Brachioradialis muscle: the forearm muscle that flexes the elbow

What it is

An elbow flexor that forms the lateral border of the cubital fossa; unusually for a flexor, it is supplied by the radial nerve.

Location

Origin: proximal two-thirds of the lateral supracondylar ridge of the humerus. Insertion: lateral surface of the distal radius near the styloid process.

Function

Flexes the forearm, most effectively in the mid-pronated position.

Body system

Muscular · paired (left and right)

FMA ID

FMA38486 (Foundational Model of Anatomy)

Key facts

• Innervation: radial nerve (C5–C6)
• Blood supply: radial recurrent artery
• Its tendon reflex (brachioradialis or supinator jerk) tests the C5–C6 segments.

Brachioradialis is the most superficial muscle on the radial (thumb) side of the forearm. It forms the lateral wall of the cubital fossa, the hollow at the front of the elbow, and lies on the border between the anterior and posterior compartments of the forearm. (Kenhub: Brachioradialis muscle.)

It arises from the upper two-thirds of the lateral supracondylar ridge of the humerus, the sharp ridge above the lateral epicondyle, and from the front of the lateral intermuscular septum of the arm. Its fibres run down the radial side of the forearm and form a thick tendon at about the middle of the forearm, which inserts on the lateral side of the radius just proximal to the radial styloid process. (Kenhub: Brachioradialis muscle.)

Radiologists describe brachioradialis as part of the superficial layer of the posterior forearm compartment even though it can be seen from the front. With extensor carpi radialis longus and brevis it makes up the 'mobile wad of Henry', a group of three muscles on the lateral side of the upper forearm. (Radiopaedia: Brachioradialis muscle.)

All anterior and posterior forearm muscles with origin, insertion, nerve and action tables. Forearm muscles.

A cadaver study measured the insertion as a teardrop or heart-shaped area about 15 mm long and 11 mm wide, ending about 17 mm above the tip of the radial styloid. The insertion forms the floor of the first dorsal extensor compartment, the tunnel that carries the tendons of abductor pollicis longus and extensor pollicis brevis. (Tirrell TF et al. Functional Consequence of Distal Brachioradialis Tendon Release: A Biomechanical Study. J Hand Surg Am. 2013.)

Just above the insertion the tendons of abductor pollicis longus and extensor pollicis brevis cross over the brachioradialis tendon on their way to the thumb. (Kenhub: Brachioradialis muscle.)

The same study found thin fibrous connections between brachioradialis, the radius and neighbouring muscles. Although easy to pierce, these connections could still transmit force to the radius when the main tendon was cut, which helps explain why releasing the tendon in surgery costs relatively little strength. (Tirrell TF et al. Functional Consequence of Distal Brachioradialis Tendon Release: A Biomechanical Study. J Hand Surg Am. 2013.)

Near the elbow, the upper part of brachioradialis is partly covered medially by brachialis, and the biceps tendon passes deep to it to reach the radial tuberosity. The radial nerve and an arterial anastomosis (involving the radial recurrent artery) pass between brachioradialis and brachialis. The cephalic vein and the lateral cutaneous nerve of the forearm cross its surface. At the wrist its tendon lies lateral to the radial artery, at the spot where the radial pulse is felt. (Kenhub: Brachioradialis muscle.)

In the cubital fossa the medial border of brachioradialis forms the lateral border of the fossa, and pronator teres forms the medial border. The radial nerve is the most lateral content of the fossa: it runs deep to brachioradialis and divides here into its superficial and deep branches. (TeachMeAnatomy: The Cubital Fossa.)

In the lower forearm the radial artery and the radial nerve lie between brachioradialis and the deep flexor muscles. (TeachMeAnatomy: Muscles in the Posterior Compartment of the Forearm.)

The superficial branch of the radial nerve, which carries only sensation, runs under brachioradialis and then along its ulnar edge. About 9 cm above the radial styloid it becomes subcutaneous by passing between the tendons of brachioradialis and extensor carpi radialis longus. (Kuschner SH, Berihun H. Robert Wartenberg Syndrome and Sign: A Review Article. Open Orthop J. 2021.)

Brachioradialis is supplied by the radial nerve, from the posterior cord of the brachial plexus. Kenhub and Radiopaedia give the roots as C5 and C6; the University of Washington atlas lists C5, C6 and C7. Sources vary in root values, but all agree on the radial nerve. (Kenhub: Brachioradialis muscle.)

The radial nerve gives its branches to triceps, brachioradialis and extensor carpi radialis longus before it divides at the elbow into its superficial and deep branches. So an injury to the deep branch or posterior interosseous nerve in the forearm does not affect brachioradialis. (TeachMeAnatomy: The Radial Nerve.)

Its blood comes from branches of the radial artery, the radial recurrent artery and the radial collateral branch of the deep (profunda) brachial artery. The University of Washington atlas names the radial recurrent artery as its main supply. (Kenhub: Brachioradialis muscle.)

Brachioradialis flexes the elbow together with biceps brachii and brachialis. It is strongest when the forearm is semi-pronated (thumb up), because its line of pull then lies in the sagittal plane; biceps is strongest with the palm up and brachialis with the palm down. From full supination it pronates the forearm to the mid-prone position, and from full pronation it supinates it back to mid-prone. (Kenhub: Brachioradialis muscle.)

OpenStax notes that brachioradialis can flex the forearm quickly or help lift a load slowly, and that it helps stabilise the elbow during a biceps curl. (OpenStax Anatomy and Physiology 2e, 11.5: Muscles of the Pectoral Girdle and Upper Limbs.)

It is the smallest of the three main elbow flexors, but it has a longer moment arm (lever) than biceps or brachialis over much of the range of motion. From muscle size and leverage, Tirrell and colleagues estimate that brachioradialis produces about 21 to 22% of elbow flexion torque at 90 degrees. (Tirrell TF et al. Functional Consequence of Distal Brachioradialis Tendon Release: A Biomechanical Study. J Hand Surg Am. 2013.)

Kenhub also describes it working eccentrically (lengthening under control) to smooth elbow extension in repetitive tasks such as hammering, and lists everyday actions such as rowing and lifting shopping bags. (Kenhub: Brachioradialis muscle.)

TeachMeAnatomy calls brachioradialis paradoxical: by origin and innervation it belongs with the extensors, but its action is elbow flexion. (TeachMeAnatomy: Muscles in the Posterior Compartment of the Forearm.)

The explanation is developmental. Kravarski and colleagues note that brachioradialis develops from the embryonic extensor muscle mass of the forearm, which explains several of its reported variations; this shared origin also fits its supply by the radial nerve, the nerve of the extensors. (Kravarski M et al. Supernumerary brachioradialis: anatomical variation with magnetic resonance imaging findings. World J Orthop. 2020.)

Brachioradialis is the fleshy bulge in the upper half of the lateral forearm. It is easiest to feel with the elbow bent and the forearm semi-pronated, thumb up. (Kenhub: Brachioradialis muscle.)

TeachMeAnatomy notes that the muscle is most visible when the elbow is flexed against resistance with the forearm half-pronated, for example when pushing up under a table edge with the thumb pointing to the ceiling. (TeachMeAnatomy: Muscles in the Posterior Compartment of the Forearm.)

The brachioradialis reflex (also called the supinator jerk) is a tendon reflex of this muscle that clinicians use to check the cervical nerve roots. In a table of findings in cervical radiculopathy, Safaee and colleagues pair the brachioradialis reflex with the C6 nerve root, the biceps reflex with C5 and the triceps reflex with C7. Because the muscle's own nerve fibres come from C5 and C6, the reflex is often described as testing the C5 to C6 segments. (Safaee MM et al. Safety and efficacy of direct nerve root decompression via anterior cervical discectomy and fusion with uncinectomy for cervical radiculopathy. J Spine Surg. 2020.)

Brachioradialis is usually a single muscle with one head, but variants exist. Kravarski and colleagues summarise cadaver studies in which an accessory brachioradialis was found in 5 of 176 limbs (2.8%) and a split brachioradialis in 9 of 150 arms (6%). (Kravarski M et al. Supernumerary brachioradialis: anatomical variation with magnetic resonance imaging findings. World J Orthop. 2020.)

They also report a 17-year-old patient with a supernumerary brachioradialis: two muscle bellies and two tendons sharing one insertion on the radius. They warn that such a variant could be stitched to the wrong tendon during repair of forearm injuries or when a radial forearm flap is raised. In a study of patients with Wartenberg syndrome that they cite, 7 of 143 (4.9%) had the superficial radial nerve trapped between the two slips of a split brachioradialis. (Kravarski M et al. Supernumerary brachioradialis: anatomical variation with magnetic resonance imaging findings. World J Orthop. 2020.)

Brachioradialis matters in distal radius fractures. Its tendon pulls on the radial styloid, and Jacobi and colleagues describe this tension as a cause of displacement of the styloid fragment that makes reduction harder. (Jacobi M, Wahl P, Kohut G. Repositioning and stabilization of the radial styloid process in comminuted fractures of the distal radius. J Orthop Surg Res. 2010.)

Surgeons often release the brachioradialis insertion to reduce and plate these fractures. In a cadaver model, releasing the tendon at the level of a typical fracture line (about 28 mm from the styloid) was predicted to cost less than 10% of brachioradialis torque. The authors estimate that even a 20% loss of brachioradialis torque would reduce total elbow flexion torque by only about 4 to 5%. (Tirrell TF et al. Functional Consequence of Distal Brachioradialis Tendon Release: A Biomechanical Study. J Hand Surg Am. 2013.)

Wartenberg syndrome is compression of the superficial branch of the radial nerve where it emerges from under brachioradialis, from the edge of the muscle, from fascial bands between brachioradialis and extensor carpi radialis longus, or from tight watch straps or handcuffs. It causes pain, tingling and altered sensation over the back of the thumb side of the wrist and hand. (Kuschner SH, Berihun H. Robert Wartenberg Syndrome and Sign: A Review Article. Open Orthop J. 2021.)

Brachioradial pruritus is an itch, most often on the skin of the upper forearm over brachioradialis in the C6 dermatome, on one or both arms. It is typically seen in fair-skinned women in late middle age. The proposed causes are cervical spine problems affecting the C5 to C6 nerve roots and sun damage to itch-carrying nerve fibres; the name describes where the itch is felt, not a disease of the muscle. (DermNet: Brachioradial pruritus.)

The nearest named structures in the 3D model, measured from the centre of each part:

• Ulnar head of pronator teres · muscular
• Supinator · muscular
• Humeral head of pronator teres · muscular
• Common interosseous artery · cardiovascular
• Extensor carpi radialis longus · muscular
• Extensor carpi radialis brevis · muscular
• Recurrent interosseous artery · cardiovascular
• Anconeus · muscular
• Anterior ulnar recurrent artery · cardiovascular
• Ulnar head of flexor carpi ulnaris · muscular