Hammer curls specifically target and effectively work the brachialis muscle. This exercise uses a neutral grip, where the palms face each other, shifting the primary focus of elbow flexion away from the biceps brachii. The hammer curl leverages arm anatomy to isolate a muscle often neglected by traditional curling exercises, which is important for developing a well-rounded training program.
The Anatomy and Function of the Brachialis Muscle
The brachialis is a large muscle located in the upper arm, lying deep beneath the visible biceps brachii. It originates from the humerus and inserts onto the ulna bone, making it a powerful elbow flexor. It is considered a “pure flexor” because its attachment point means it does not assist with forearm rotation (supination).
The biceps brachii, conversely, attaches to the radius, enabling it to both flex the elbow and supinate the forearm. The brachialis is often cited as the strongest elbow flexor, providing about 50% more power than the biceps in this action. When developed, the brachialis pushes the biceps up from underneath, contributing noticeably to the overall girth and thickness of the upper arm.
The Mechanism Behind Hammer Curl Activation
The effectiveness of the hammer curl is rooted in biomechanics, specifically how the neutral grip alters the mechanical advantage of the arm muscles. When the forearm is in the neutral position, the biceps brachii is mechanically disadvantaged in its ability to supinate. This grip minimizes the biceps’ secondary function, forcing it to focus primarily on elbow flexion.
This shift in leverage allows the brachialis to become the primary mover during the curling action. Since the brachialis is a dedicated elbow flexor, the neutral grip places it in an optimal position to bear the majority of the load. The brachioradialis, a forearm muscle that also acts as an elbow flexor, is highly activated as well. By reducing the biceps’ involvement, the hammer curl ensures the brachialis and brachioradialis receive the stimulus needed for growth, leading to a thicker appearance in the upper arm and forearm.
How Grip Position Changes Arm Muscle Targeting
The three main forearm grip positions—supinated, pronated, and neutral—uniquely distribute the workload among the arm flexors.
The supinated grip (palms upward), used in the traditional bicep curl, strongly favors the biceps brachii. This grip allows the biceps to maximize its ability to both supinate and flex the elbow, making it the most dominant muscle in this variation.
The pronated grip (palms downward), used for reverse curls, places the biceps in a mechanically weak position. This shifts the focus heavily toward the brachioradialis and the brachialis.
The neutral grip of the hammer curl (palms facing each other) is the most direct way to isolate the brachialis. It places the brachialis and brachioradialis under high tension while reducing the contribution of the biceps. Regularly rotating through these grip variations ensures complete and balanced development of all arm flexor muscles.
Optimizing Hammer Curl Technique for Brachialis Growth
To maximize the brachialis-building benefits of the hammer curl, proper technique is essential, focusing on control and maximizing time under tension. Begin by holding the dumbbells with a neutral grip, ensuring your palms remain facing each other throughout the set. Keep your elbows pinned close to your sides to isolate the arm flexors and prevent the shoulders from taking over the movement.
Avoid using momentum or “swinging” to lift the weights, as this reduces tension on the target muscle. Use a controlled tempo, lifting smoothly until your forearms are nearly vertical. The eccentric phase (lowering the weight) is important for muscle growth and should be performed slowly, taking two to three seconds to return to the starting position. Maintain a neutral wrist position to keep the tension focused squarely on the brachialis and brachioradialis.