Hammer curls: what the thumbs-up grip changes, and what it doesn't

A hammer curl is a dumbbell curl with your palms facing each other and your thumbs on top, the way you'd hold a hammer. Three muscles bend your elbow: the biceps, the brachialis underneath it, and the brachioradialis, which runs from the upper arm down the thumb side of the forearm. The hammer curl's reputation is that the thumbs-up grip moves the work from the first muscle to the other two.

This page covers the setup, what the grip changes in the studies and what it doesn't, hammer curls against ordinary curls, the versions, the common faults, and how many sets. How curls fit next to your rows and presses, and how to count them, is on the arm workout; wrist curls, reverse curls and grip work are on the forearm workout.

How to do a hammer curl

1. Stand tall with a dumbbell in each hand, arms straight at your sides, palms facing your thighs.
2. Pin your elbows to your sides. They stay there for the whole set.
3. Curl the weights up by bending only at the elbows, thumbs leading and palms still facing each other, until your forearms are close to vertical.
4. Pause briefly at the top, then lower under control until your arms are straight again. That's one rep.
5. Both arms together or one at a time. Alternating lets you watch each arm and keep the other one still.

Your wrists stay straight, in line with your forearms. If they tip back or curl in as the set gets hard, the weight is too heavy for the grip.

What the thumbs-up grip changes

Start with the anatomy, because it's where the idea comes from. The biceps has two jobs: besides bending the elbow, it turns the palm up.¹ The brachialis has one. It attaches to the ulna, the forearm bone that doesn't rotate, so it works the same however your forearm is turned, and its only job is bending the elbow.² Take the palm-up turn out of the curl, the reasoning goes, and the biceps does less of the work.

The studies that put electrodes on the arm are less tidy:

• Palms up came out ahead in trained lifters. In 10 competitive bodybuilders curling an 8-rep-max load, the biceps was about 12% more active with the palms up than with the hammer grip, and the brachioradialis about 6% more.³
• Palms down, not thumbs up, in another. In 16 healthy adults bending the elbow slowly, the brachioradialis was more active palms down than palms up or thumbs up, while biceps activity stayed the same.¹
• No clear difference in two more. With fine-wire electrodes in 10 healthy adults, nine men and one woman, brachioradialis activity didn't differ between palms up, palms down and thumbs up.⁴ In 15 young healthy men curling as hard as they could, the biceps, brachialis and brachioradialis were all very active in every position, and the brachioradialis slightly more so with the palms up.⁵

So the thumbs-up grip doesn't switch the biceps off or turn the brachioradialis on. All three elbow flexors work in every grip, and the differences between grips were small and pointed in different directions. These studies measured activity during a set, not growth over weeks.

Do hammer curls build the brachialis?

That's the question the grip is chosen for, and no trial has answered it directly: none that trained one group with hammer curls and another with palms-up curls turned up in a PubMed search for this page. What the growth trials do show is that the brachialis and brachioradialis grow from curls in general:

• In 21 untrained adults who trained one arm with incline dumbbell curls and the other with preacher curls for 12 weeks, MRI showed the brachialis grew 17.1% and 21.1% and the brachioradialis 13.5% and 17.6%, with the preacher curl ahead on both.⁶
• In 15 young men who trained one arm with a preacher-style cable curl and the other with a cable curl behind the body for 10 weeks, the brachialis grew 10% and 8%, with no significant difference between the two.⁷

So the brachialis doesn't need a special grip to grow. Whether the hammer grip grows it more than an ordinary curl hasn't been tested. The forearm workout lands in the same place for the brachioradialis: the grips don't settle it, so use more than one across the week.

Hammer curls vs regular curls

Choose by what you'll keep doing. If you only have room for one curl, the palms-up curl gives up nothing in the studies above. If you want two, the hammer curl is a different grip for the same three muscles, and a reasonable swap for a block of weeks.

Dumbbell, cable, rope and cross-body

The cross-body and rope versions turned up in no study in a PubMed search for this page, so pick by the equipment you have. The shoulder angle matters less than it looks: in the 15-man cable trial above, curling with the arm in front of the body or behind it grew the biceps and brachialis about equally.⁷ The same thumbs-up grip shows up outside the curl, too, in a rope row or a sled pulled hand over hand, which the sled push covers.

Common mistakes

• Swinging the weight up. Leaning back hands part of the work to your hips. Go lighter and keep your torso still.
• Elbows drifting forward. The front of the shoulder joins in: in the bodybuilder study, it was more active with the thumbs up than with the palms up.³ Keep your elbows at your sides.
• Half reps. Lower until your arms are straight before the next rep.
• Bending at the wrist. Keep it straight; if it won't stay straight, the dumbbell is too heavy.
• Dropping the weight on the way down. Take about two seconds to lower it.
• Curling through something that hurts. Sore the next day is normal; a sharp or lasting hurt in the elbow or wrist isn't. Stop, and see a doctor or a physical therapist.

How many sets and reps, and where it goes

• Two or three sets of 8 to 15, stopping with a rep or two in reserve, or closer: curls are a fine place to go near failure. (How to judge reps left.) The American College of Sports Medicine's 2026 position stand advises at least two sets per exercise.⁸
• Count it like any curl. It's a direct set for the elbow flexors in the arm workout's weekly count, and the same stand found muscle growth was enhanced by ten or more sets per muscle group a week.⁸
• After the rows and pulldowns, at the end of a pull day or an upper-body day, where the arm workout puts curls.
• One grip or two. Run palms-up curls in one block of weeks and hammer curls in the next, or one of each in the same week.
• Add weight at the top of the range. When every set reaches 15 clean reps, take the next dumbbell up and start again near 8. (Progressive overload covers how much.)

If you'd rather not plan it alone

A curl is easy to learn; deciding how many direct arm sets your week can carry on top of the rows and presses is the part a page can't do. Coaching is the version where your workouts are built around the days you actually have, and a weekly call looks at the week you just had and rebuilds the next one. Coaching to build muscle explains how that works, and here's how to apply. Otherwise: at your next upper-body session, swap your second curl for three sets of hammer curls and write down the weight.