Topic Overview

Muscles of the Back of the Forearm

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Overview

• All extensor muscles of the forearm lie in the posterior compartment.

• They are divided into two layers:

  1. Superficial group

  2. Deep group

• Nerve supply: Entirely by radial nerve or its posterior interosseous branch.

• Main actions: Extension of wrist and fingers, supination of forearm, abduction of thumb.

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I. Superficial Muscles

👉 Mnemonic (from lateral → medial):
“BRAE — BRE — DI — MI — UL”
(Brachioradialis, Extensor Carpi Radialis Longus, Extensor Carpi Radialis Brevis, Extensor Digitorum, Extensor Digiti Minimi, Extensor Carpi Ulnaris)

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1. Brachioradialis

• Origin: Upper two-thirds of lateral supracondylar ridge of humerus.

• Insertion: Lateral side of distal radius (styloid process).

• Nerve Supply: Radial nerve (C5–C6).

• Action: Flexes elbow (best in mid-prone position).

• Clinical Note: Though in extensor group, it acts as flexor of forearm.

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2. Extensor Carpi Radialis Longus (ECRL)

• Origin: Lower part of lateral supracondylar ridge.

• Insertion: Base of 2nd metacarpal bone.

• Nerve Supply: Radial nerve (C6–C7).

• Action: Extends and abducts wrist.

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3. Extensor Carpi Radialis Brevis (ECRB)

• Origin: Common extensor origin (lateral epicondyle).

• Insertion: Base of 3rd metacarpal.

• Nerve Supply: Posterior interosseous nerve.

• Action: Extends wrist, helps in abduction.

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4. Extensor Digitorum

• Origin: Common extensor origin (lateral epicondyle).

• Insertion: Dorsal digital expansion of medial four fingers.

• Nerve Supply: Posterior interosseous nerve.

• Action: Extends fingers at MCP, PIP, and DIP joints.

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5. Extensor Digiti Minimi

• Origin: Common extensor origin.

• Insertion: Dorsal digital expansion of little finger.

• Nerve Supply: Posterior interosseous nerve.

• Action: Extends little finger.

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6. Extensor Carpi Ulnaris

• Origin: Lateral epicondyle and posterior border of ulna.

• Insertion: Base of 5th metacarpal.

• Nerve Supply: Posterior interosseous nerve.

• Action: Extends and adducts wrist.

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Clinical Note (Superficial Extensors)

• Lateral Epicondylitis (Tennis Elbow):

  • Inflammation of common extensor origin → pain on resisted wrist extension and grip.

• Posterior interosseous nerve entrapment:

  • Causes weakness in finger extension without wrist drop.

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II. Deep Muscles

👉 Mnemonic (from lateral → medial):
“Sup – AB – EPB – EPL – EI”
(Supinator, Abductor Pollicis Longus, Extensor Pollicis Brevis, Extensor Pollicis Longus, Extensor Indicis)

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1. Supinator

• Origin: Lateral epicondyle, radial collateral ligament, annular ligament, and supinator crest of ulna.

• Insertion: Upper third of radius (lateral and anterior surfaces).

• Nerve Supply: Deep branch of radial nerve (before it becomes posterior interosseous).

• Action: Supinates forearm (especially when extended).

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2. Abductor Pollicis Longus (APL)

• Origin: Posterior surfaces of radius, ulna, and interosseous membrane.

• Insertion: Base of 1st metacarpal.

• Nerve Supply: Posterior interosseous nerve.

• Action: Abducts and extends thumb at CMC joint.

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3. Extensor Pollicis Brevis (EPB)

• Origin: Posterior surface of radius and interosseous membrane.

• Insertion: Base of proximal phalanx of thumb.

• Nerve Supply: Posterior interosseous nerve.

• Action: Extends thumb at MCP joint.

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4. Extensor Pollicis Longus (EPL)

• Origin: Posterior surface of ulna and interosseous membrane.

• Insertion: Base of distal phalanx of thumb.

• Nerve Supply: Posterior interosseous nerve.

• Action: Extends thumb at IP, MCP, and CMC joints.

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5. Extensor Indicis (EI)

• Origin: Posterior surface of ulna and interosseous membrane.

• Insertion: Dorsal digital expansion of index finger (joins extensor digitorum tendon).

• Nerve Supply: Posterior interosseous nerve.

• Action: Extends index finger independently.

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Clinical Note (Deep Extensors)

• De Quervain’s Tenosynovitis:

  • Involves 1st dorsal compartment tendons (APL & EPB).

  • Pain on lateral wrist and during thumb abduction.

• Intersection Syndrome:

  • Friction between APL/EPB and ECRL/ECRB → pain 4–6 cm above wrist.

• Posterior Interosseous Nerve Palsy:

  • Loss of extension of digits but wrist extension preserved.

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✋ Dorsal Digital (Extensor) Expansion

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Definition

• A triangular aponeurosis on the dorsum of fingers formed by extensor digitorum, lumbricals, and interossei tendons.

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Structure

• Base (proximal): Over MCP joint, receives tendon of extensor digitorum.

• Sides: Joined by lumbricals and interossei.

• Apex (distal): Divides into three slips:

  • Central slip → inserted into base of middle phalanx.

  • Two lateral slips → unite and attach to base of distal phalanx.

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Functions

• Coordinates extension of MCP, PIP, and DIP joints.

• Maintains balance between extensors and flexors during finger movements.

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Clinical Anatomy

• Mallet Finger (Baseball Finger):

  • Avulsion of extensor tendon from distal phalanx → finger tip droops.

• Boutonnière Deformity:

  • Rupture of central slip → flexed PIP and hyperextended DIP joint.

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🧠 Dissection of Muscles of Back of Forearm

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Steps

1. Place limb prone and make longitudinal incision along posterior forearm.

2. Reflect superficial fascia → identify cutaneous veins and nerves (posterior cutaneous nerve of forearm).

3. Expose superficial extensor muscles (BRAE BRE DI MI UL) from lateral to medial.

4. Reflect these to reveal deep muscles (Sup, APL, EPB, EPL, EI).

5. Identify posterior interosseous nerve entering deep between supinator and APL.

6. Note extensor retinaculum compartments and tendon sheaths near wrist.

7. Trace extensor tendons to dorsal digital expansions over MCP joints.