The extensor indicis is a muscle located in the posterior compartment of the forearm. It is considered an extrinsic muscle of the hand in that it is situated in the forearm but acts on a joint in the hand. Specifically, this muscle is responsible for extension or straightening of the index finger, though because it crosses the wrist joint, it also aids other forearm muscles in extending the wrist, or bending it back so that the dorsal side of the hand approaches the forearm.
Found on the posterior side of the forearm — the same side as the back of the hand — the extensor indicis is nestled deep in the arm between the ulna bone and the extensor pollicis longus, which extends the thumb. Like the other muscles located here, it is a long, straplike muscle that runs parallel to the forearm. Originating on the lower third of the ulna on its posterior surface, the fibers of the extensor indicis converge at the wrist to form a tendon that crosses the wrist or radiocarpal joint.
Running just deep to one of the four tendons of the extensor digitorum muscle, which extends the two interphalangeal joints in each of the four fingers, the tendon of the extensor indicis then inserts on the back of the proximal, middle, and distal phalanges, the three bones in the index finger, to form what is known as the extensor hood. The extensor hood is a type of aponeurosis, a broad expansion of tendinous fibers, that spans the dorsal surface of these bones and is an extension of the tendons of both the indicis and the extensor digitorum. Through this attachment to the finger, the muscles can act on multiple joints as once, extending the entire finger until it forms a straight line.
Like many of the extensor muscles of the forearm, the extensor indicis acts not just on the finger joints but on the other hand and wrist joints. Since its tendon crosses the metacarpophalangeal joint at the base of the index finger, the articulation between the second proximal phalanx and second metacarpal in the hand that is visible as a large knuckle when making a fist, it extends this joint as well. Additionally, as it crosses the wrist or radiocarpal joint this muscle plays a small role in extending the wrist, though wrist extension is largely the result of contractions of the extensor carpi radialis and extensor carpi ulnaris muscles in the forearm.