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Muscles of the Posterior or Back of Forearm

The muscles of the posterior of the forearm are categorized into two classes:

  • superficial
  • deep

The muscles that form the back of the forearm are commonly known as Extensor Muscles. These are provided by the radial nerve.

Superficial Muscles of the back of Forearm

The superficial muscles of the back of forearm are seven in number. From lateral to medial these are:

  • Brachioradialis.
  • Extensor carpi radialis longus (ECRL).
  • Extensor carpi radialis brevis (ECRB).
  • Extensor digitorum (ED).
  • Extensor digiti minimi (EDM).
  • Extensor carpi ulnaris (ECU).
  • Anconeus.

The superficial muscles of the back of the forearm are further categorized into two groups: lateral and posterior. Each group consists of three muscles:

Lateral group of superficial extensors

  • Brachioradialis.
  • Extensor carpi radialis longus.
  • Extensor carpi radialis brevis.

Posterior group of superficial extensors

  • Extensor digitorum.
  • Extensor digiti minimi.
  • Extensor carpi ulnaris.
  • Anconeus.

 

Tendon from the tip of lateral epicondyle of the humerus (known as common extensor origin) commonly give occurance to four of the superficial muscles (extensor carpi radialis brevis, extensor digitorum, extensor digiti minimi, and extensor carpi ulnaris). All the seven muscles cross the elbow joint.

Clinical Significance

Lateral epicondylitis is the overuse syndrome in the elbow. It’s an injury involving the extensor muscles of the forearm. There is degeneration of the attachment of the tendon, placing stress and weakening the site.

It’s also referred to as lateral epicondylitis but this is a misnomer because, in general evaluation of the tendons doesn’t show signs of inflammation, but rather degeneration and collagen disarray.

Tennis elbow is the most common reason that people see their doctors for elbow pain. It is most common at about any age, although it can come up in people of the age of 40, on an average.

Origin, insertion, nerve supply, and actions of the superficial muscles of the back of the forearm (superficial extensors)

MuscleOriginInsertionNerve supplyActions
Lateral group
BrachioradialisUpper two-third of the lateral supracondylar ridge of the humerusLateral surface of the distal end of radius just above the styloid processRadial nerve
  • Flexes the elbow joint.
  • Pronates the supinated forearm to midprone position
  • Supinates the pronated forearm to midprone position
Extensor carpi radialis longus (ECRL)Lower one-third of the lateral supracondylar ridge of the humerusLateral side of the dorsal surface of the base of second metacarpal boneRadial nerve
  • Acting with extensor carpi ulnaris extends the wrist
  • Acting with flexor carpi radialis abducts the wrist
Extensor carpi radialis brevis (ECRB)By a common tendon from the lateral epicondyle of the humerus and lateral ligament of the elbow jointLateral side of the dorsal surface of the base of third metacarpal bonePosterior interosseous nerve before piercing the supinator-do-
Posterior group
Extensor digitorumBy a common tendon from the lateral epicondyle
  • Gives rise to four tendons for medial four digits.
  • By the extensor expansion it is inserted into the dorsum of middle and terminal phalanges
Posterior interosseous nerveExtends the medial four digits. Can also extend the wrist
Extensor digiti minimiBy the common tendon from the lateral epicondyle
  • Lies medial to the extensor digitorum tendon for the little finger.
  • Through the extensor expansion, it is inserted into the dorsum of middle and terminal phalanges of little finger
Posterior interosseous nerve
  • Extends the little finger
  • Helps in the extension of the wrist
Extensor carpi ulnaris (ECU)By the common tendon from the lateral epicondyle and by an aponeurosis from the upper two-third of the posterior border of ulna along with flexor carpi ulnaris and flexor digitorum profundusInto a tubercle on the medial side of the dorsal surface of the base of the fifth metacarpalPosterior interosseous nerve
  • Acting with extensor carpi radialis it extends the wrist
  • Acting with flexor carpi ulnaris it adducts the wrist
AnconeusFrom the back of the lateral epicondyleLateral side of the olecranon process and upper one- fourth of the posterior surface of the ulnaNerve to anconeus, which arises from radial nerve in spiral groove and descends through medial head of the triceps brachiiWeak extensor of the elbow joint

Deep Muscles of the Back of Forearm

There are five deep muscles of the back of forearm, from above these are:
  • Supinator.
  • Abductor pollicis longus(APL).
  • Extensor pollicis brevis (EPB).
  • Extensor pollicis longus (EPL).
  • Extensor indicis.

The three deep extensors of the forearm, which function on thumb (abductor pollicis longus, extensor pollicis brevis, and extensor pollicis longus) are located deep to the superficial extensors and in order to acquire insertion on the three short long bones of thumb crop out’ (emerge) from the furrow in the lateral element of the forearm between lateral and posterior groups of superficial extensor. These three muscles are therefore called outcropping muscles.

 

 

None of the deep muscles of the back of forearm cross the elbow joint. All of them emerge from the radius, ulna, and interosseous membrane. All of them are delivered by the posterior interosseous nerve (deep branch of the radial nerve).

Clinical Significance

Wrist drop, also known as radial nerve palsy, is a condition where a person cannot extend their wrist and it hangs flaccidly. The individual may experience many symptoms including numbness in the wrist and forearm when there’s an injury to this nerve. The individual may find it hard to work with the hand in day to day tasks like objects. There may be weakness. The individual may find it tough to bend the wrist back.

Symptoms may include a burning or sharp pain sensations in fingers and the thumb. It is common to experience numbness, tingling, and trouble straightening your arm. You may realize that you straighten or cannot extend fingers and your wrist. This is called “wrist drop” or “finger drop,” and it does not happen in all circumstances.

Treatment

Initial management includes splinting of the wrist for support together with Osteopathic medicine, Physiotherapy and Occupational Therapy. In some instances, operative removal of bone spurs or other anatomical defects that may be impinging on the nerve may be warranted. If injury was caused by pressure from use of other mechanisms of injury or fitted crutches, then the symptoms of wrist fall will resolve within 8 – 12 weeks.

Origin, insertion, nerve supply, and actions of the deep muscles of the back of the forearm (deep extensors of forearm)

MuscleOriginInsertionNerve supplyAction
Supinator
  • Lateral epicondyle
  • Lateral ligament of the elbow joint
  • Annular ligament
  • Supinator crest of ulna and from the triangular area in front of it
Upper one-third of the posterior, lateral, and anterior surfaces of the radiusPosterior interosseous nerve before piercing the supinatorSupination of the forearm
Abductor pollicis longus (APL)
  • Lateral part of the posterior surface of ulna below the anconeus
  • Middle one-third of the posterior surface of radius (below the posterior oblique line) and intervening posterior surface of interosseous membrane
Lateral side of the base of first metacarpalPosterior interosseous nerveAbducts the thumb
Extensor pollicis brevis (EPB)From a small area on the posterior surface of radius below the origin of abductor pollicis longus and from adjoining interosseous membraneDorsal surface of the base of proximal phalanx of thumbPosterior interosseous nerveExtends the thumb at metacarpophalangeal joint and extends the carpometacarpal joint
Extensor pollicis longusFrom lateral part of middle one-third of the posterior surface of ulna and adjoining interosseous membraneDorsal surface of the base of distal phalanx of thumbPosterior interosseous nerve
  • Extends the joints of thumb
  • Helps in the extension of the wrist
Extensor indicisFrom the posterior surface of ulna below the origin of extensor pollicis longus and also from the adjoining interosseous membrane
  • The tendon lies medial to the extensor digitorum tendon for the index finger
  • Through the extensor expansion, it is inserted into the dorsum of middle and distal phalanges of the index finger
Posterior interosseous nerve
  • Extends the index finger
  • Helps in the extension of the wrist
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