The brachialis is a muscle in the upper arm that plays a fundamental role in arm movement. While other arm muscles might receive more attention, the brachialis serves as a consistent and powerful contributor to everyday actions. It is a key component of the arm’s musculature, enabling a range of motions at the elbow joint.
The Brachialis: Primary Action
The brachialis muscle is recognized as the strongest flexor of the elbow joint, often called the “workhorse” of the elbow. Its primary and sole function is to bend the arm at the elbow, drawing the forearm closer to the upper arm. This muscle generates a significant amount of force during elbow flexion, reportedly producing about 50% more power than the biceps in this specific action.
A unique aspect of the brachialis is its ability to function consistently regardless of the forearm’s position. Whether the forearm is turned palm-up (supinated) or palm-down (pronated), the brachialis performs its flexion duty without being affected. Its attachments prevent it from participating in the rotational movements of the forearm. The brachialis is always active when the elbow is bent or held in a flexed position, even during slow, controlled lowering movements, which involve an eccentric contraction.
Location and Structure
The brachialis muscle is situated in the anterior compartment of the upper arm, lying deep to the more superficial biceps brachii muscle. It covers the lower half of the humerus, the long bone of the upper arm. The muscle originates from the anterior surface of the distal half of the humerus, just below the deltoid muscle’s insertion point.
From its broad origin on the humerus, the muscle fibers converge to form a thick tendon. This tendon then inserts onto two specific points on the ulna, one of the two bones of the forearm: the coronoid process and the tuberosity of the ulna. Its thick, fusiform shape contributes to its powerful role in elbow flexion.
Distinguishing the Brachialis from Other Arm Muscles
The brachialis muscle often receives less recognition than the biceps brachii, yet it holds a distinct and primary role in elbow flexion. While the biceps is more visible and also contributes to bending the arm, the brachialis is considered the pure flexor of the elbow. Unlike the biceps, the brachialis does not attach to the radius, the other forearm bone, meaning it does not assist with forearm rotation (supination or pronation). The biceps, in contrast, is a powerful supinator of the forearm in addition to being an elbow flexor.
This functional difference highlights the specialized role of the brachialis. When the forearm is in a pronated position, the biceps’ ability to flex the elbow is mechanically disadvantaged, making the brachialis even more prominent as the primary mover. The brachialis is the consistent and reliable muscle for all elbow flexion movements, regardless of forearm orientation.