The hammer curl is a specialized variation of the common bicep curl, distinguished by the neutral hand position used throughout the movement. This exercise is a powerful tool for developing the upper arm and forearm. Understanding which specific parts of the arm are targeted by this unique grip is important for balanced muscle development, as the hammer curl emphasizes muscles that traditional curls often neglect.
Understanding the Target Muscles
The movement of bending the elbow, known as elbow flexion, involves three main muscles. The Biceps Brachii is the most visible muscle on the front of the upper arm and is the main target of standard curls. It is a strong flexor that also plays a large role in rotating the forearm outward.
The Brachialis lies beneath the Biceps Brachii, originating from the lower half of the upper arm bone and inserting onto the ulna. This muscle is considered the workhorse of elbow flexion because it is active regardless of the hand’s rotation or grip position. The Brachioradialis is the third significant flexor, running along the thumb-side of the forearm. It contributes significantly to forearm mass and assists in flexing the elbow, particularly when the hand is in a neutral position.
The Role of the Neutral Grip
The difference in muscle activation between a standard curl and a hammer curl is due entirely to the hand position. A standard curl uses a supinated grip, where the palm faces up, which places the Biceps Brachii in its most mechanically advantageous position. Since the Biceps Brachii is responsible for both flexing the elbow and supinating the forearm, the palm-up position allows it to maximize its strength.
The hammer curl utilizes a neutral grip, where the palms face inward toward the torso. This rotation places the Biceps Brachii at a biomechanical disadvantage, reducing its ability to contribute maximally to the lift. The neutral grip minimizes the Biceps Brachii’s leverage, effectively shifting the majority of the load.
This shift in leverage forces the Brachialis to take on a greater percentage of the workload. Because the Brachialis attaches only to the bones of the upper arm and forearm, its function is solely to flex the elbow, making it a pure elbow flexor. Isolating this muscle promotes the development of arm thickness, as the Brachialis sits underneath the Biceps Brachii.
Furthermore, the neutral grip is the optimal position for the Brachioradialis to contribute to elbow flexion. This muscle works most efficiently when the forearm is in a neutral or mid-prone position, which is precisely the orientation of the hammer grip. The increased activation of the Brachioradialis makes the hammer curl effective for building mass along the length of the forearm. The neutral hand position intentionally limits the Biceps Brachii, maximizing the recruitment of the Brachialis and the Brachioradialis.
Proper Execution for Maximum Isolation
To ensure the hammer curl effectively targets the Brachialis and Brachioradialis, strict attention to form is necessary. Begin by standing with the dumbbells held at your sides, palms facing your body, maintaining a slight bend in your elbows. Keep the upper arms stationary throughout the movement, fixing the elbows close to the sides of the torso to isolate the flexor muscles.
As you curl the weight upward, focus on bending only at the elbow joint and avoid leaning back or using momentum. The wrist should remain in a neutral, straight position throughout the lift, which helps to maintain tension on the target muscles. Control the weight during the lowering phase, taking about three seconds to return to the starting position. This controlled eccentric motion is effective for promoting growth in the Brachialis. Maintaining a stable torso and controlling the tempo prevents the Biceps Brachii from taking over the lift.