Both hammer curls and bicep curls work the biceps brachii muscle, which is a large muscle in the front of the upper arm. The bicep muscle has two heads that adhere to the scapula and connect with a common insertion point. The bicep’s basic function is moving your arms forward, upward, and to the sides. The bicep stabilizes the shoulder when carrying objects or supporting weight. It also assists with pulling and curling motions.

Hammer curls target the long head of the bicep as well as the brachialis (another muscle in the upper arm) and the brachioradialis (one of the key forearm muscles). Hammer curls work your forearms and outer biceps muscles (long head). Although your forearms also contribute, biceps curls isolate your inner biceps muscles (short head) more effectively. Since hammer curls require more extensive buy-in from your forearms, they can give your overall arms a thicker appearance. Hammer curls are ideal for working multiple muscles in your arm, including a small benefit for your triceps. The traditional bicep curl targets the short head of the biceps brachii, but the hammer curl offers a more comprehensive arm exercise for building muscle by activating the long head of the biceps brachii and the brachialis, or elbow flexor muscles.

In summary, hammer curls work the long head of the bicep, the brachialis, and the brachioradialis, while bicep curls isolate the short head of the biceps brachii.