The Back of the Forearm
Muscles of the Posterior Compartment of the Forearm
There are 12 muscles on the back of the forearm which are divided into superficial and deep groups.
1) The superficial group
The superficial group consists of seven muscles which are arranged as follows from the lateral to the medial side, the:
a) Brachioradialis,
b) Extensor carpi radialis longus,
c) Extensor carpi radialis brevis,
d) Extensor digitorum,
e) Extensor digiti minimi,
f) Extensor carpi ulnaris, and
g) Anconeus.
2) The deep group
The deep group consist of five muscles which are arranged as follows from above downwards, the:
a) Supinator,
b) Abductor pollicis longus,
c) Extensor pollicis brevis,
d) Extensor pollicis longus, and
e) Extensor indicis.
Except for anconeus, brachioradialis and extensor carpi radialis longus, which are supplied by the radial nerve, all of the remaining muscles are supplied by the posterior interosseous nerve.
Muscles of the Posterior Compartment of the Forearm
There are 12 muscles on the back of the forearm which are divided into superficial and deep groups.
1) The superficial group
The superficial group consists of seven muscles which are arranged as follows from the lateral to the medial side, the:
a) Brachioradialis,
b) Extensor carpi radialis longus,
c) Extensor carpi radialis brevis,
d) Extensor digitorum,
e) Extensor digiti minimi,
f) Extensor carpi ulnaris, and
g) Anconeus.
2) The deep group
The deep group consist of five muscles which are arranged as follows from above downwards, the:
a) Supinator,
b) Abductor pollicis longus,
c) Extensor pollicis brevis,
d) Extensor pollicis longus, and
e) Extensor indicis.
Except for anconeus, brachioradialis and extensor carpi radialis longus, which are supplied by the radial nerve, all of the remaining muscles are supplied by the posterior interosseous nerve.
The superficial group of muscles
Brachioradialis
Origin
The upper two-thirds of the lateral supracondylar ridge with the adjacent lateral intermuscular septum.
Insertion
Inserted in the lateral surface of the distal part of the radius.
Nerve supply
Radial nerve.
Action
Flexor of the elbow in midprone forearm.
Figure1: Brachioradialis and extensor carpi radialis longus.
Extensor carpi radialis longus
Origin
Lower third of the lateral supracondylar ridge and the adjacent lateral intermuscular septum.
Insertion
Dorsum of the base of the second metacarpal.
Nerve supply
Radial nerve.
Action
Extension and abduction of the hand.
Extensor carpi radialis brevis
Origin
Common extensor tendon from the front of the lateral epicondyle of the humerus and adjacent intermuscular septum
Insertion
Dorsum of the base of the third metacarpal.
Nerve supply
Deep branch of radial nerve.
Actions
Extension and abduction of the hand at the wrist joint.
Figure2: Extensor carpi radialis brevis.
Extensor digitorum
Origin
Common extensor tendon from the front of the lateral epicondyle of the humerus and adjacent intermuscular septum.
Insertion
By four tendons into the dorsal surface of the bases of the middle and distal phalanges of the medial four fingers through the extensor expansion.
Nerve supply
Posterior interosseous nerve.
Actions
Extension of the medial four fingers and extension of the hand at the wrist joint.
Figure3: Extensor digitorum.
Extensor digiti minimi
Origin
Common extensor tendon from the front of the lateral epicondyle of the humerus and the adjacent part of the intermuscular septum.
Insertion
In the extensor expansion of the little finger.
Nerve supply
Posterior interosseous nerve.
Actions
Extension of the little finger.
Origin
Common extensor tendon from the front of the lateral epicondyle of the humerus and the adjacent part of the intermuscular septum.
Insertion
In the extensor expansion of the little finger.
Nerve supply
Posterior interosseous nerve.
Actions
Extension of the little finger.
Figure4: Extensor digiti minimi.
Extensor carpi ulnaris
Origin
By a common extensor tendon from the front of the lateral epicondyle and from the posterior border of the ulna.
Insertion
Medial side of the base of the fifth metacarpal.
Nerve supply
Posterior interosseous nerve.
Actions
Extension and adduction of the hand at the wrist joint.
Origin
By a common extensor tendon from the front of the lateral epicondyle and from the posterior border of the ulna.
Insertion
Medial side of the base of the fifth metacarpal.
Nerve supply
Posterior interosseous nerve.
Actions
Extension and adduction of the hand at the wrist joint.
Figure5: Extensor carpi ulnaris.
Figure6: Extensor carpi ulnaris.
Anconeus
Origin
From the lateral epicondyle of the humerus.
Insertion
In the posterolateral surface of the olecranon process and the upper third of the posterior surface of the ulna.
Nerve supply
Nerve to anconeus from the radial nerve, which reaches the muscle via a branch to the medial head of the triceps.
Action
Assists the triceps to extend the elbow and abduction of the ulna in pronation.
Figure7: Anconeus.
Figure8: Anconeus.
Deep group of muscles
Abductor pollicis longus
Origin
The upper part of the posterior surface of the ulna and radius and the related interosseous membrane.
Insertion
Lateral side of the base of the first metacarpal bone.
Nerve supply
Posterior interosseous nerve.
Actions
Abducts the thumb at the carpometacarpal joint and assists in the extention of the thumb.
Figure9: Abductor pollicis longus.
Extensor pollicis brevis
Origin
The posterior surface of the radius and the adjacent interosseous membrane.
Insertion
Dorsum of the base of the proximal phalanx of the thumb.
Nerve supply
Posterior interosseous nerve.
Action
Extends the thumb at the metacarpophalangeal joint and carpometacarpal joint.
Figure10:Extensor pollicis brevis.
Figure11: Extensor tendons ( posterior compartment of the forearm).
Extensor pollicis longus
Origin
From the posterior surface of the ulna and the adjacent interosseous membrane.
Insertion
Dorsal surface of the base of the distal phalanx of the thumb.
Nerve supply
Posterior interosseous nerve.
Action
Extends the thumb at interphalangeal, carpometacarpal and metacarpophalangeal joints.
Origin
From the posterior surface of the ulna and the adjacent interosseous membrane.
Insertion
Dorsal surface of the base of the distal phalanx of the thumb.
Nerve supply
Posterior interosseous nerve.
Action
Extends the thumb at interphalangeal, carpometacarpal and metacarpophalangeal joints.
Figure12: Extensor pollicis longus.
Extensor indicis
Origin
From posterior surface of the ulna.
Insertion
Extensor expansion of the index finger.
Nerve supply
Posterior interosseous nerve.
Action
Extends the index finger.
Figure13: Extensor indicis.
Figure14: Posterior compartment of the forearm.
Figure15: Extensor digitorum, Extensor pollicis longus and extensor indicis.
Supinator
Origin
It consists of superficial part which originates from the lateral epicondyle, radial collateral and the annular ligament of the elbow joint, and deep part which arise from the supinator crest of the ulna.
Insertion
Upper third of the lateral surface of the radius superior to the anterior oblique line.
Nerve supply
The posterior interosseous nerve
Action
Supinates the hand.
Origin
It consists of superficial part which originates from the lateral epicondyle, radial collateral and the annular ligament of the elbow joint, and deep part which arise from the supinator crest of the ulna.
Insertion
Upper third of the lateral surface of the radius superior to the anterior oblique line.
Nerve supply
The posterior interosseous nerve
Action
Supinates the hand.
Figure16: Supinator.
Figure17: Supinator and deep branch of radial nerve.
The posterior Interosseous Nerve
The posterior interosseous nerve is a branch of the radial nerve in the cubital fossa. It leaves the cubital fossa and reaches the back of the forearm by passing between the two heads of supinator. It winds around the lateral side of the radius then emerges from the posterior surface of supinator to the posterior compartment of the forearm where it descends between the superficial and the deep muscles of the back of the forearm. At the distal border of extensor pollicis brevis, it passes deep to extensor pollicis longus to run on the posterior surface of the interosseous membrane where it joins the anterior interosseous artery. Finally, it passes to the dorsal surface of the wrist joint.
Branches
1) Muscular:
Before piercing supinator it innervates extensor carpi radialis brevis and supinator. After piercing supinator it innervates extensor digitorum, extensor digiti minimi, extensor carpi ulnaris, abductor pollicis longus, extensor pollicis brevis, extensor pollicis longus and extensor indicis.
2) Articular:
The areticular branches to the wrist, inferior radioulnar and the carpal joints.
The posterior interosseous nerve is a branch of the radial nerve in the cubital fossa. It leaves the cubital fossa and reaches the back of the forearm by passing between the two heads of supinator. It winds around the lateral side of the radius then emerges from the posterior surface of supinator to the posterior compartment of the forearm where it descends between the superficial and the deep muscles of the back of the forearm. At the distal border of extensor pollicis brevis, it passes deep to extensor pollicis longus to run on the posterior surface of the interosseous membrane where it joins the anterior interosseous artery. Finally, it passes to the dorsal surface of the wrist joint.
Branches
1) Muscular:
Before piercing supinator it innervates extensor carpi radialis brevis and supinator. After piercing supinator it innervates extensor digitorum, extensor digiti minimi, extensor carpi ulnaris, abductor pollicis longus, extensor pollicis brevis, extensor pollicis longus and extensor indicis.
2) Articular:
The areticular branches to the wrist, inferior radioulnar and the carpal joints.
Figure18: Posterior interosseous nerve and artery.
Posterior Interosseous Artery
The posterior interosseous artery is the smaller of the two terminal branches of the common interosseous artery in the cubital fossa. It enters the back of the forearm by passing above the upper border of the interosseous membrane. In the posterior compartment of the forearm it passes between supinator and abductor pollicis longus and thereafter accompanies the posterior interosseous nerve between the superficial and deep muscles. In the distal part of the posterior compartment it ends by anastomosing with the anterior interosseous artery which reaches the posterior compartment by piercing the interosseous membrane above pronator quadratus.
Branches
1) Muscular:
2) Recurrent Interosseous artery:
It runs upwards to the back of the lateral epicondyle, where it anastomoses with the posterior descending branch of the profunda brachii artery.
