Muscles of the posterior forearm- deep layer

Supinator

• Proximal origin
  • Lateral epicondyle of the humerus; radial collateral and anular ligaments; supinator fossa and crest of the ulna
• Distal insertion
  • Posterolateral and anterior surfaces of the proximal third of the radius
• Innervation
  • Deep branch of Radial nerve– (C7) C8
• Action
  • Supinates the forearm

Extensor Indicis

• Proximal origin
  • Posterior surface of the distal third of the ulna and interosseous membrane
• Distal insertion
  • Extensor expansion of the 2nd digit
• Innervation
  • Posterior interosseous nerve- (C7) C8
• Action
  • Extends the 2nd digit (enables independent extension)
  • helps to extend the hand at the wrist

The next 3 muscles begin in the deeply but ‘outcrop’ superficially.

Abductor Pollicis longus

• Proximal origin
  • Posterior surface of the proximal halfves of the ulna, radius and interosseous membrane
• Distal insertion
  • Base of 1st metacarpal
• Innervation
  • Posterior interosseous nerve– (C7) C8
• Action
  • Abducts the thumb and extends it at CMC joint

Extensor Pollicis Longus

• Proximal origin
  • Posterior surface of middle third of the ulna and interosseous membrane
• Distal insertion
  • Dorsal aspect of base of distal phalanx of the thumb
• Innervation
  • Posterior interosseous nerve- (C7) C8
• Action
  • Extends the thumb at IP, MCP and CMC joints

Extensor pollicis brevis

• Proximal origin
  • Proximal surface of the distal third of the radius and interosseous membrane
• Distal insertion
  • Dorsal aspect of the base of the proximal phalanx of the thumb
• Innervation
  • Posterior interosseous nerve- (C7) C8
• Action
  • Extends the thumb at MCP and CMC joints.