Forearm extensors

Function

Two groups on the back of the forearm

The forearm has a front (flexor) compartment and a back (extensor) compartment⁶. Together they hold twenty muscles that act on the elbow, the wrist and the joints of the hand⁶. The back compartment’s muscles mainly extend the wrist, fingers and thumb¹. Wrist extension means bending the wrist back. Gray’s Anatomy calls these muscles the direct opponents of the flexor muscles and of the muscles that turn the palm down².

Gray describes a superficial group and a deep group². Sources count the superficial group in different ways:

One anatomy reference counts seven superficial muscles, the seven in the table below⁷.
Another sets three apart as the “mobile wad”: the brachioradialis and the extensor carpi radialis longus and brevis, the two radial wrist extensors¹. It lists the other four as the superficial group¹.

Many of the wrist extensors share a starting point at or near the lateral epicondyle⁴. This is the outer bony bump at the elbow end of the upper arm bone.

Engraving of the back of the forearm and hand. The extensor carpi radialis brevis, extensor digitorum, extensor digiti minimi and extensor carpi ulnaris run down the forearm from the lateral epicondyle, the labelled bump on the outer side of the elbow, and become long tendons that cross the back of the wrist into the hand. Brachioradialis and extensor carpi radialis longus lie along the thumb side, with anconeus and the lower triceps at the elbow. — Superficial muscles on the back of the forearmHenry Vandyke Carter, in Henry Gray, Anatomy of the Human Body (1918), Fig. 418. Public domain, via Wikimedia Commons. (source, Public domain)

Muscle	Starts on	Ends on	What it does
Brachioradialis	The upper two-thirds of a ridge above the outer elbow, on the humerus (upper arm bone)²	The outer side of the radius (the forearm bone on the thumb side), at the base of its styloid process²	Bends the elbow and turns the forearm both palm-down and palm-up⁸
Extensor carpi radialis longus (ECRL)	The lower third of the same ridge²	The back of the base of the second metacarpal, one of the long bones of the hand²	Bends the wrist back and tilts the hand toward the thumb²
Extensor carpi radialis brevis (ECRB)	The lateral epicondyle, by a tendon shared with three other muscles⁹	The back of the base of the third metacarpal⁹	Bends the wrist back and may tilt the hand slightly toward the thumb²
Extensor digitorum	The lateral epicondyle, by the common extensor tendon²	The middle and end bones of the fingers, by four tendons²	In Gray’s account, straightens the fingers, acting mainly on the first finger bones²
Extensor digiti minimi	The common extensor tendon, by a thin slip²	The back of the first bone of the little finger, where it joins the extensor digitorum’s tendon sheet²	Straightens the little finger and helps bend the wrist back²
Extensor carpi ulnaris	The lateral epicondyle, by the common extensor tendon, and the back border of the ulna^2,7	The base of the fifth metacarpal^2,7	Bends the wrist back and tilts the hand toward the little finger^2,7
Anconeus	The back of the lateral epicondyle, by its own tendon²	The side of the olecranon (the bony point of the elbow) and the upper fourth of the back of the ulna²	Called a weak elbow extensor by the authors of a study of ten healthy volunteers¹⁰

The ulna is the forearm bone on the little-finger side. A styloid process is a slender, pointed part of a bone. The brachioradialis is counted as an extensor-compartment muscle although it works as a flexor⁸.

The deep group

The deep group has five muscles¹.

Engraving of the back of the forearm and hand with the superficial extensors removed. The supinator wraps round the upper radius below the lateral epicondyle, with anconeus beside the olecranon. Below it the abductor pollicis longus, extensor pollicis brevis and extensor pollicis longus slant down toward the thumb, and the extensor indicis runs to the index finger. The cut tendons of the extensor carpi radialis longus and brevis are labelled on the thumb side and the extensor carpi ulnaris tendon at the wrist. Flexor digitorum profundus and flexor carpi ulnaris show along the ulnar side. — Deep muscles on the back of the forearmHenry Vandyke Carter, in Henry Gray, Anatomy of the Human Body (1918), Fig. 419. Public domain, via Wikimedia Commons. (source, Public domain)

Muscle	Starts on	Ends on	What it does
Supinator	The lateral epicondyle, two ligaments of the elbow and a ridge on the ulna²	The upper radius, which its superficial fibres wrap round²	Helps the biceps turn the hand palm-up²
Abductor pollicis longus	The back of the shaft of the ulna and of the radius²	The base of the first metacarpal, on its thumb side²	In Gray’s account, mainly carries the thumb sideways away from the palm²
Extensor pollicis brevis	The back of the radius, and the interosseous membrane (a sheet of tough tissue joining two side-by-side bones)²	The base of the first bone of the thumb²	Straightens the first bone of the thumb²
Extensor pollicis longus	The middle third of the back of the ulna²	The base of the end bone of the thumb²	Straightens the end bone of the thumb²
Extensor indicis	The back of the ulna, and the interosseous membrane²	The extensor digitorum tendon to the index finger, which it joins²	Straightens the index finger and helps bend the wrist back²

How the tendons cross the wrist

A strong fibrous band, the extensor retinaculum, lies across the back of the wrist². Between the band and the bones are six compartments for tendons, each with its own lubricating sheath². Each compartment carries set tendons:

First: two thumb tendons, the abductor pollicis longus with the extensor pollicis brevis, covered on the first dorsal compartment page².
Second: the two radial wrist extensors².
Third: the extensor pollicis longus².
Fourth: the extensor digitorum and extensor indicis².
Fifth: the extensor digiti minimi².
Sixth: the extensor carpi ulnaris tendon².

Their nerve

A review states that the radial nerve supplies all the muscles of the back of the arm and forearm³. In Gray’s account, the nerve itself supplies the brachioradialis, the two radial wrist extensors and the anconeus². Its deep branch supplies the rest². The review instead places the ECRB on the deep branch³.

The deep branch lies between the two layers of the supinator². It emerges as the posterior interosseous nerve, which supplies the muscles of the back of the forearm¹¹.

What they do in a grip

The wrist extensors add to stability during gripping by holding the wrist slightly extended⁴. This lets the flexors produce their best force⁴. In 20 healthy subjects making a free maximal grip, the wrist settled at 35 degrees of extension¹². Grip strength was significantly lower in any wrist position moved away from the one they chose¹².

One study used surface electromyography (EMG), which measures a muscle’s activity by picking up its electrical signals¹³. It recorded three wrist flexors and three wrist extensors during combined grip and wrist forces of 15% to 60% of maximum¹³. Across the grip and wrist tasks, flexor activity changed significantly from task to task, while extensor activity stayed consistently high¹³.

Breakdown

How they tire in grip work

In nine women doing on-and-off gripping at 25% of maximum, EMG recorded three extensor muscles¹⁴. At every work-rest pattern tested, at least two of them showed significant signs of fatigue¹⁴. The two flexor muscles showed them at one pattern only: 30 seconds of work to 10 of rest¹⁴.

At the outer elbow

Lateral epicondylitis, pain where the forearm extensor tendons attach at the outer elbow, is commonly called tennis elbow¹⁵. A clinical reference describes it as an overuse injury of the common extensor tendon, particularly where the ECRB starts¹⁵. Studies of work disagree on what brings it on^16,17. A systematic review of five worker studies found no significant link with gripping, wrist extension or repeated movements at work¹⁶. In a Finnish sample aged 30 to 64, arm work both repetitive and forceful carried 5.6 times the odds, compared with doing neither¹⁷.

Where the thumb muscles cross the wrist extensors

The ECRL tendon runs under two thumb muscles, the extensor pollicis brevis with the abductor pollicis longus, before it reaches the wrist band². A 2017 review describes intersection syndrome as a rare sports overuse injury¹⁸. In the review’s account, it arises from friction where the first and second compartments cross¹⁸. The review lists rowing, canoeing, horseback riding, skiing, weight training and racket sports as sports in which it is reported, each with repeated wrist extension¹⁸.

The review reports an open debate about its nature¹⁸. One side holds it is a chronic tendon degeneration without inflammation, the other an inflammatory process of the tendon sheaths¹⁸.

A case series, a report on a group of patients with no comparison group, came from a national rowing squad¹⁹. About 5% of the squad had intersection syndrome over three years¹⁹. One study compared patients with intersection syndrome and patients with De Quervain’s tenosynovitis, painful swelling around two thumb tendons at the wrist²⁰. Compared with De Quervain’s tenosynovitis, intersection syndrome affected men and the dominant hand more often²⁰.

Pressure inside the forearm

Compartment syndrome is a rise in pressure inside a closed space of muscle wrapped by tough tissue and bone. In the chronic exertional form, pressure inside the forearm compartments rises during repeated activities²¹. It is rare in the general population but can be seen in motorcycle racers, climbers and rowers²².

When the nerve is hurt

A review describes “wrist drop” as the loss of wrist straightening after radial nerve injury³. It reads this as a sign of weakness of the two radial wrist extensors³.

A review describes “finger drop” when the posterior interosseous nerve is impaired³. The knuckle joints of the fingers and thumb do not straighten properly, while skin feeling stays normal³. Posterior interosseous nerve syndrome means trouble with that deep branch. A clinical reference states that wrist straightening is then expected to be kept, because the radial nerve itself supplies the ECRL¹¹.

Recovery

Growth with use

One MRI study measured muscle volume in 7 male tennis players of about 11, before puberty⁵. The forearm extensors of the playing arm were 25% larger than those of the other arm⁵. The side-to-side difference was significantly greater in the players than in 7 non-active controls⁵. The authors worded their conclusion as an association⁵. The funders, a Real Madrid and Madrid university research chair and the Spanish science ministry, had no role in it, the study states⁵.

What training studies found

In 13 healthy subjects, 8 weeks of isometric wrist extension training at 70% of maximum raised wrist extension strength²³. Isometric means the muscle works but the joint does not move. Grip strength on the trained side rose too²³.

A four-week trial randomly assigned experienced climbers to finger flexion, finger extension or alternating training on a hang board²⁴. A hang board is a board with small edges to grip. Only the extension-based training affected finger extension strength²⁴. One author declared a role in developing a training product, and the paper states that the firm took no part in the research²⁴.

As the nerve comes back

One study followed 14 arms of seven healthy men as a radial nerve block wore off²⁵. A nerve block is an injection that numbs a nerve for a short time. Grip strength was very strongly correlated with wrist extension strength²⁵. Making a fist stopped pulling the wrist into flexion (bending forward) once wrist extension strength was back to a mean of 51% of normal²⁵.

Connections

The forearm flexors

The extensors and the forearm flexors also work at the same time, which is called co-contraction¹³. In the surface EMG study of grip and wrist forces, co-contraction at the wrist was significantly higher when the wrist extensors were the opposing muscles¹³.

Sources differ on which muscles share the common extensor tendon^9,26. One account has the ECRB sharing it with three muscles: the extensor carpi ulnaris, the extensor digitorum and the extensor digiti minimi⁹. An MRI and anatomical study of 10 cadaver elbows found a shared origin for the ECRB, extensor digitorum and extensor digiti minimi²⁶. In that study, the tendon of the extensor carpi ulnaris came from the lower back part of the lateral epicondyle²⁶.

Exercises and stretches that load them

Related exercise pages: wrist extension and dead hang^27,28. Studies recorded the following:

Movement	What studies found
Wrist extension	In 40 healthy arms holding a wrist-extension effort at 75% of maximum, surface EMG showed the ECRB working harder with the forearm turned palm-down²⁷
Dead hang	In 11 male climbers hanging from a 20 mm edge until failure, the authors classed the finger extensor as a motor muscle²⁸

The wrist extensor stretch page covers stretching²⁹. A 2015 anatomical paper describes the stretch used in lateral epicondylitis as bending the wrist forward with the elbow straight and the palm down²⁹.