The brachialis is a muscle positioned deep within the anterior compartment of the upper arm, a region often dominated by the more visible biceps brachii. It is a flat, fusiform muscle that forms the floor of the cubital fossa. This muscle is recognized as the primary muscle responsible for bending the elbow joint.
The Origin of the Brachialis Muscle
The fixed, or proximal, attachment point for the brachialis muscle is located on the humerus. Specifically, the muscle fibers begin on the anterior surface of the distal half of the humerus bone. This origin site is quite extensive, starting near the insertion point of the deltoid muscle, which creates a characteristic V-shape at its uppermost boundary. The origin extends downward to within a few centimeters of the articular surface of the elbow joint. Because the origin is on the humerus, which is relatively stationary during an isolated elbow curl, it serves as the anchor point for the muscle’s pulling action.
The Specific Insertion Point
The insertion is the mobile, or distal, attachment point. The muscle fibers converge near the elbow to form a tendon. This tendon crosses the elbow joint and attaches to two specific bony landmarks on the ulna. The primary insertion is onto the ulnar tuberosity, a roughened area just below the elbow joint on the upper part of the ulna. It also attaches to the coronoid process of the ulna, a triangular projection of bone that sits near the joint.
The insertion onto the ulna, rather than the radius, is a significant anatomical detail that defines the muscle’s function. The ulna does not rotate when the hand turns palm-up (supination) or palm-down (pronation). This physical attachment ensures that the brachialis acts as a pure flexor of the elbow regardless of the forearm’s rotational position.
Primary Action and Function
The brachialis muscle is often called the “workhorse” of the elbow flexors because it initiates and maintains the bending motion. It is active during every type of elbow flexion. The muscle is capable of generating significant force. This muscle contrasts with the biceps brachii, which is not only an elbow flexor but also a powerful supinator.
Because the brachialis inserts on the non-rotating ulna, its action is confined entirely to flexion. This makes the brachialis uniquely positioned to continue flexing the elbow even when the forearm is in a pronated position, where the biceps is at a mechanical disadvantage. The brachialis contributes to functional movements such as bringing a cup to the mouth or lifting objects close to the body. Without the brachialis, the ability to bend the arm against resistance would be severely compromised, especially when the hand is turned palm-down.
Clinical Significance and Strengthening
The brachialis can suffer from strains or tears, particularly during sudden, forceful lifting movements or heavy eccentric loading. In cases of severe elbow trauma, the muscle can sometimes be involved in a condition called myositis ossificans, where bone tissue forms within the muscle fibers following injury. For individuals seeking to increase the size and strength of their arms, targeting the brachialis is an effective strategy.
Because the brachialis lies underneath the biceps, strengthening it pushes the biceps up, contributing to greater arm thickness and width. Exercises that minimize the involvement of the biceps are effective for isolating the brachialis. Exercises like the hammer curl (neutral grip) or the reverse curl (pronated grip) are recommended. These grips reduce the leverage of the biceps and force the brachialis to handle the majority of the load for elbow flexion. Including these variations in a workout routine ensures this muscle is fully developed.