Brachialis muscle

Anatomical relations

The brachialis is located on the anterior surface of the shaft of the humerus, deep to the muscle belly of biceps brachii and distally to its tendon. The biceps brachii muscle is located immediately anterior to the brachialis, as are the brachial vessels, the musculocutaneous, and median nerves. The humerus and the capsule of the elbow joint lie posterior to the muscle.

Medially, the brachialis is separated from the triceps brachii and the ulnar nerve by the medial intermuscular septum and pronator teres. Laterally it is related to the radial nerve in the radial groove, the brachioradialis and the extensor carpi radialis longus muscles. The tendon of brachialis forms part of the floor of the cubital fossa.

Origin and insertion

The brachialis muscle originates from the anterior surface of the distal half of the humerus, just distal to the insertion of the deltoid muscle. It is also attached to the intermuscular septa of the arm on either side, with a more extensive attachment to the medial intermuscular septum.

The fibers of brachialis extend distally to converge on a strong tendon. The tendon inserts onto the tuberosity of ulna and onto a rough depression on the anterior surface of the coronoid process of the ulna–passing between two slips of the flexor digitorum profundus muscle. It sometimes has an accessory attachment to the radius or the bicipital aponeurosis.

Function

The brachialis is known as the workhorse of the elbow. It is a major flexor of the forearm at the elbow joint, flexing the elbow while it is in all positions. The brachialis is the only pure flexor of the elbow joint–producing the majority of force during elbow flexion. It is not affected by pronation or supination of the forearm, and does not participate in pronation and supination due to its lack of attachment to the radius.

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During controlled extension of the elbow joint, the brachialis steadies the movement by relaxing at an even pace. This is an eccentric contraction of the muscle. This motion is used for precision movements such as lowering a teacup onto a flat surface carefully. The brachialis is also responsible for holding the elbow in the flexed position, thus, when the elbow joint is flexed, the brachialis is always contracting. 

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Innervation and blood supply

The brachialis is primarily supplied by the musculocutaneous nerve (C5, C6). In addition, a small lateral portion of the muscle is innervated by the radial nerve (C7).
Branches of the brachial artery and the radial recurrent artery supply the brachialis with contribution from accessory arteries. Occasionally, branches from the superior and inferior ulnar collateral arteries also contribute to the arterial supply of the brachialis muscle. The accessory arteries are small and highly variable. They can arise as branches from the brachial artery directly, the profunda brachii, or the superior and inferior ulnar collateral arteries. Venous drainage of the brachialis is by venae comitantes, mirroring the arterial supply and ultimately drain back into the brachial veins.