Brachialis muscle

The brachialis (brachialis anticus) is a muscle in the upper arm that flexes the elbow joint. It lies just deep to biceps brachii, and is a more powerful flexor of the elbow. It makes up part of the floor of the region known as the cubital fossa.

Origin and insertion
The brachialis originates from the lower half of the front of the humerus, near the insertion of the deltoid muscle, which it embraces by two angular processes. Its origin extends below to within 2.5 cm. of the margin of the articular surface of the humerus. It also arises from the intermuscular septa of the arm, but more extensively from the medial than the lateral; it is separated from the lateral below by the brachioradialis and extensor carpi radialis longus muscles.

Its fibers converge to a thick tendon, which is inserted into the tuberosity of the ulna and the rough depression on the anterior surface of the coronoid process of the ulna.

Innervation
The brachialis muscle is innervated by the musculocutaneous nerve, which runs on its superficial surface, between it and the biceps brachii. Part of it is also innervated by the radial nerve which allows it to be split during certain approaches to the arm. The divide between the two innervations is at the insertion of the deltoid.

Actions
The brachialis is the strongest flexor of the elbow. Unlike the biceps, the brachialis does not insert on the radius, and therefore cannot participate in pronation/supination of the forearm.

Pronation/supination of the forearm does not affect its action.

Variations
Occasionally doubled; additional slips to the supinator, pronator teres, biceps brachii, lacertus fibrosus, or radius are more rarely found.