The hammer curl is a specialized variation of the traditional bicep curl, distinguished by the neutral grip, where the palms face each other (the handshake position). Unlike a standard curl, which uses a supinated grip, this subtle change in wrist orientation fundamentally alters the muscle recruitment pattern during elbow flexion. The neutral grip shifts the mechanical emphasis away from forearm rotation, engaging other significant muscles that contribute to arm size and strength. This exercise provides a more complete development of the musculature surrounding the elbow joint.
The Primary Muscle Targets
The hammer curl shifts the majority of the training stimulus onto two muscles that are often secondary movers in a traditional curl. The first is the brachialis, which lies beneath the biceps brachii. Because it inserts directly onto the ulna, the brachialis acts solely as a powerful elbow flexor without any role in forearm rotation. Strengthening this muscle adds significant thickness to the upper arm, as its growth pushes the overlying biceps upward.
The second primary beneficiary is the brachioradialis, the most prominent muscle of the outer forearm. This muscle is heavily engaged because it is optimally positioned to flex the elbow when the forearm is held in a neutral position. Development of the brachioradialis contributes substantially to the visible mass and width of the forearm.
The biceps brachii remains active during the hammer curl, but its role becomes less dominant. While the two heads of the biceps are still involved in bending the elbow, the neutral grip places them at a mechanical disadvantage. The hammer curl tends to target the long head of the biceps more effectively than the short head, ensuring balanced development across the front of the arm.
Biomechanics of the Neutral Grip
The neutral grip changes muscle activation due to the functional anatomy of the arm’s flexors. The biceps brachii is a bi-articular muscle, crossing both the shoulder and elbow, and performs two main actions at the elbow: flexion and supination. In a standard, supinated curl, the biceps is in its strongest mechanical position, making it the primary muscle of the lift.
By using a neutral grip, the hammer curl removes the supination component of the movement. This action shortens the working length of the biceps and places it at a mechanical disadvantage for elbow flexion. This shift in leverage forces the recruitment of the brachialis and brachioradialis to compensate for the biceps’ reduced contribution.
The brachialis is the strongest elbow flexor overall and is always active during any curl, but the neutral grip allows it to take over the majority of the load. Consequently, the hammer curl is particularly effective for increasing the sheer circumference and thickness of the upper arm.
Proper Execution for Maximum Activation
To ensure the brachialis and brachioradialis receive maximum stimulus, the hammer curl must be performed with precise control. Begin by standing upright with a stable torso and shoulders held back, minimizing unwanted movement. Hold the dumbbells with a firm, neutral grip so the palms face inward toward the sides of the legs.
The movement is initiated by flexing the elbow, drawing the weights straight up toward the shoulders. Keep the elbows pinned close to the sides of the body throughout the lifting phase. Allowing the elbows to travel forward engages the shoulder muscles and reduces isolation on the arm flexors.
Once the weights reach peak contraction (thumbs near the shoulder), pause briefly to maximize tension. The lowering phase, or eccentric contraction, should be performed slowly and with full control, taking two to three seconds to return to the starting position. Maintaining a fixed neutral grip sustains the mechanical disadvantage on the biceps, keeping tension concentrated on the target muscles.