The supinator muscle is located in the forearm, playing a distinct role in lower arm movement. Its primary function involves a specific rotational action, essential for many daily tasks.
Precise Anatomical Placement
The supinator muscle is a broad, curved muscle situated deep within the posterior compartment of the forearm, wrapping around the upper third of the radius bone. It is considered a deep muscle, meaning it lies closer to the bone rather than near the surface of the skin. The muscle has two layers of fibers, superficial and deep, between which the deep branch of the radial nerve passes.
The supinator originates from several points around the elbow joint. These include the lateral epicondyle of the humerus, a bony prominence on the outer side of the upper arm bone. It also attaches to the supinator crest and fossa of the ulna, the longer forearm bone, and to the radial collateral and annular ligaments that stabilize the elbow and superior radioulnar joints.
From these origins, the muscle fibers spiral around the proximal third of the radius, the other forearm bone. It then inserts onto the anterior, lateral, and posterior surfaces of the proximal third of the radius, superior to the insertion of the pronator teres muscle. This winding path around the radius allows it to perform its specific rotational action.
The Supinator’s Primary Action
The primary action of the supinator muscle is to supinate the forearm. Supination is the rotational movement that turns the palm of the hand upwards or forwards, as if holding a bowl of soup. This action is the opposite of pronation, which turns the palm downwards.
When performing slow, unresisted supination, the supinator muscle acts as the prime mover. For quick, strong, or forceful supination, the biceps brachii muscle assists. The biceps brachii is most effective as a supinator when the elbow is flexed to about 90 degrees, while the supinator can act in all positions of elbow flexion and extension.
Supination is integral to many everyday activities. Examples include turning a doorknob, using a screwdriver, or rotating your hand to catch an object. The coordinated effort of the supinator allows for precise control over forearm rotation.
Nerve Supply and Clinical Significance
The supinator muscle receives its nerve supply from the deep branch of the radial nerve. This deep branch, often referred to as the posterior interosseous nerve after it passes through the supinator, originates from nerve roots C5, C6, C7, and C8. The nerve travels between the two layers of the supinator muscle, making it susceptible to compression or entrapment.
Compression of the deep branch of the radial nerve as it passes through the supinator muscle can lead to Supinator Syndrome, also known as Radial Tunnel Syndrome. This condition involves irritation or inflammation of the nerve due to friction or pressure. The radial nerve can be compressed at various points along its path, with the supinator muscle being a common site.
Symptoms of Supinator Syndrome typically include pain on the outer side of the elbow and forearm, which can radiate towards the wrist and fingers. This pain often intensifies with activities involving forearm rotation or heavy lifting. While it primarily causes pain, some individuals may experience weakness in finger extensors or in the supination movement itself, though significant motor weakness is less common than in other nerve compression syndromes.