Extensor Carpi Ulnaris

Forearm · Upper Limb

The Extensor Carpi Ulnaris is a Upper Limb muscle within the Forearm group. Originating at Lateral epicondyle of humerus and posterior border of ulna, it inserts at Base of 5th metacarpal. Its chief action is that it extension and adduction (ulnar deviation) of wrist. Innervation is provided by the Posterior interosseous nerve (C7, C8).

Origin, Insertion, Action & Nerve

Origin

Lateral epicondyle of humerus and posterior border of ulna

Insertion

Base of 5th metacarpal

Action

Extension and adduction (ulnar deviation) of wrist

Nerve

Posterior interosseous nerve (C7, C8)

Attachments explained

The Extensor Carpi Ulnaris 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

Lateral epicondyle of humerus and posterior border of ulna

Insertion

Base of 5th metacarpal

Action & function

When the Extensor Carpi Ulnaris contracts, it produces the following movement: Extension and adduction (ulnar deviation) of wrist.

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 Carpi Ulnaris 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 Carpi Ulnaris, impairing the movements it normally produces (extension and adduction (ulnar deviation) of wrist). This is why knowing the innervation is central to localising nerve lesions in clinical practice.

How to study the Extensor Carpi Ulnaris (the OIANS method)

OIANS stands for Origin, Insertion, Action and Nerve, the four facts that uniquely define every skeletal muscle. To learn the Extensor Carpi Ulnaris, 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 Carpi Ulnaris 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.