The extensor indicis is one of the muscles of the deep layer of the posterior compartment of the forearm. It is an accessory extensor of the index finger.

The posterior compartment of the forearm consists of a superficial layer (7 muscles) and a deep layer (5 muscles).

All the muscles of the posterior compartment are innervated by the radial nerve.

The muscles of the deep layer include:

Origin
  • Posterior surface of the ulna and adjacent interosseus membrane (distal to extensor pollicis longus origin)
Insertion
  • Extensor expansion/hood of the index finger together with the extensor digitorum
Innervation
  • Posterior interosseus nerve
Blood Supply
  • Superficial surface – branches from the posterior interosseus artery
  • Deep surface – branches from the anterior interosseus artery
Action
  • Extension of the index finger (and wrist). Enables independent extension of the index finger.