The hammer curl is a variation of the traditional bicep curl performed with a neutral grip, meaning the palms face each other throughout the movement. This exercise is primarily intended to train the elbow flexor muscles on the front of the upper arm and forearm, and it is not an exercise for the triceps. The common confusion stems from the fact that while the hammer curl targets the biceps, its unique hand position shifts the emphasis to different, deeper muscles compared to a standard curl. The triceps, located on the back of the arm, are responsible for the opposite action: straightening the elbow.
The Muscles Primary to the Hammer Curl
The hammer curl strongly recruits three specific muscles that govern elbow flexion: the biceps brachii, the brachialis, and the brachioradialis. While the biceps brachii on the front of the upper arm is a primary mover, the neutral grip is especially effective at activating the underlying brachialis muscle. The brachialis is considered the strongest elbow flexor and is a pure flexor, meaning its function is not affected by the rotation of the forearm.
The brachialis is situated beneath the two heads of the biceps, and its development contributes to the overall thickness of the upper arm. The third muscle heavily involved is the brachioradialis, which is the most prominent muscle on the thumb-side of the forearm. This muscle assists in flexing the elbow, particularly when the forearm is held in the neutral position used during the hammer curl. By distributing the load across these three muscles, the hammer curl promotes well-rounded development of the arm flexors.
How the Neutral Grip Changes Arm Mechanics
The way the hand is positioned—the grip—determines which specific flexor muscles bear the greatest load during the curling motion. A typical bicep curl uses a supinated grip, where the palms face upward. The biceps brachii is a powerful elbow flexor, but its secondary and equally important function is to supinate, or rotate the forearm outward.
When the hand is already supinated, the biceps is placed in a position of maximum mechanical advantage for both flexion and rotation. Conversely, the hammer curl uses a neutral grip, where the palms face inward, which significantly minimizes the biceps brachii’s supination role. This neutral alignment places the biceps in a mechanically weaker position for pure elbow flexion, which then forces the brachialis and brachioradialis to take on a greater percentage of the lifting load.
The brachialis is uniquely suited for this role because it inserts directly onto the ulna, a bone in the forearm that does not rotate, making it an unwavering flexor regardless of hand position. The brachioradialis, which runs from the upper arm to the forearm, is also biomechanically favored when the hand is in the neutral position. Therefore, the neutral grip shifts the focus from the superficial biceps to the deeper brachialis and the forearm-based brachioradialis.
Common Triceps Exercises for Comparison
The triceps brachii muscle, located on the back of the upper arm, is the direct antagonist to the muscles worked by the hammer curl. Its primary function is elbow extension, the movement that straightens the arm. The triceps are composed of three heads—the long, lateral, and medial heads—all of which must be trained for full arm development.
Exercises that effectively target the triceps involve pushing the forearm away from the upper arm against resistance. The triceps pushdown works the lateral and medial heads by keeping the elbow pinned at the side. The overhead triceps extension places greater emphasis on the long head by requiring the weight to be lowered behind the head. The close-grip bench press is a compound movement that heavily recruits all three triceps heads.