm.
It cannot be palpated or observed, except in dissections, because it is the deepest muscle in the anterior aspect of the forearm.
It "clothes" the distal fourth of the radius and ulna and the interosseous membrane between them.
The pronator quadratus is the only muscle that is attached proximally only to the ulna and is attached distally on the radius.

Proximal attachments are: distal fourth of anterior surface of ulna.
Distal attachments are: distal fourth of anterior surface of radius.
Innervation: anterior interosseous nerve from median (C8 and T1).

It pronates the forearm at the superior and inferior radioulnar joints and is the prime mover in pronation of the forearm.
The pronator quadratus initiates pronation and is assisted by the pronator teres when more speed and power are needed.
The pronator quadratus also helps the interosseous membrane to hold the radius and ulna together, particularly when upward thrusts are transmitted through the wrist.

Deep Flexors

The Flexor Digitorum Profundus Muscle (p. 566)

This is a long, thick (L. profundus, deep) muscle.
It is the only one that can flex the distal interphalangeal joints of the digits (i.e., it flexes all these joints).
It has an extensive proximal attachment to the ulna and interosseous membrane, and "clothes" the anterior aspect of the ulna.

Proximal attachments are: proximal 3/4 of medial and anterior surfaces of the ulna and interosseous membrane.
Distal attachments are: bases of the distal phalanges of medial four digits.
Innervation: medial part-ulnar nerve (C8 and T1), lateral part-median nerve (C8 and T1).

The flexor digitorum profundus flexes the 2^nd to 5^th digits after the flexor digitorum superficialis muscle has flexed the middle phalanges.

The flexor digitorum profundus divides into 4 parts that ends as four tendons.
They pass posterior to the tendons of the flexor digitorum superficialis and the flexor reticulum.

The Flexor Pollicis Longus Muscle (pp. 566-71)

This is the long flexor of the thumb (L. pollex).
It lies lateral to the flexor digitorum profundus, where it "clothes" the anterior aspect of the radius.
Its flat tendon passes deep to the flexor retinaculum, enveloped in its own synovial sheath on the lateral side of the common flexor synovial sheath.

Proximal attachments are: anterior surface of the radius and adjacent interosseous membrane.
Distal attachments are: base of the distal phalanx of the thumb.
Innervation: anterior interosseous nerve from the median nerve (C8 and T1).

It flexes the distal phalanx of the thumb and, secondarily, the proximal phalanx and the first metacarpal bone.
The flexor pollicis longus is the only muscle that flexes the interphalangeal joints of the thumb.

It also flexes the metacarpophalangeal and carpometacarpal joints of the thumb, and it may assist with the flexion of the wrist.

Intermediate Layer of Flexors

The Flexor Digitorum Superficialis Muscle (p. 566)

This is the superficial muscle in the forearm and it forms an intermediate layer between the superficial and deep groups of forearm muscles.
It has two heads.
The median nerve and ulnar artery passes deep to the heads of this muscle.

Near the wrist, the flexor digitorum superficialis gives rise to four tendons, which pass deep to the flexor retinaculum.
The tendons are surrounded by a common flexor synovial sheath.
The superficial pair of tendons passes to the 3^rd and 4^th digits.

Proximal attachments are: humeroulnar head-medial epicondyle of humerus, ulnar collateral ligament, and coronoid process of the ulna, radial head-superior half of anterior border of radius.
Distal attachments are: bodies of the middle phalanges of the medial four digits.
Innervation: median nerve (C7, C8, and T1).

This muscle flexes the middle phalanges of the medial four digits by flexing the proximal interphalangeal joints.
It also flexes the proximal phalanges (i.e., metacarpophalangeal joints) and the wrist joint.

The Superficial Layer of Flexors

The Pronator Teres Muscle (p. 565)

This is an elongated and narrow muscle and is a pronator of the forearm and a flexor of the elbow joint.
It forms the medial boundary of the cubital fossa and has two heads of proximal attachment.

Proximal attachments are: medial epicondyle of humerus and coronoid process of ulna.
Distal attachments are: middle of lateral surface of radius.
Innervation: median nerve (C6 and C7).

The pronator teres pronates the forearm and flexes it.
It also assists the pronator quadratus during pronation and is a weak flexor of the forearm.

Flexor Carpi Radialis Muscle (p. 565)

This is a long, fusiform muscle that is located medial to the pronator teres.
In the middle of the forearm its fleshy belly is replaced by a long, flattened tendon that becomes cord-like as it approaches the wrist, where it is readily palpable.

Proximal attachments are: medial epicondyle of the humerus.
Distal attachments are: base of 2^nd and 3^rd metacarpal bone.
Innervation: median nerve (C6 and C7).

To reach its insertion, its long tendon passes through a canal in the lateral part of the flexor retinaculum, and through the vertical groove in the trapezium.

The flexor carpi radialis flexes the hand and abducts it.

The Palmaris Longus Muscle (p. 565)

This is a small fusiform muscle that is absent on one or both sides (usually the left) in about 14% of people, but its actions are not missed.
It may have two bellies and two tendons.
When present, the palmaris longus tendon is readily visible and palpable.
Its long, thin tendon passes superficial to the flexor retinaculum.

Proximal attachments are: medial epicondyle of humerus.
Distal attachments are: distal half of flexor retinaculum and the palmar aponeurosis.
Innervation: median nerve (C7 and C8).

The palmaris longus flexes the hand and tightens the palmar aponeurosis.

Flexor Carpi Ulnaris Muscle (pp. 565-6)

This is the most medial of the superficial flexor muscles in the forearm.
It has 2 heads of proximal attachment, between which the ulnar nerve passes distally in the forearm.

Proximal attachments are: humeral head-medial epicondyle of humerus, ulnar head-olecranon and posterior border of ulna.
Distal attachments are: pisiform bone, hook of hamate bone and the 5^th metacarpal bone.
Innervation: ulnar nerve (C7 and C8).

The flexor carpi ulnaris flexes and adducts the hand.
It also fixes the pisiform bone when the hypothenar muscles act.

The Lateral Group

The Brachioradialis Muscle (pp. 559, 560, 582)

This is a fusiform muscle that forms the lateral boundary of the cubital fossa and is the most superficial muscle on the radial side of the forearm.

Proximal attachments are: proximal two-thirds of lateral supracondylar ridge of humerus.
Distal attachments are: lateral surface of distal of end of radius.
Innervation: radial n. (C5, C6, C7).

The brachioradialis flexes the forearm, especially when quick movement is required or when a weight is lifted during slow flexion of the forearm.
It is used to give power and speed and acts to its best advantage when the forearm is in the midprone position. It is therefore capable of initiating both pronation and supination.

The Extensor Carpi Radialis Longus Muscle (p. 582)

This is a fusiform muscle that is partly overlapped by the brachioradialis with which it is often blended.

Proximal attachments are: lateral supracondylar ridge of humerus.
Distal attachments are: base of 2^nd metacarpal bone.
Innervation: radial nerve (C6 and C7).

It extends and abducts the hand at the wrist joint.

The Extensor Carpi Radialis Brevis Muscle (p. 582)

This is a fusiform muscle and it is shorter than the extensor carpi radialis longus, which covers it.

Proximal attachments are: lateral epicondyle of humerus.
Distal attachments are: base of 3^rd metacarpal bone.
Innervation: deep branch of radial nerve (C7 and C8).

It extends and abducts the hand at the wrist joint.

The Extensor Muscles of the Forearm

For the purposes of description, the muscles of the posterior of the forearm are divided into the superficial and deep groups.

Four muscles in the superficial group of extensor muscles (extensor carpi radialis brevis, extensor digitorum, extensor digiti minimi, and extensor carpi ulnaris) are attached by a flattened common extensor tendon.
It is attached to the lateral epicondyle of the humerus, the adjacent fascia, and the lateral supracondylar ridge.

The extensor tendons are held in place by a strong fibrous band, called the extensor retinaculum.
This retinaculum prevents bow stringing of the long extensor tendons when the hand is hyper-extended at the wrist joint.

Superficial Extensor Muscles

The Extensor Digitorum Muscles (p. 582)

This is the principal extensor of the medial four digits and occupies much of the posterior surface of the forearm.
It divides into four tendons proximal to the wrist, which passes though a common synovial sheath, deep to the extensor retinaculum.

Proximal attachments are: lateral epicondyle of the humerus.
Distal attachments are: extensor expansions of the medial four digits.
Innervation: posterior interosseous nerve (C7 and C8), a branch of the radial nerve.

It extends the proximal phalanges and through its collateral reinforcements, the middle and distal phalanges as well.
It also helps to extend the hand at the wrist joint after exerting its traction primarily on the digits.

The Extensor Digiti Minimi Muscle (p. 582)

This slip of muscle is a partially detached part of the extensor digitorum.
Its tendon runs through a separate compartment in the extensor retinaculum and then divides into 2 slips; the lateral one is joined to the tendon of the extensor digitorum.

Proximal attachments are: lateral epicondyle of humerus.
Distal attachments are: extensor expansion of the 5^th digit.
Innervation: posterior interosseous nerve (C7 and C8), a branch of the radial nerve.

The extensor digiti minimi extends the proximal phalanx of the 5^th digit at the metacarpophalangeal joint and assists in the extension of the interphalangeal joints.
It also assists with extension of the hand after exerting its traction primarily on the 5^th digit.

The Extensor Carpi Ulnaris Muscle (p. 582)

This is a long fusiform muscle, located on the medial border of the forearm.
It has 2 heads.
Its tendon runs in a groove between the head and styloid process of the ulna, within a special compartment of the extensor retinaculum.

Proximal attachments are: lateral epicondyle of humerus and posterior border of the ulna.
Distal attachments are: base of 5^th metacarpal bone.
Innervation: posterior interosseous nerve (C7 and C8), a branch of the radial nerve.

The extensor carpi ulnaris extends and adducts the hand at the wrist joint.
Acting with the extensor carpi radialis, it extends the hand.
Acting with the flexor carpi ulnaris, it adducts the hand.

The Deep Extensor Muscles

The deep extensors of the forearm consist of 3 muscles that act on the 1^st digit or thumb (abductor pollicis longus, extensor pollicis brevis, and extensor pollicis longus) and the extensor indicis.

The Abductor Pollicis Longus Muscle (p. 588)

This is a long fusiform abductor of the thumb (L. pollex).
It lies just distal to the supinator muscle and is closely related to the extensor pollicis brevis muscle.

Proximal attachments are: posterior surfaces of the ulna and radius, and the interosseous membrane.
Distal attachments are: base of the 1^st metacarpal bone.
Innervation: posterior interosseous nerve (C7 and C8).

It abducts and extends the thumb at the carpometacarpal joint.
This muscle acts with the abductor pollicis brevis during abduction of the thumb and with the extensor pollicis brevis during extension of this digit.

Its tendon passes deep to the extensor retinaculum in a common synovial sheath with the tendon of the extensor pollicis brevis.

The Extensor Pollicis Brevis Muscle (p. 588)

This is a short fusiform extensor of the thumb that lies distal to the long abductor (abductor pollicis longus), and is partly covered by it.

Proximal attachments are: posterior surface of radius and interosseous membrane.
Distal attachments are: base of proximal phalanx of thumb.
Innervation: posterior interosseous nerve (C7 and C8).

It extends the proximal phalanx of the thumb at the metacarpophalangeal joint.
It also helps to extend the metacarpal bone of the thumb and to extend and abduct the hand.

The Extensor Pollicis Longus Muscle (p. 588)

This is the long extensor of the thumb and it is larger, and its tendon longer than that of extensor pollicis brevis.

Proximal attachments are: posterior surface of middle 1/3 of ulna and interosseous membrane.
Distal attachments are: base of distal phalanx of thumb.
Innervation: posterior interosseous nerve (C7 and C8).

It extends the distal phalanx of the thumb, and in continued action, it extends the metacarpophalangeal and interphalangeal joints of the thumb.
It also adducts the extended thumb and rotates it laterally.
It can also abduct the hand.

The Extensor Indicis Muscle (p.588)

This is a narrow, elongated muscle that lies medial to and alongside the extensor pollicis longus.

Proximal attachments are: posterior surface of ulna and interosseous membrane.
Distal attachments are: extensor expansion of 2^nd digit (index finger).
Innervation: posterior interosseous nerve (C7 and C8).

Action with the extensor digitorum, it extends the index finger at the proximal interphalangeal joint.
This puts the index finger into the "pointing position".
The extensor indicis also assists in extension of the hand.