Despite the bulk of the muscle body being visible from the anterior aspect of the forearm, the brachioradialis is a posterior compartment muscle and consequently is innervated by the radial nerve. Of the muscles that receive innervation from the radial nerve, it is one of only four that receive input directly from the radial nerve. The other three are the triceps, anconeus, and extensor carpi radialis longus. (All other posterior compartment muscles that receive radial innervation are supplied by the deep branch of the radial nerve.)[citation needed]
The brachioradialis flexes the forearm at the elbow.[1][2] When the forearm is pronated, the brachioradialis tends to supinate as it flexes.[2] In a supinated position, it tends to pronate as it flexes.[2] This also assists the biceps brachii.[2]
The brachioradialis is a stronger elbow flexor when the forearm is in a midposition between supination and pronation at the radioulnar joint. When pronated, the brachioradialis is more active during elbow flexion since the biceps brachii is in a mechanical disadvantage.
With the insertion of the muscle so far from the fulcrum of the elbow, the brachioradialis does not generate as much joint torque as the brachialis or the biceps. It is effective mainly when those muscles have already partially flexed at the elbow. The brachioradialis flexes the forearm at the elbow, especially when quick movement is required and when a weight is lifted during slow flexion of the forearm.
The muscle is used to stabilize the elbow during rapid flexion and extension while in a midposition, such as in hammering. The brachioradialis is synergistic with the brachialis and biceps brachii; the triceps brachii and anconeus are antagonistic.[5][6]
Anatomy figure: 07:01-09 at Human Anatomy Online, SUNY Downstate Medical Center - "Transverse section through the left arm just proximal to the elbow."
Anatomy figure: 07:03-07 at Human Anatomy Online, SUNY Downstate Medical Center - "Superficial muscles of the anterior (flexor) compartment of the left forearm."
Anatomy figure: 09:02-02 at Human Anatomy Online, SUNY Downstate Medical Center - "Superficial muscles of the posterior (extensor) compartment of the left forearm."
