The hammer curl is a dumbbell arm exercise defined by its unique neutral grip, where the palms face each other throughout the movement. This grip variation changes the muscle recruitment pattern compared to a standard curl, leading many to question its effect on the lower arm. The specific positioning of the wrist and forearm in the hammer curl definitively engages the forearm muscles, making it a valuable exercise for overall arm development and building arm thickness.
The Forearm Muscle Targeted
The primary forearm muscle targeted by the hammer curl is the brachioradialis, a major elbow flexor. This muscle is located on the thumb side of the forearm, running from the humerus near the elbow down to the radius bone near the wrist. Unlike other forearm muscles that primarily move the wrist, the brachioradialis visibly contributes to the mass and thickness of the upper forearm.
The neutral grip places the forearm in a semi-pronated position, which is the optimal angle to maximize tension on the brachioradialis muscle. This position reduces the mechanical advantage of the biceps brachii, forcing the brachioradialis to take on a significantly larger portion of the load to bend the elbow. Therefore, the hammer curl is the best exercise to specifically develop the size and strength of this forearm flexor.
Hammer Curls as an Upper Arm Movement
While the hammer curl effectively targets the brachioradialis, it works all three major elbow flexors. The movement significantly engages the brachialis, a muscle that lies beneath the biceps brachii. The brachialis is the strongest flexor of the elbow joint, and its development adds width and girth to the upper arm by pushing the biceps outward.
The neutral grip is particularly effective at isolating the brachialis because it prevents the biceps from fully assisting, as the biceps are also responsible for forearm supination. Although less emphasized than in a standard supinated curl, the biceps brachii remains active during the hammer curl. The neutral grip ensures a balanced workload across both the long and short heads of the biceps, contributing to overall arm development.
Comparing Grip Types for Forearm Activation
The degree of forearm muscle activation is determined by the grip used during a curling movement. The neutral grip of the hammer curl forces maximal activation of the brachioradialis while still engaging the upper arm flexors, making it a balanced choice for simultaneous upper arm girth and upper forearm development.
In contrast, the supinated grip (palms up), used in a standard biceps curl, places the greatest emphasis on the biceps brachii and minimizes brachioradialis recruitment. Conversely, the pronated grip (palms down), or reverse curl, places the least tension on the biceps and shifts the focus to the forearm extensor muscles on the top of the forearm.
The hammer curl is superior to both the standard and reverse curl for targeting the brachioradialis, making it ideal for balanced arm thickness. However, because the hammer curl primarily targets only one forearm muscle, it does not replace dedicated isolation work like wrist curls for the flexors or reverse wrist curls for the extensors. It effectively bridges the gap between upper arm and upper forearm training, leading to a more complete and proportioned arm appearance.