The hammer curl is the arm exercise that builds the thickness a regular curl misses — hitting a muscle under your biceps and your forearms at the same time. If your arms look flat from the side, this is likely what's missing. Here's the honest guide. A hammer curl is a curl performed with a neutral grip (palms facing each other), which trains the biceps, the brachialis underneath, and the forearms — building arm thickness and grip that a standard palms-up curl doesn't. Here's how to do it and why it belongs in your arm training.
What the hammer curl trains (the honest mechanics)
The hammer curl uses a neutral grip — palms facing each other, like holding two hammers — and this hand position changes which muscles work. It trains the biceps, but crucially it also heavily hits the brachialis (a muscle that sits underneath the biceps) and the brachioradialis (a forearm muscle). Building the brachialis actually pushes the biceps up and adds thickness and width to the arm, while the forearm involvement builds grip. This makes the hammer curl a key complement to standard curls for complete, thick arms — a natural partner to the preacher curl and incline curl in any bigger-arms plan.
Proper form (the movement)
Stand holding a dumbbell in each hand, arms at your sides, palms facing your body (neutral grip). Then: curl the weights up while keeping your palms facing each other the whole time (don't rotate your wrists). Keep your elbows tucked at your sides and your upper arms still. Squeeze at the top, then lower under control to a full stretch. You can curl both arms together or alternate. The cues: neutral grip throughout, elbows pinned at your sides, control the weight, full range. The palms-facing position is what makes it a hammer curl — keep it.
Hammer curl vs regular curl (the honest difference)
The distinction is the grip, and it matters. A regular (supinated) curl has palms facing up, which maximally targets the biceps peak. A hammer curl has palms facing each other, which shifts emphasis to the brachialis and forearms while still working the biceps. The practical result: regular curls build the biceps peak, hammer curls build arm thickness and forearm size. Honest takeaway: they're complementary, not competing — do both. Regular curls for the peak, hammer curls for thickness and grip. Most people only do palms-up curls and miss the thickness hammer curls provide, which is why their arms look flat from the side.
The swing-and-ego sin (why it stops working)
The classic curl mistake applies here too: swinging your body and using momentum to heave the weights up. Because hammer curls let you use slightly more weight than regular curls (the brachialis is strong), people load too heavy and start rocking their torso and flinging the dumbbells, which takes the work off the target muscles. The fix: keep your elbows pinned at your sides, your torso still, and curl with control. If you're swinging or your elbows are drifting forward, lighten the weight. Hammer curls reward strict form and full range, not ego. Feel the biceps and forearms work on every rep.
Where it fits (pair accordingly)
Use hammer curls as a bicep and forearm accessory on an arm or pull day. Pair them with palms-up curls (standing, incline, or preacher) so you build both the biceps peak and arm thickness, and balance with tricep work. As a bonus, they double as forearm and grip builders. Use controlled, moderate weight for moderate-to-high reps. Progress with progressive overload. They build the muscle; a calorie deficit reveals the definition.
Quick answers
What does the hammer curl work?
The biceps, plus the brachialis (underneath the biceps) and forearms — building arm thickness and grip.
Hammer curl or regular curl?
Both — regular curls build the biceps peak, hammer curls build thickness and forearms; they're complementary.
Why do hammer curls build thicker arms?
They target the brachialis, which pushes the biceps up and adds width, plus the forearms.
Should I rotate my wrists?
No — keep your palms facing each other the whole time; rotating turns it into a regular curl.
Can I use more weight on hammer curls?
Usually slightly more than regular curls, since the brachialis is strong — but stay controlled.
Do hammer curls work forearms?
Yes — the neutral grip heavily involves the forearm muscles.
How many reps?
Moderate-to-high reps (8–15) with controlled form work well.
Palms-up curls build the peak. Hammer curls build the thickness. Skip the hammers and your arms stay flat from the side.
The hammer curl is the simple grip change that builds the arm thickness and forearm size a standard curl misses. Keep a neutral grip, pin your elbows, and control every rep. Pair it with palms-up curls for complete arms, and you'll build biceps that look full from every angle — not just flat from the front.