Deep muscle of posterior compartment of forearm. Wraps around upper end of radius. Does not cross elbow joint.

Origin, Insertion and Nerve Supply

Action

• Supinates the forearm — rotates radius so palm faces anteriorly/upwards
• Supinator of choice when elbow is extended (biceps brachii is more powerful with elbow flexed)

Key Relation — Posterior Interosseous Nerve

• Supinator has two layers (superficial and deep) from a common origin
• Posterior interosseous nerve (deep branch of radial nerve) passes between these two layers to reach posterior compartment of forearm
• Proximal edge of superficial layer = thickened musculoaponeurotic structure = arcade of Frohse

Applied Anatomy

• PIN Syndrome: Posterior interosseous nerve compressed at arcade of Frohse or within supinator
• Results in weakness of wrist and finger extensors (wrist drop with radial deviation; cannot extend fingers at MCP joints)
• No sensory loss — posterior interosseous nerve is purely motor at this level (arises distal to cutaneous branches of radial nerve)

Supinator is a deep muscle of the posterior compartment of the forearm, wrapping around the upper end of the radius. It does not cross the elbow joint.

Origin, Insertion and Nerve Supply

Action

• Supinates the forearm — rotates the radius so that the palm faces anteriorly (or upwards, if the elbow is flexed to 90°).
• Acts as the supinator of choice when the elbow is extended (in contrast to biceps brachii, which is the more powerful supinator when the elbow is flexed).

Key Relation — Posterior Interosseous Nerve

• Supinator has two layers (superficial and deep), arising from a common origin.
• The posterior interosseous nerve (deep branch of the radial nerve) passes between these two layers of supinator to reach the posterior compartment of the forearm.
• The proximal edge of the superficial layer, where the nerve enters, is a thickened musculoaponeurotic structure called the arcade of Frohse.

Applied Anatomy

• Posterior Interosseous Nerve (PIN) Syndrome: The posterior interosseous nerve may be compressed either at the arcade of Frohse or within the substance of the supinator muscle.
• This results in weakness of the extensors of the wrist and fingers (wrist drop with radial deviation, inability to extend fingers at the MCP joints), without sensory loss, since the posterior interosseous nerve is purely motor at this level (it arises distal to the cutaneous branches of the radial nerve).

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