Extensor indices are a narrow muscle, which lies medial and parallel to extensor pollicis longus muscle. It arises from the posterior surface of the ulna, distal to extensor pollicis longus and also the adjoining interosseous membrane. The tendon is formed just proximal to the wrist and passes under the extensor retinaculum in the fourth compartment with the tendon of extensor digitiorum.

The Opposite to the head of the second metacarpal, it joins the ulnar side of the tendon of extensor digitorum, and it is inserted to dorsum of the middle and distal phalanges of the index finger through extensor expansion. An extensor index occasionally sends accessory slips to the extensor tendons of the other digits.