Hammer Curls for Arm Thickness: Brachialis and Brachioradialis Training Explained

Hammer curls for arm thickness are worth doing, but the evidence is more modest than gym lore suggests. The neutral grip (palms facing each other) lowers biceps excitation compared with a supinated curl, so the brachialis and the forearm’s brachioradialis carry a relatively bigger share of the work. Those muscles grow with training. What nobody has shown yet is that hammers beat other curls for size.

This article sits inside our complete arm and forearm training guide. If you want the anatomy first, read how the arm flexors work and come back. Here we stick to what the exercise does, what the research supports, and how to perform it well.

What do hammer curls actually work?

Three muscles bend your elbow: the biceps brachii, the brachialis underneath it, and the brachioradialis, which runs down the thumb side of the forearm. In a standard supinated curl (palms up) the biceps does a lot of the job. Rotate your palms to face each other and the job gets shared differently.

Why the biceps? It does two things: it flexes the elbow and it turns the palm up (supination). Cadaver measurements and a computer model show the biceps flexion moment arm is larger when the forearm is supinated (Murray et al., 1995). In plain terms, the biceps has a better lever when your palm faces up and a worse one when it does not. That is a mechanical argument, drawn from two specimens and a model, so treat it as a plausible explanation and not a proof.

The brachialis attaches to the ulna, which does not rotate, so its job is simply to bend the elbow whatever your grip, which is why coaches call it the “workhorse” of elbow flexion.

Does the neutral grip really shift work off the biceps?

Partly, and the data are less tidy than the usual story.

In a 2023 EMG study, 10 competitive bodybuilders did bilateral curls (6-rep sets with an 8-RM load) using supinated, pronated and neutral grips. Biceps excitation on the way up was higher with the supinated grip than with the neutral grip, by about 12% (ES 1.24). Surprisingly for the usual narrative, brachioradialis excitation was also slightly higher with the supinated grip (by about 6%) than with the neutral grip (Coratella et al., 2023).

An older study points the other way for the deep muscles. When nine healthy subjects slowly rotated the forearm while holding the elbow bent, brachialis and brachioradialis EMG was higher with the forearm prone, while biceps was higher with the forearm supine (Naito et al., 1998). A third study in 10 trained men found similar activation across brachialis, both biceps heads and brachioradialis in both supination and pronation on a preacher bench, with no significant differences (Bankoff et al., 2007).

So three small studies, three slightly different answers. Why might that be? Different loads, different tasks (a rotation test versus a full curl), different electrode sites, and surface EMG on deep muscles like the brachialis is hard to interpret. A cautious reading: the biceps seems to do somewhat less in a hammer curl, the brachialis and brachioradialis are always involved, and the exact redistribution is not settled.

Hammer curls vs regular curls: which is better?

There is no head-to-head hypertrophy trial that I could find, so nobody can honestly say one is better.

What we do know: hammer curls were ranked lowest for biceps surface EMG among four curl types (concentration, dumbbell, hammer, incline) in a 2026 study of 15 adults at light-to-moderate loads. The ranking was stable across four sessions at the group level, but one-third of individuals showed only fair-to-moderate consistency (Croskery et al., 2026). Lower biceps activity at light load is not the same as worse growth, though. It may simply mean the work is spread more widely.

Here is the practical comparison.

	Supinated curl (barbell, EZ, dumbbell)	Hammer curl
Grip	Palms up	Palms facing each other
Biceps excitation (EMG)	Higher	Lower in the studies above
Brachialis / brachioradialis	Involved	Involved; share may rise
Wrist comfort	Can be harder on wrists	Often more comfortable
Hypertrophy evidence vs the other	None found	None found

The sensible answer is not “one or the other”. Use both in the same week. If you want a full comparison of angle and shoulder position, our incline vs preacher curl breakdown covers that side of arm training.

Do the brachialis and brachioradialis really grow?

Yes. This is the part that is well supported. In a 2026 MRI study, 21 untrained adults trained one arm with incline dumbbell curls and the other with preacher dumbbell curls for 12 weeks (5 sets of 10 reps at 70% 1RM, twice weekly). In the preacher arm, brachialis volume rose 21.1%, brachioradialis 17.6% and biceps 18.9%. In the incline arm the figures were 17.1%, 13.5% and 22.1% (Kobayashi et al., 2026).

Two cautions. These were untrained people, who grow quickly, so your numbers will be smaller. And the exercises were not hammer curls, so this tells us the muscles are trainable, not that hammers are the best way to train them. Note too that the different exercise setups favoured different muscles: incline curls slightly favoured the biceps, preacher curls the other two. That is a hint that variety across exercises may spread growth around.

Will a bigger brachialis make my arm look wider?

You will hear that the brachialis “pushes the biceps up”. Anatomically it lies directly beneath the biceps brachii, so a larger brachialis could add to the thickness of the front of the upper arm. That reasoning is sensible. But I found no study that measured visible arm width after hammer curl training, so it is an inference, not a finding.

My coaching view: do not expect hammer curls alone to change how your arm looks. Arm size mostly follows total elbow flexor and triceps volume, progressive overload, enough food, and time. The triceps makes up a large share of the upper arm too, so direct versus indirect arm volume matters more than any single curl variation.

Dumbbell hammer curl form

The cable version above is the same movement pattern. For the dumbbell version:

1. Stand tall with feet hip width, a dumbbell in each hand, palms facing your thighs. Brace your abs lightly.
2. Pin your elbows beside your ribs. They can drift forward slightly at the top; they should not swing.
3. Curl until your forearm is vertical or the dumbbell nears your shoulder. Keep wrists straight.
4. Squeeze for a beat, then lower over 2-3 seconds until your arm is fully straight.
5. Stop the set when the next rep would need body English.

Range of motion matters. A systematic review found that full-range reps, or partial reps in the stretched part of the range, produced more growth at distal biceps brachii and brachialis sites than partial reps in the shortened part (Kassiano et al., 2023). So lower fully until the elbow is almost straight instead of stopping halfway.

Rope cable hammer curls

Set a low pulley with a rope attachment. Hold the rope ends with a neutral grip, elbows at your sides, and curl until the rope ends reach shoulder height. Lower slowly.

Why use it? The cable keeps tension on the muscle throughout the rep, which many lifters find more consistent than a dumbbell at the very top and bottom. It is also easier to do strict reps for 10-15 reps. There is no study showing it grows more muscle than the dumbbell version, so the real reason to use it is feel and variety.

Cross-body and pinwheel hammer curls

In a cross-body (pinwheel) curl you curl one dumbbell across your torso toward the opposite shoulder, palm facing in, then alternate arms. It changes the path of the rep and many lifters feel it more in the outer biceps and forearm.

That is a feel claim, not a measured one. I know of no research on pinwheel curls specifically. Use them as a lighter variation, 10-15 reps, and keep the upper arm still so it does not become a swinging shoulder exercise.

How many sets and reps for hammer curls?

Start with 3-4 hard sets per week of hammers, in addition to your regular curls and the biceps work you already get from rows and pull-ups. That is a starting point, not a prescription.

The volume research is reasonably consistent. A meta-regression of 15 studies found a graded dose-response between weekly sets and muscle growth: each extra weekly set was associated with a small increase in gain, about 0.37% (Schoenfeld et al., 2017). Returns do diminish, and recovery counts. For reps, the repetition continuum has been revisited: muscle can grow across a wide range of loads when sets are taken near failure (Schoenfeld et al., 2021). So 8-15 reps with 1-3 reps in reserve is a good range.

Here is one way to place them in a pull day:

• Lat pulldown or row for 3 sets
• Supinated curl (EZ bar or incline dumbbell) for 3 sets of 8-12
• Hammer curl for 3 sets of 10-15

Our free pull day plan is built this way. Track effort with the RIR and RPE calculator if you are unsure how close to failure you are going.

Vegetarian lifters: do you need more protein to grow arms?

Arms respond to total training and total protein, not to a single exercise. If you eat mostly dal, curd, paneer, roti and rice, the main issue is usually total protein, not curl technique. Use the protein calculator to see your daily target, then spread it over 3-4 meals. Our biceps and triceps pairing guide shows how to fit arm sets into a busy session.

When to see a doctor

Muscle soreness for 1-3 days is normal. Sharp pain at the front of the elbow, pain that persists for weeks, a sudden pop in the upper arm, bruising, swelling, or tingling and numbness in the hand are not. Stop training the arm and get it examined by a doctor or physiotherapist. This article is education, not a diagnosis.

The bottom line

Hammer curls are a useful, wrist-friendly way to train the three elbow flexors with a slightly different emphasis. They are not a proven shortcut to bigger arms. Use them for 3-4 hard sets per week next to supinated curls, keep the elbows still, lower slowly, and judge the result by how your logbook moves over months. For the wider plan, return to the complete arm and forearm guide.