Supinator Muscle Anatomy

Location & Overview

The supinator is a paired muscle of the proximal forearm. Its official Latin name is musculus supinator, and its official English term is supinator muscle.^[1]

The supinator lies in the deep part of the posterior compartment of the forearm. The posterior forearm compartment is also described as the extensor–supinator compartment, and in the proximal forearm it sits more posterolaterally than directly posterior.^[1,2]

At the elbow, the supinator lies deep in the cubital fossa, the anterior hollow at the elbow, and helps the brachialis form the floor of that space.^[2] The muscle is broad and sheet-like, wrapping around the neck and proximal shaft of the radius.^[3,2]

The supinator has superficial and deep parts. The radial nerve’s deep motor pathway passes between these parts as it travels into the posterior forearm, which makes the muscle an important landmark for the posterior interosseous nerve.^[3,2]

The arcade of Frohse is a named arch between the two layers of the supinator that transmits the posterior interosseous nerve.^[3] Benes et al. (2021), in a systematic review and meta-analysis, estimated that this arch is present in about 66% of adults. That means it is common, but not a universal structure.^[4]

Small accessory muscle slips can occur around the superior radioulnar joint. Paraskevas and Ioannidis (2011) dissected 36 elbow joints and reported a lateral tensor of the anular ligament in 16.6%, a medial tensor of the anular ligament in 11.1%, and an accessory supinator muscle in 16.6% of elbows.^[5] Hast and Perkins (1986) reported different frequencies in a smaller anatomical study, with a lateral tensor in 7 of 10 cadavers, a medial tensor in 2 of 10 cadavers, and an accessory supinator in 4 of 10 cadavers.^[6] These structures are best understood as variants rather than standard parts of the supinator.^[3,5,6]

Origin & Insertion

The supinator has a broad origin from the lateral side of the elbow. It arises from the lateral epicondyle of the humerus, radial collateral ligament, anular ligament of the proximal radioulnar joint, and the supinator crest/fossa region of the ulna.^[3,2]

The ulnar part of the origin needs careful wording because the supinator crest and adjacent fossa form a closely related attachment region. The supinator crest is the raised ridge on the ulna related to the muscle, while the adjacent fossa is a shallow depression. The supinator may also arise from the triangular depression anterior to the supinator crest and from an aponeurosis covering the muscle. An aponeurosis is a broad, sheet-like tendon.^[3]

The muscle inserts on the proximal third of the radius. Its attachment includes the lateral, anterior, and posterior radial surfaces, which fits its spiral course around the radius.^[3,2]

More precise insertion boundaries place the radial attachment between the anterior oblique line and the fainter posterior oblique ridge, extending distally to the insertion of the pronator teres.^[3] This detail helps explain why the supinator can rotate the proximal radius without crossing the wrist or fingers.

The oblique cord is a small, inconstant fascial band associated with the deep head of the supinator. It runs from the lateral side of the ulnar tuberosity to the radius just distal to the radial tuberosity, but its functional significance is unclear.^[3]

Action & Function

The supinator supinates the forearm by rotating the radius. In anatomical position, this turns the palm anteriorly; when the elbow is flexed, it turns the palm superiorly.^[3,2,1]

The muscle is especially important during slow, unopposed supination. Biceps brachii contributes strongly when supination is rapid, forceful, or resisted, particularly when the elbow is flexed.^[3,2]

Haugstvedt, Berger and Berglund (2001), in a cadaveric mechanical study of 8 upper extremities with the elbow fixed at 90 degrees, found that both biceps brachii and supinator acted as supinators. In that test position, biceps brachii generated four times more torque than the supinator when the forearm was pronated.^[7]

Functionally, the supinator can rotate the forearm from a pronated starting position as the elbow flexes, such as when an object is lifted and brought toward the eyes.^[3] The supinator does not flex or extend the elbow as its main action; its main action is rotation at the radioulnar joints.^[3,2]

Innervation

The supinator is innervated by the radial nerve’s deep motor pathway. The supplying nerve may be described as the posterior interosseous nerve or as the deep branch of the radial nerve, depending on where the naming boundary is placed as the nerve passes through the supinator region.^[3,2]

This difference is mostly a naming-boundary issue. The deep branch of the radial nerve passes through the supinator and is commonly referred to as the posterior interosseous nerve after it exits the muscle in the posterior compartment.^[2] Jeon et al. (2021) also discussed variation in how the radial nerve, deep branch, and posterior interosseous nerve are described around the supinator.^[8]

The root values reported for the supinator vary between C6-C7 and C7-C8. Because of this variation, the clearest general description is that the supinator is supplied through the deep radial nerve/posterior interosseous nerve pathway.^[3,2]

The small branches entering the supinator vary. Duquin, Chavan and Bisson (2010), in an anatomical study of 12 fresh male upper-extremity specimens, reported 2-9 posterior interosseous nerve branches to the supinator.^[9] Tubbs et al. (2013), in a cadaveric study of 52 limbs, reported that the posterior interosseous nerve entered the supinator either as a single nerve or as two approximately equal-size branches, with further variation within the muscle.^[10]

Blood Supply

The supinator receives blood from recurrent and interosseous arterial branches around the elbow and posterior forearm. The superficial part is supplied by branches from the radial recurrent artery.^[3]

The deep part is supplied by branches from the posterior interosseous artery and posterior interosseous recurrent artery.^[3] This fits the regional anatomy of the posterior forearm, where the posterior interosseous artery courses between the superficial and deep extensor layers with the posterior interosseous nerve.^[2]

Among anatomy sources, there is no consistent prevalence pattern for arterial variants. The clearest general anatomy description is the named arterial supply: radial recurrent branches for the superficial part, and posterior interosseous plus posterior interosseous recurrent branches for the deep part.^[3]