Hammer curls primarily target the biceps, specifically the long head of the biceps brachii, as well as the brachialis and brachioradialis muscles in the upper arm and forearm. The brachioradialis muscle is particularly involved as a secondary muscle group. Hammer curls are a variation of the traditional bicep curl, and the neutral grip with palms facing each other during the full range of motion is what sets it apart from the traditional bicep curl. Hammer curls are a good exercise for building forearm and grip strength, and they can help make everyday movements easier.