The brachialis muscle is situated deep beneath the more prominent biceps brachii. It serves a singular and powerful purpose, making it an indispensable part of arm movement. This muscle is fundamental for the movement of the forearm, specifically at the elbow joint. Understanding its location and mechanics reveals why it is so effective in controlling the action of the arm.
Anatomical Location and Structure
The brachialis muscle is positioned in the anterior compartment of the upper arm, lying flat against the humerus bone. It is situated deep to the biceps brachii. This deep location allows it to form the floor of the cubital fossa, the triangular space at the front of the elbow.
The muscle originates broadly from the distal half of the anterior surface of the humerus. Its fibers then converge and travel downward to insert onto the ulna, the forearm bone on the pinky-finger side. Specifically, it attaches to the tuberosity of the ulna and the coronoid process. The brachialis is primarily innervated by the musculocutaneous nerve, although a significant proportion of people also receive additional nerve supply from the radial nerve.
The Primary Action: Elbow Flexion
The sole action of the brachialis is to flex the forearm at the elbow joint. This movement involves decreasing the angle between the upper arm and the forearm, such as when bringing the hand toward the shoulder. It is widely considered the strongest and main flexor of the elbow.
This muscle is sometimes referred to as the “workhorse” of elbow flexion because it provides consistent force throughout the entire range of motion. The mechanical reason for this consistent action lies in its insertion point: the ulna. Since the ulna does not rotate during pronation or supination of the forearm, the brachialis’s ability to flex the elbow is completely independent of the hand’s position.
Whether the palm is facing up, down, or sideways, the brachialis contracts to bend the arm. It has a large cross-sectional area and a direct line of pull, allowing it to exert tremendous force. This makes it the steadfast driver of flexion, even when performing slow or controlled movements where stability is necessary.
Comparison to Other Forearm Flexors
The arm contains other muscles that contribute to bending the elbow, mainly the biceps brachii and the brachioradialis. The brachialis is distinct because it is a single-joint muscle, meaning it only crosses the elbow joint. This contrasts with the biceps brachii, which crosses both the elbow and the shoulder joint, giving it a multi-faceted function.
The biceps brachii not only flexes the elbow but is also the primary muscle responsible for supination, the action of turning the palm upward. When the forearm is in a pronated position (palm down), the biceps is less efficient at flexion, leaving the brachialis to handle the majority of the load. The brachioradialis is most active when the forearm is in a neutral or “hammer” grip position. The brachialis, therefore, maintains its role as the pure flexor.
Functional Role and Targeted Strengthening
The functional importance of the brachialis extends to nearly all activities that involve pulling or lifting objects toward the body. Its consistent force is necessary for stable movements, such as carrying groceries or climbing. It is responsible for initiating and sustaining the powerful bending motion of the arm.
Targeting this muscle during strength training can enhance overall arm development and elbow stability. Exercises that place the forearm in a neutral or pronated position intentionally minimize the involvement of the biceps’ supination function. Movements like hammer curls and reverse curls are effective because they force the brachialis to become the dominant elbow flexor. This focused approach helps develop balanced strength and contributes to the overall size of the arm.