Supinator muscle

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.

Location

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.

Structure

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.

• Origin:
  • Lateral epicondyle of the humerus (superficial head)
  • Radial collateral ligament and annular ligament of the radius
  • Supinator crest and fossa of the ulna (deep head)
• Insertion: Lateral, posterior, and anterior surfaces of the proximal third of the radius.
• Fiber Orientation: Oblique and spiral around the upper radius, forming a muscle sheath.

Function

The primary function of the supinator muscle is:

• Forearm Supination: Rotates the radius laterally around the ulna to turn the palm anteriorly (anatomical position) or upward when the elbow is flexed.
• Stabilization: Assists in stabilizing the proximal radioulnar joint during forearm rotation.

Innervation

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.

Blood Supply

• Radial recurrent artery
• Posterior interosseous artery

Relations

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.

Surface Anatomy

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.

Development

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.

Anatomical Variations

• Accessory slips from the supinator may extend to adjacent muscles or fascial planes.
• The deep head may be partially fused with the extensor muscles in some individuals.

Clinical Significance

• Radial Tunnel Syndrome: Compression of the deep branch of the radial nerve as it passes through the supinator can lead to pain and weakness — a condition sometimes mistaken for lateral epicondylitis.
• Nerve Entrapment: The arcade of Frohse (fibrous arch in the superficial head of the supinator) is a common site for compression of the posterior interosseous nerve.
• Surgical Landmark: The muscle must be carefully dissected during surgical exposure of the radial head and neck.

Comparative Anatomy

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.