Extensor Indicis

Forearm · Upper Limb

Located in the Forearm of the Upper Limb, the Extensor Indicis is a key muscle for anatomy study. Originating at Posterior surface of the distal ulna and interosseous membrane, it inserts at Extensor expansion of the index finger (joining the extensor digitorum tendon). It is responsible for several movements: Extends the index finger; assists wrist extension. Its nerve supply is 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)

OIANS stands for Origin, Insertion, Action and Nerve, the four facts that uniquely define every skeletal muscle. To learn the Extensor Indicis, work through them in order: picture its origin, trace the muscle to its insertion, reason out the action that shortening between those two points must create, then add the nerve that drives it.

Most students remember the Extensor Indicis fastest by linking its action back to its attachments rather than memorising each fact in isolation. Once the origin and insertion make sense, the action usually follows logically.