The main muscles worked by hammer curls are the brachioradialis muscle of the forearm and the biceps brachii. Stabilizer muscles stressed while performing hammer curls include the anterior deltoid and the upper and middle trapezius.
Dumbbell hammer curls may be performed by alternating one arm at a time or curling both arms simultaneously. They are similar to dumbbell biceps curls, except that the thumb is facing upward toward the lifter and the wrist is locked in place with no supination. This altered grip places extra stress on the forearms and de-emphasizes the biceps.
To perform an alternating dumbbell hammer curl, a trainer holds a dumbbell in each hand while standing with his feet approximately shoulder width apart. With his palms facing in towards his leg and his elbows locked tightly against his body, the trainer raises a dumbbell until his thumb faces his shoulder. After pausing for a moment at peak contraction, the dumbbell is lowered, and the lift is repeated with the other arm.
To properly stress the forearms and biceps, it is important for lifters to use controlled form and avoid using momentum to swing the weight up. Additionally, the wrist must be locked in position at all times.