Curls primarily work the biceps brachii, the two-headed muscle on the front of your upper arm. But they also recruit several supporting muscles, including the brachialis (a deeper elbow flexor underneath the biceps), the brachioradialis (which runs along the top of your forearm), the anterior deltoid (the front of your shoulder), and the smaller flexor and extensor muscles of your forearm and wrist. How much each of these muscles contributes depends on your grip, arm position, and the type of curl you choose.
The Biceps Brachii: Your Primary Mover
The biceps has two heads, a long head on the outer side of your arm and a short head on the inner side, and both work together to bend your elbow and rotate your forearm so your palm faces up. During a standard dumbbell curl with palms facing forward, the biceps can reach activation levels up to 95% of its maximum capacity in the final portion of the lift. That makes curls one of the most effective ways to load this muscle through a full range of motion.
The two heads respond slightly differently depending on grip width. A narrow grip on a barbell places more emphasis on the short (inner) head, which contributes to the peak of the biceps when viewed from the front. A wider-than-shoulder-width grip shifts more work to the long (outer) head, which adds width and fullness when the arm is viewed from the side. Both heads are always active during any curl, but adjusting your grip can bias one over the other.
The Brachialis and Brachioradialis
Sitting directly underneath the biceps, the brachialis is actually the most powerful elbow flexor in the arm. It doesn’t care about forearm rotation the way the biceps does, so it fires hard regardless of your grip. Because it lies beneath the biceps, building the brachialis pushes the biceps outward and adds thickness to your upper arm that’s visible from the side.
The brachioradialis is the prominent muscle running from your outer elbow up toward the thumb side of your forearm. It assists with elbow flexion during all curls, but its role grows significantly when you change your grip. During a hammer curl (neutral grip, palms facing each other), the biceps produces about 12% less activation compared to a standard palms-up curl. That missing workload gets redistributed to the brachialis and brachioradialis. Reverse curls, performed with a palms-down grip, shift even more demand onto the brachioradialis and largely take the biceps out of the starring role.
How Grip Changes the Muscles Involved
Your hand position is the single biggest variable that determines which muscles a curl emphasizes. Here’s how each grip compares:
- Supinated (palms up): The classic curl grip. Produces the highest activation in both the biceps brachii and the brachioradialis. This is your best option for overall biceps development.
- Neutral (palms facing each other): The hammer curl grip. Biceps activation drops, but the brachialis and brachioradialis pick up more of the load. The anterior deltoid also works harder with this grip, likely compensating for the reduced biceps contribution to stabilize the shoulder.
- Pronated (palms down): The reverse curl grip. The biceps are at a significant mechanical disadvantage here. The brachioradialis becomes the primary driver, making this variation a forearm-focused exercise more than a biceps exercise.
Shoulder and Forearm Stabilizers
The anterior deltoid, at the front of your shoulder, stays active during every curl variation to stabilize the upper arm against the load. Its contribution is relatively small during palms-up curls but increases noticeably with neutral and pronated grips. Research published in Sports found that anterior deltoid activation jumped about 9% higher with a neutral grip compared to a supinated grip, a large effect size that reflects real differences in how hard the shoulder works across variations.
Your forearm flexors and extensors fire throughout every curl to keep your wrist stable. If the weight pulls your wrist into extension, the flexors on the underside of your forearm contract to hold it in place. This is why heavy curls can leave your forearms feeling pumped even though you’re not specifically targeting them. The heavier you go and the thicker the bar or handle, the more your grip muscles contribute.
Your core also plays a subtle stabilizing role during standing curls. Bracing your abdominals prevents your torso from swaying backward as you lift, which is why you’ll sometimes feel your midsection working during heavier sets. Seated and supported variations reduce core involvement but increase biceps isolation.
How Arm Position Shifts the Demand
Where your upper arm sits in space changes which part of the range of motion is hardest, and that affects muscle recruitment patterns in meaningful ways.
During a preacher curl, your arm is positioned in front of your body on an angled pad. This setup creates peak biceps activation at the very bottom of the movement, near full elbow extension, and the demand drops off quickly as you curl higher. The biceps long head works hardest at stretched positions close to full extension. Peak activation during preacher curls reaches about 80% of maximum, compared to 95% for standard and incline curls. That doesn’t make preacher curls inferior. It means they load the biceps differently, emphasizing the stretched portion of the range where many other exercises are weakest.
An incline dumbbell curl, performed on a bench set to about 45 degrees, places your arms slightly behind your torso. This stretches the long head of the biceps at the start of each rep and produces peak activation (up to 95% of maximum) in the final phase of the curl. If you want to train the biceps through a long range of motion with high activation at the top, incline curls are one of the most effective options available.
Concentration Curls for Maximum Isolation
An ACE-sponsored study comparing eight common biceps exercises found that the concentration curl produced significantly higher biceps activation than every other exercise tested. The reason comes down to isolation: your upper arm is braced against your inner thigh, which prevents swinging and removes the anterior deltoid from the equation almost entirely. In other exercises, the front of the shoulder tends to kick in naturally to stabilize or assist the lift, which dilutes how much work the biceps actually performs. Pinning the arm against the leg eliminates that compensation, forcing the biceps to handle the load on its own.
This doesn’t mean concentration curls are the only curl worth doing. They simply produce the purest biceps contraction. Standing curls, hammer curls, and preacher curls each train different supporting muscles and load different portions of the range, so a mix of variations covers more ground than any single exercise.
Putting the Variations Together
Every curl works the biceps, brachialis, brachioradialis, anterior deltoid, and forearm muscles to some degree. The variation you pick determines the ratio. A practical way to think about it:
- Standard supinated curls maximize total biceps activation.
- Hammer curls shift work toward the brachialis and brachioradialis, building forearm and upper arm thickness.
- Reverse curls prioritize the brachioradialis and forearm extensors.
- Preacher curls emphasize the stretched position of the biceps, loading the bottom of the range hardest.
- Incline curls also emphasize the stretch but peak at the top, hitting up to 95% activation in the final phase.
- Concentration curls isolate the biceps most completely by eliminating shoulder and momentum contributions.
If your goal is well-rounded arm development, combining two or three of these variations across your training covers both heads of the biceps, the brachialis, and the forearm muscles without needing a dozen different exercises.