Supinator

Musculus supinator

A deep, sheet-like muscle that wraps around the upper radius. It supinates the forearm, and the deep branch of the radial nerve passes between its two layers.

Show on body map Muscles — Posterior View

Overview

Supinator arises from the lateral epicondyle, the radial collateral and annular ligaments, and the supinator crest and fossa of the ulna. Its two layers (superficial and deep) wrap around the neck and upper shaft of the radius, inserting on its lateral, posterior and anterior surfaces above the pronator tuberosity. Contracting, it unwinds the radius and supinates the forearm. It acts alone in slow, unresisted supination and in supination with the elbow extended. Biceps takes over when speed or force is needed with the elbow flexed.

The deep branch of the radial nerve pierces the muscle between its layers. It enters under the fibrous upper edge of the superficial layer (the arcade of Frohse) and emerges at the back as the posterior interosseous nerve.

Parts & subdivisions

Superficial layer
From the lateral epicondyle and ligaments. Its upper edge forms the arcade of Frohse.
Deep layer
From the supinator crest and fossa of the ulna.

Anatomy

Origin: Lateral epicondyle of the humerus, radial collateral and annular ligaments, supinator crest and fossa of the ulna.
Insertion: Lateral, posterior and anterior surfaces of the proximal third of the radius.
Action: Supination of the forearm.
Innervation: Deep branch of the radial nerve.
Nerve roots: C5–C6 (C7)
Arterial supply: Radial recurrent artery and recurrent interosseous artery.

Function

Supinates the forearm (especially slow, unresisted, or with the elbow extended)

Clinical significance

Posterior interosseous nerve syndrome / radial tunnel syndrome

Compression of the deep branch at the arcade of Frohse. Motor PIN syndrome causes weakness of finger and thumb extension with radial deviation on wrist extension, and no sensory loss. Radial tunnel syndrome causes lateral forearm pain that mimics tennis elbow.

Test yourself

Which muscle is the most powerful supinator of the forearm when the elbow is flexed?

Supinator
Brachioradialis
Biceps brachii
Pronator teres

Answer: Biceps brachii

Biceps brachii is the most powerful supinator when the elbow is flexed, which is how a right-handed person drives a screw. Supinator acts alone in slow, unresisted supination and when the elbow is extended.