Supinator muscle wraps around the proximal ulna and radius to rotate the forearm into supination.
The supinator is a deep muscle of the posterior compartment of the forearm responsible for forearm supination — the rotation of the forearm that turns the palm upward or anteriorly. It wraps around the upper third of the radius and plays a crucial role in rotating the radius back into anatomical position.
The supinator is located in the proximal part of the forearm, deep to the superficial extensor muscles. It lies wrapped around the proximal radius and posterior ulna, forming part of the floor of the cubital fossa.
The muscle is broad and flat, with fibers encircling the upper third of the radius in a spiral fashion. It has two heads — superficial and deep — that originate from the humerus and ulna, respectively.
The primary function of the supinator muscle is:
The muscle is innervated by the deep branch of the radial nerve (C6–C7), which passes between the two heads of the muscle. After passing through the supinator, the nerve continues as the posterior interosseous nerve.
The radial nerve (deep branch) passes between the superficial and deep heads of the muscle — a potential site of nerve entrapment. The muscle lies deep to the brachioradialis, extensor carpi radialis longus, and extensor digitorum proximally.
The supinator is not palpable externally due to its deep position, but it contributes to the contour of the lateral forearm just below the elbow, especially in movements involving resisted supination.
The supinator develops from the dorsal muscle mass of the embryonic upper limb. Its dorsal origin is consistent with radial nerve innervation and its extensor compartment location.
In quadrupeds, the supinator is often smaller or fused with surrounding muscles, reflecting reduced need for fine forearm rotation. In humans, it works in coordination with the biceps brachii to produce powerful and precise supination, especially when the elbow is extended.