Extensor Indicis
Forearm · Upper Limb
The extensor indicis is a deep extensor that gives the index finger its own extensor, allowing it to be pointed independently of the other fingers. It arises from the posterior surface of the distal ulna and the interosseous membrane and its tendon joins the extensor digitorum tendon to reach the extensor expansion of the index finger. It extends the index finger and assists in extending the wrist. It is supplied by the posterior interosseous nerve (C7, C8).
Origin, Insertion, Action & Nerve
Origin
Posterior surface of the distal ulna and interosseous membrane
Insertion
Extensor expansion of the index finger (joining the extensor digitorum tendon)
Action
Extends the index finger; assists wrist extension
Nerve
Posterior interosseous nerve (C7–C8)
Attachments explained
The Extensor Indicis is defined first by where it attaches. Its origin is the more fixed anchor, usually the proximal or more stable end that stays put during contraction, while its insertion is the more mobile point that is pulled toward the origin when the muscle shortens.
Origin
Posterior surface of the distal ulna and interosseous membrane
Insertion
Extensor expansion of the index finger (joining the extensor digitorum tendon)
Action & function
When the Extensor Indicis contracts, it produces the following movements: Extends the index finger; assists wrist extension.
As part of the Forearm group of the Upper Limb, it works alongside neighbouring muscles to generate smooth, coordinated movement and to stabilise the structures it acts on.
Nerve supply & clinical relevance
The Extensor Indicis receives its nerve supply from the Posterior interosseous nerve (C7–C8). This nerve carries fibres from spinal segment(s) C7, C8.
Because a muscle can only contract when its nerve is intact, injury to the Posterior interosseous nerve or to its spinal roots (C7, C8) can weaken or paralyse the Extensor Indicis, impairing the movements it normally produces (extends the index finger). This is why knowing the innervation is central to localising nerve lesions in clinical practice.
How to study the Extensor Indicis (the OIANS method)
Think of extensor indicis as the muscle that lets you point: it is the index finger's own extensor, supplementing extensor digitorum so the index can straighten by itself. Its deep origin on the distal ulna and interosseous membrane distinguishes it from the superficial common extensor.
Pair it with extensor digiti minimi, the equivalent private extensor for the little finger, so the two independent-finger muscles are learned together. Both run on the posterior interosseous nerve, which reinforces the extensor-compartment innervation pattern.
Frequently asked questions
Where is the Extensor Indicis located?
The Extensor Indicis is a muscle of the Forearm group, located in the Upper Limb.
What is the origin of the Extensor Indicis?
Posterior surface of the distal ulna and interosseous membrane
What is the insertion of the Extensor Indicis?
Extensor expansion of the index finger (joining the extensor digitorum tendon)
What movements does the Extensor Indicis produce?
Extends the index finger; assists wrist extension
What nerve supplies the Extensor Indicis?
Posterior interosseous nerve (C7–C8)
Is the Extensor Indicis free to study in OIANS?
Yes. The Extensor Indicis is free in OIANS: its full origin, insertion, action and nerve details are open to everyone in the Muscle Directory, and because it belongs to the Upper Limb it is also covered by the free Quiz and Flashcard decks. The other regions, the "All Muscles" deck, and the Progress and Explore tools are unlocked with a one-time Lifetime purchase.
Related muscles
Study the Extensor Indicis
Test yourself on Upper Limb muscles with interactive quizzes and flashcards in the free OIANS app.
Quiz yourself → Flashcards Open the OIANS app
Browsing is always free. A one-time Lifetime unlock adds every region's quizzes, flashcards, progress tracking and explore tools.