Hammer Curls

The hammer curl uses a neutral grip (thumbs up) throughout the entire range of motion, mechanically eliminating the supination function of the biceps brachii. This grip redistributes elbow flexor demand: the brachialis and brachioradialis become primary movers, while biceps brachii contributes as a secondary flexor without its most efficient mechanical advantage. Hammer curls are the primary training stimulus for the brachialis — a muscle that sits under the biceps and contributes to upper arm size regardless of supination capability.

Execution

1. Stand with dumbbells at the sides in a neutral grip (palms facing each other)
2. Keep the neutral grip throughout the entire movement — do not rotate into supination
3. Curl both dumbbells simultaneously or alternating to shoulder height
4. Lower under control; maintain the neutral wrist position through the eccentric

The Neutral Grip Mechanics

The biceps brachii is a powerful forearm supinator. In a supinated grip (palms up), the biceps can exert both flexion and supination torque simultaneously, producing high activation. In a neutral grip:

• Supination is eliminated as a mechanical input
• Biceps brachii activation decreases significantly
• Brachialis activation increases (it has no supination function and flexes the elbow regardless of grip)
• Brachioradialis — which prefers a neutral grip — becomes more active

The practical result: hammer curls are a brachialis-first exercise, not a biceps-first exercise.

Why Train the Brachialis

The brachialis sits deep to the biceps brachii and does not contribute to the "peak" shape of the biceps. However, a well-developed brachialis pushes the biceps upward, increasing overall upper arm circumference and visual height from the side. Including hammer curls ensures the brachialis — which is undertrained by all supinated curl variations — receives direct work.

Data Note

Jahizi 2023 (n=30) confirmed the grip-specific activation shift but did not report absolute %MVIC values. The muscle priority assignments (brachialis/brachioradialis primary) are based on mechanical analysis of the neutral grip position supported by the study's qualitative findings.

For system-specific training applications, see each system's lens entry.