Extensor indicis extends the index finger and assists wrist extension.
The extensor indicis is a deep, slender muscle in the posterior compartment of the forearm. It functions to extend the index finger independently of the other digits. It lies medial to the extensor pollicis longus and shares the same extensor hood system as the extensor digitorum but is anatomically and functionally distinct.
The extensor indicis is located in the distal third of the posterior forearm, deep to the extensor digitorum and extensor carpi ulnaris. Its tendon passes beneath the extensor retinaculum and joins the extensor expansion of the second digit (index finger).
The muscle originates from the ulna and interosseous membrane and becomes tendinous as it approaches the wrist.
The extensor indicis provides:
Supplied by the posterior interosseous nerve (C7–C8), which is the continuation of the deep branch of the radial nerve.
The extensor indicis lies medial to the extensor pollicis longus and deep to the extensor digitorum. Its tendon merges into the dorsal digital expansion of the index finger, often appearing slightly medial to the extensor digitorum tendon of the same digit.
Though not individually visible on the surface, the extensor indicis contributes to the tendon group visible on the dorsal hand during active index finger extension, particularly during isolated extension of the index finger.
The extensor indicis arises from the dorsal muscle mass of the forearm during limb development. It represents a specialization of the extensor system that allows finer, independent digit control in humans.
In other primates and mammals, this muscle is often absent or not distinctly developed. In humans, it has evolved to enhance independent control of the index finger, contributing to complex tool use and fine hand function.