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Extensor Indicis Muscle
*Musculus extensor indicis*
For medical students2 min readUpdated 2026-10-10
The extensor indicis is a deep posterior forearm muscle that provides independent extension of the index finger. Its tendon travels to the dorsal hand within a shared compartment with other extensors, ensuring precise motor control and assisting with wrist extension.
OriginPosterior surface of the ulna
InsertionDorsal digital expansion (extensor hood) of the index finger
ActionExtension of the index finger
InnervationRadial nerve (*n. radialis*)
Blood supplyPosterior interosseous artery
Attachments and Topography
The muscle is located in the deep layer of the posterior forearm muscle group.
Origin (Origo): Muscle fibers arise from the distal third of the posterior surface of the ulna and the adjacent interosseous membrane.
Tendon Course: Traveling distally, the tendon of m. extensor indicis passes beneath the extensor retinaculum. It runs through the fourth osseofibrous compartment together with the tendons of the m. extensor digitorum.
Insertion (Insertio): Upon reaching the dorsum of the hand, the tendon runs toward the index finger and firmly blends into its dorsal digital expansion (extensor hood).
Biomechanics and Functions
The primary function of this muscle is directly reflected in its name.
Isolated Extension: Contraction of its muscle fibers pulls on the extensor hood, allowing selective extension of the index finger even when the other fingers are flexed into a fist.
Joint Synergy: Crossing the radiocarpal joint, the muscle also acts as a synergist to assist with overall wrist extension.
Blood Supply and Innervation
The muscle is supplied and controlled by standard structures of the posterior forearm:
Innervation is supplied entirely by the radial nerve (n. radialis), specifically originating from spinal nerve segments C7–C8.
Blood supply is delivered via branches of the posterior interosseous artery (a. interossea posterior), which courses through the deep layer of the forearm.
Lateral Muscle Group of the Forearm
When studying the extensors, it is important to remember the adjacent lateral muscle group, which topographically overlaps with them. These muscles are also innervated by the radial nerve (n. radialis):
Brachioradialis (m. brachioradialis): Flexes the forearm and brings the forearm into a mid-prone (neutral) position. Supplied by the radial artery (a. radialis).
Extensor carpi radialis longus and brevis (m. extensor carpi radialis longus et brevis): Their function is to extend the wrist and abduct it radially (radial deviation). Supplied by the radial artery (a. radialis) and radial recurrent artery (a. recurrens radialis).
Mnemonic
To easily remember the function of the m. extensor indicis, think of pointing gesture: this muscle allows you to keep your index finger straight (extending it) while the other fingers remain curled.
Frequently asked questions
Which muscles make up the deep layer of the posterior forearm group?
The deep layer of the posterior forearm group includes the supinator, extensor indicis, and muscles acting on the thumb. They include:
Supinator (m. supinator) — located in the upper lateral aspect of the forearm.
Abductor pollicis longus (m. abductor pollicis longus) — originates from the posterior surfaces of both forearm bones and the interosseous membrane.
Extensor pollicis brevis (m. extensor pollicis brevis) — inserts onto the base of the proximal phalanx of the thumb.
Extensor pollicis longus (m. extensor pollicis longus) — inserts onto the base of the distal phalanx of the thumb.
Extensor indicis (m. extensor indicis) — originates from the posterior surface of the ulna and interosseous membrane.
Which compartment does the extensor indicis tendon pass through?
Its tendon passes through the fourth osseofibrous compartment beneath the extensor retinaculum.
Which tendon shares this compartment with the extensor indicis?
In the fourth compartment, the tendon of m. extensor indicis runs alongside the tendons of the m. extensor digitorum.
What is the origin of the extensor indicis?
The muscle originates from the posterior surface of the ulna (in its distal third) and the interosseous membrane.
Which nerve innervates this muscle?
Like most posterior forearm extensors, it is innervated by the radial nerve (n. radialis, segments C7–C8).
Go deeper
Topography of the dorsal wrist osseofibrous compartments (1st to 6th compartments)
Anatomy and functions of the extensor digitorum muscle
Branches of the radial nerve in the forearm
Structure of the dorsal digital expansion (extensor hood) of the fingers
Mechanism of radial deviation of the wrist (action of the radial wrist extensors)