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The superficial extensors of the forearm

The superficial extensors of the forearm are group of three muscles located at the posterior side of the forearm. Due to their superficial course, they form the surface of the lateral side of the posterior forearm where they can be easily palpated, especially during hand extension.

They all share a common extensor tendon arising from the lateral epicondyle of the humerus together with the extensor carpi radialis brevis from the radial musculature, and all of them insert at the specific points at the dorsum of the hand.

Key Facts
Extensor digitorum Origin - lateral epicondyle of the humerus
Insertion - posterior surfaces of the middle and distal phalanges (2nd-5th)
Innervation - posterior interosseus nerve (branch of the radial nerve)
Action - extension of the index, middle, ring and little fingers
Extensor digiti minimi Origin - lateral epicondyle of the humerus
Insertion - dorsal aponeurosis of the little finger
Innervation - posterior interosseus nerve (branch of the radial nerve)
Action - extension of the little finger
Extensor carpi ulnaris Origin - lateral epicondyle of the humerus, posterior border of the ulna
Insertion - medial side of the base of the metacarpal V
Innervation - posterior interosseus nerve (branch of the radial nerve)
Action - extension and adduction of the wrist

This article will discuss the anatomy and function of all three superficial extensor muscles.

Recommended video: Superficial extensors of the forearm
Origins, insertions, innervation and functions of the superficial extensors of the forearm.

Extensor digitorum muscle

The extensor digitorum muscle originates from the lateral epicondyle of the humerus. Distally it divides into four smaller tendons which insert at the posterior surface of the middle and distal phalanges of the second to fifth fingers.

Hereby the insertion tendons build a sheet-like tendinous expansion, the dorsal aponeurosis, covering the medial, posterior and lateral surfaces of the distal phalanges.

The dorsal aponeurosis serves as a place of insertion for various ligaments and muscles.

Extensor digiti minimi and extensor carpi ulnaris muscles

The extensor digiti minimi muscle arises from the lateral epicondyle of the humerus and inserts at the dorsal aponeurosis of the little finger. Its' muscle belly often blends with the extensor digitorum.

When it's about the extensor carpi ulnaris muscle, it has its' origin at both the lateral epicondyle of the humerus (humeral head) and the posterior side of the ulna (ulnar head). This muscle's insertion is located at the base of the fifth metacarpal bone

Both of the muscles extend their respective digits and the wrist joint.

Innervation

Like all extensors of the forearm, these three muscles are innervated by the radial nerve. The radial nerve divides into a superficial and deep branch at the height of the radial head. While the superficial branch continues along the brachioradialis the deep branch penetrates the supinator muscle after which it branches off the posterior interosseus nerve (C6-C8). There upon the posterior interosseus nerve supplies the superficial extensors with motor branches.

Radial nerve

Function

As the name suggests, all superficial extensors of the forearm fulfill a dorsal extension in the wrist joint.

Furthermore, the extensor digitorum extends the metacarpophalangeal (MCP), proximal interphalangeal (PIP) and distal interphalangeal joints (DIP) of the second to fifth fingers. Due to the fact that its' insertion tendons are partly attached to each other through the dorsal aponeurosis, an extension of one finger simultaneously leads to a slight extension of the neighboring fingers.

The extensor digiti minimi extends the MCP, DIP and PIP of the little finger. The contraction of the extensor carpi ulnaris pulls the hand medially (ulnar abduction).

Ulnar abduction or ulnar flexion

Clinical Notes

Approximately 2-3% of all people have so called 'small brother' of the extensor digitorum, the extensor digitorum brevis manus. It originates around the area of the distal radius and inserts at the dorsal aponeurosis of one or more fingers. This accessory muscle is normally harmless but can compress the posterior interosseus nerve during hypertrophy causing pain in the wrist joint. During clinical examination, it appears like a bulge at the dorsum of the hand, which is why it is often mistaken as a tendon ganglion, cyst or even a soft-tissue tumor. In order to prevent misdiagnoses and unnecessary surgeries, one should always consider an accessory muscle as a differential diagnosis when facing pain in the wrist joint.

The cause for the development lies in the incomplete migration of the forearm muscles during embryogenesis. It is assumed that this muscle anomaly represents an atavism, a “throwback” to our phylogenetic ancestors. It reminds us that our distant ancestors controlled their hands and fingers solely by short, intrinsic hand muscles. The takeover of this function by forearm muscles with long tendons has only developed through the course of evolution.

The superficial extensors of the forearm - want to learn more about it?

Our engaging videos, interactive quizzes, in-depth articles and HD atlas are here to get you top results faster.

Sign up for your free Kenhub account today and join over 852,397 successful anatomy students.

“I would honestly say that Kenhub cut my study time in half.” – Read more. Kim Bengochea Kim Bengochea, Regis University, Denver

Show references

References:

  • M. Schünke/E. Schulte/U. Schumacher: Prometheus – LernAtlas der Anatomie – Allgemeine Anatomie und Bewegungssystem, 2. Auflage, Thieme Verlag (2007), S. 312-313
  • J. E. Muscolino: The muscular system manual – The skeletal muscles of the human body, 2.Auflage, Elsevier Mosby (2005), S. 609-617, 648
  • A. V. Ranade/R. Rai/L. V. Prabhu et al.: Incidence of extensor digitorum brevis manus muscle, Hand/NY (Dezember 2008), Issue 3(4), S. 320-323
  • S. R. Bolla/V. R. Vollala/B. Bovindala et al.: Extensor digitorum brevis manus – It’s clinical significance and morphology, International Journal of Anatomical Variations (2008), Issue 1, S. 32-34

Author:

  • Achudhan Karunaharamoorthy

Illustrators:

  • First Illustration Gallery - Yousun Koh, Irina Münstermann
  • Second - Third Illustration Galleries - Yousun Koh
  • Radial nerve - Yousun Koh
  • Ulnar abduction or ulnar flexion - Paul Kim
© Unless stated otherwise, all content, including illustrations are exclusive property of Kenhub GmbH, and are protected by German and international copyright laws. All rights reserved.

Related Atlas Images

Extensors of the forearm

Radius and ulna

Main muscles of the upper extremity

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