Extensor Indicis Anatomy

Extensor Indicis Muscle

Origin:  Posterior surface of the ulna and the interosseous membrane
Insertion:  Dorsal surface of the base of the middle phalanx (central slip) and distal phalanx (lateral bands) of the index digit.
Actions: Extension of the second digit
Innervation: Radial nerve (C6 C7 C8)
Blood Supply: Posterior interosseous artery from the ulnar artery

Primary Actions of the Extensor Indicis

1. Extension of the second digit at the metacarpophalangeal joint

  Agonists:

  • Extensor digitorum

  Antagonists:

  • First dorsal interossei
  • First Palmar Interosseous
  • First Lumbricals (hand)
  • Flexor Digitorum Superficialis
  • Flexor Digitorum Profundus

2. Extension of the second digit at the proximal interphalangeal joint

  Agonists:

  • Extensor Digitorum
  • First Lumbrical
  • First Dorsal Interosseous
  • First Palmar Interosseous

  Antagonists:

  • Flexor Digitorum Profundus
  • Flexor Digitorum Superficialis

3. Extension of the second digit at the distal interphalangeal joint

  Agonists: 

  • Extensor Digitorum
  • First Dorsal Interossei
  • First Palmar Interossei
  • First Lumbricals

  Antagonists:

  • Flexor Digitorum Profundus

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Secondary Actions of the Extensor Indicis:

1. Assists with adduction of the second digit at the carpometacarpal joint

  Agonists:

  • First Palmar Interosseous

  Antagonists:

  • First Dorsal Interosseous