Hammer Curls vs Bicep Curls: Muscles Worked, What Each Builds and How to Program Both

You've got one pair of dumbbells and two curls to choose from. Turn your palms up and it's a bicep curl. Turn your thumbs up and it's a hammer curl. One video says hammer curls are the secret to thick arms, another says they barely touch your biceps, and a forum thread says to do both and stop worrying.

When the advice keeps changing, it's easy to wonder if months of curls went to the wrong muscle, or if your arms aren't growing because you picked the wrong grip. Your grip probably isn't the problem. Arms grow from hard sets repeated for months, and most arm plans fall apart long before the choice of curl could matter.

The research on these two curls is smaller and less tidy than most articles suggest. This guide covers what part of the bicep hammer curls work, whether hammer curls are better than bicep curls, and how to program both.

Hammer Curls vs Bicep Curls at a Glance

The differences are small, and no row here justifies dropping either curl. The studies behind each row are below.

Hammer Curls vs Bicep Curls: Muscles Worked

Both curls work the same three muscles, because all three bend the elbow. Only the way your forearm is turned changes.

• Biceps brachii. The two-headed muscle on the front of your upper arm. It attaches to the radius, the forearm bone on the thumb side, so it bends your elbow and also rotates your palm upward.
• Brachialis. A flat muscle under the biceps that attaches to the ulna, the forearm bone that stays put when your forearm rotates. It only bends the elbow, and researchers credit it with the biggest share of elbow-bending force (Date 2021).
• Brachioradialis. The long muscle on the thumb side of your forearm. It crosses the elbow and works during elbow bending in every forearm position (Boland 2008).

That anatomy explains why grip matters to the biceps at all. As your palm turns down, the biceps tendon wraps around the radius and the muscle loses force (Kleiber 2015). The brachialis pulls on the ulna, so it doesn't have that problem. A spinal reflex from the brachioradialis also damps the biceps, most with the palm down, less with the thumb up and least with the palm up (Barry 2008).

What Part of the Bicep Do Hammer Curls Work?

Hammer curls work both parts of the bicep, the long head and the short head, plus the brachialis underneath. You'll often read that they single out the long head. We couldn't find a study that recorded the two heads separately during a hammer curl, so treat that as gym lore. In the Barry study, that reflex acted on both heads alike.

What the EMG Studies Found, Muscle by Muscle

Electromyography (EMG) records how hard a muscle is working during a rep.

Biceps: a small edge for palms up

Coratella and colleagues (2023) had 10 competitive bodybuilders do standing cable curls with each grip's 8-rep-max weight, using a bar for palms up and a rope for the hammer grip. While lifting, biceps activity was 12% higher with the palms up. While lowering, the grips didn't differ.

A 2024 conference paper by Jahizi and colleagues points the same way: in 30 trained men using dumbbells, biceps activity was highest in the traditional curl, then the hammer curl, then the reverse curl. The gap can also disappear. In a slow elbow-bending test of 16 adults, with no added weight described, Kleiber's group found no biceps difference between palm up and thumb up.

Brachialis: no proven hammer-grip boost

Hammer curls are sold as the brachialis exercise, but the data don't show a boost. Date and colleagues ran fine-wire electrodes into the brachialis of six men, and its activity pattern looked similar with the palm up, thumb up or palm down. That test used no weight and didn't compare grips statistically, so it's a small piece of evidence, though it fits the anatomy. In the Jahizi paper, the reverse curl drew more brachialis activity than either the hammer or the traditional curl.

Brachioradialis: no clear hammer-grip boost either

This is the muscle most often credited to the hammer curl, yet three studies fail to show it. It was 6% more active with the palms up than with the thumbs up while lifting in the Coratella study. With fine-wire electrodes in 10 healthy adults, Boland and colleagues found no difference between forearm positions during elbow bending. Kleiber's group also found no difference between palm up and thumb up. They did see more activity with the palm down, but the Coratella study didn't.

These studies are small and used different setups, and EMG shows how hard a muscle works during a rep, not how much it grows.

Which Curl Builds What?

We found no trial that compared a hammer grip with a palms-up grip for muscle growth, so nobody can say which one grows which muscle faster. What we do have shows that a palms-up curl can grow the brachialis on its own.

Attarieh and colleagues (2025) trained 15 untrained men twice a week for 10 weeks, with cable preacher curls on one arm and Bayesian curls, where the arm sits behind the body, on the other. The paper specifies a palms-up grip for its strength tests and says training matched those positions. On ultrasound, the biceps grew 7 to 9% and the brachialis 8 to 10%, with no significant difference between arms.

So expect both curls to build the biceps and the brachialis, with a small activity edge for the biceps on palms-up curls. The brachialis adds size under the biceps whichever curl grew it. Forearm size takes more than curls, and our forearm guide covers the wrist work they leave out.

Are Hammer Curls Better Than Bicep Curls?

Not overall. Each curl wins a few narrow comparisons, and none of the gaps is large.

• Biceps activity: bicep curl. It was 12% higher while lifting in the Coratella study.
• Reps: hammer curl. The 30 men in the Jahizi paper completed the most reps with hammer curls.
• Strength: about even. In a static elbow-bending test of 11 men, peak force was 243.6 newtons with the thumb up and 213.6 with the palm up, a gap that wasn't statistically significant. Palm down fell to 113.6 (Kohn 2018).
• Soreness after hard lowering: hammer curl. Lau and colleagues (2021) had 10 untrained men do maximal lowering sets, one arm palm up and the other thumb up. The palm-up arm got sorer and lost more strength.
• Less shoulder work: bicep curl. The front of the shoulder worked 9% harder with the neutral grip while lifting in the Coratella study.
• Wrist comfort: hammer curl, for many people. We found no study on this, but the thumbs-up grip is the usual swap when palms-up curls bother a wrist or elbow.

How to Program Hammer Curls and Bicep Curls With Dumbbells

You don't need an arm day. Add these curls to the end of two workouts a week, two or three days apart, with 60 to 90 seconds of rest between sets. Each session swaps which curl goes first, so both get your freshest sets.

End each set with one or two good reps left. When you hit the top of the rep range on every set, move up a dumbbell, or slow the lowering to three seconds if you don't have a heavier pair. That steady increase is progressive overload.

That's 10 curl sets a week, and your rows and pull-ups add more. Our guide to building bigger arms explains how to count them, and our 30-minute dumbbell arm workout adds the triceps. For form, see our bicep curl and hammer curl guides.

Hammer Curls or Bicep Curls: Which Should You Pick?

For most people, both. If you have to choose, pick by goal:

• Bigger biceps: lead with bicep curls and keep hammer curls second.
• Thicker arms overall: do both, and rely on hard sets rather than grip to grow the brachialis.
• Wrists or elbows that dislike palms-up curls: switch to hammer curls, and get any pain that lasts checked.
• Short on time: pick the curl you'll keep doing, because the gap between them is small.

Knowing which curl to do is the easy part. Doing them twice a week for months is where most arm plans stall. In FitCraft, your AI coach demonstrates each exercise with an interactive 3D demo you can zoom in on and says your name while you train. Your program adapts as you progress, and streaks keep the sessions from slipping. If you train at home, a dumbbell workout app can carry the plan and the progression for you.

The Bottom Line

You didn't waste your curls on the wrong grip. The arms you want come from the same few sets repeated for months. As Mike put it: "The streak system got me hooked. I didn't want to break my chain."