brachialis

The brachialis is a thick muscle of the anterior compartment of the upper arm, lying deep to the biceps brachii and crossing the elbow joint. It plays a primary role in elbow flexion and helps stabilize the joint during movement.

Origin and insertion: It arises from the distal half of the anterior surface of the humerus. Its fibers descend to insert mainly on the ulna, attaching to the ulnar tuberosity and the anterior part of the coronoid process.

Innervation and blood supply: The musculocutaneous nerve provides most of the innervation (typically C5–C6, with possible contribution from C7). The brachial artery and its branches supply the muscle with blood.

Function: The brachialis flexes the elbow by pulling the ulna toward the humerus. Because it inserts on the ulna rather than the radius, it acts as an elbow flexor in all forearm positions, but it is particularly important when the forearm is pronated or when flexion strength is needed independent of supination. It works in concert with the biceps brachii and brachioradialis, with the biceps being more effective in supination.

Clinical notes: Injury or compression affecting the musculocutaneous nerve can weaken elbow flexion, especially with a supinated forearm. Because the brachialis lies deep to the biceps, isolated pathology is less common than injuries to surrounding structures, but fractures of the distal humerus or elbow joint injuries can involve the muscle or its nerve supply. Variations in distal attachments exist but the standard insertion remains on the ulna via the coronoid process and ulnar tuberosity.