The supinator muscle is located deep in the proximal posterior forearm. It lies deep to the brachioradialis and common extensor tendon at the lateral elbow. The supinator is a relatively flat muscle originating primarily on the dorsal surface of the proximal ulna, but also on the lateral epicondyle of the humerus, anterior fibers of the humeroulnar joint capsule, and proximal radioulnar ligament. The muscle wraps around the lateral surface of the radius, inserting broadly on the proximal radial surfaces, just distal to the radial head.
Proximally, the supinator is divided into superficial and deep layers. This division creates a channel for the deep portion of the radial nerve, also known as the posterior interosseous branch. The tendon of the biceps brachii is also located near this Y-shaped portion of the supinator. Caution should be exercised when palpating the insertion of the biceps brachii and the fibers of the supinator to avoid compressing the radial nerve.
When the supinator is activated, it rotates the radius posteriorly and laterally, supinating the forearm. It works synergistically with the biceps brachii and brachioradialis to supinate the forearm, turning the hand palm up. Unlike the biceps brachii, the supinator is strongest when the elbow is extending or extended. Some of its fibers attach to the lateral epicondyle of the humerus, allowing it to assist with this movement.
The supinator reverses the action of the pronator teres and quadratus when turning a screwdriver or wrench. It is also activated when throwing a curveball in baseball. Here, the elbow extends while the forearm supinates, generating the characteristic spin of the curve ball. Excessive force production when throwing, carrying objects in a palm-down position, flipping objects from a palm-down to a palm-up position, or twisting movements may stress or injure the supinator.
Prolonged positioning with the forearm pronated, such as when typing, writing, or using a computer mouse, may lead to elongated tension in the supinator and associated muscles. Entrapment of the deep radial nerve, trigger-point activation, and poor joint mechanics may result. Trigger points in the supinator muscle refer pain to the lateral elbow and into the web of the thumb on the dorsal side.

Palpating the Supinator
Position: client seated with forearm neutral (thumb up).
1. Passively flex the elbow and supinate the forearm to slacken the tissue.
2. Locate the lateral epicondyle with your thumb, pushing the brachioradialis aside laterally.
3. Follow the muscle distally and slightly anteriorly onto the radial head. (Note: avoid compressing the radial nerve, which resides in this location.)
4. Gently resist as the client supinates the forearm to ensure proper location.

Client Homework:
Active Forearm Supination

1. With or without a small weight, sit with your arm supported, elbow bent, and palm down.
2. Keeping your shoulder and elbow relaxed, rotate your forearm, turning the palm up.
3. Return your forearm to the palm-down position.
4. Repeat until you feel a loosening and slight fatigue in the elbow.

Christy Cael is a licensed massage therapist, certified strength and conditioning specialist, and instructor at the Bodymechanics School of Myotherapy & Massage in Olympia, Washington. Her private practice focuses on injury treatment, biomechanical analysis, craniosacral therapy, and massage for clients with neurological issues. She is the author of Functional Anatomy: Musculoskeletal Anatomy, Kinesiology, and Palpation for Manual Therapists (Lippincott Williams & Wilkins, 2009). Contact her at functionalbook@hotmail.com.

Editor’s note: The Client Homework element in Functional Anatomy is intended as a take-home resource for clients experiencing issues with the profiled muscle. The stretches identified in Functional Anatomy should not be performed within massage sessions or progressed by massage therapists, in order to comply with state laws and maintain scope of practice.