) is a muscle
in the upper arm
that flexes the elbow joint
. It lies just deep of the biceps brachii
, and is a synergist that assists the biceps brachii
in flexing at 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 at the elbow joint. 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
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.
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.
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
of the forearm.
Pronation/supination of the forearm does not affect its action.
Occasionally doubled; additional slips to the supinator
, pronator teres
, biceps brachii
, lacertus fibrosus
, or radius
are more rarely found.