Reverse curls primarily work the brachioradialis, the thick muscle running along the top of your forearm, along with the brachialis and biceps brachii. The simple act of flipping your grip from palms-up to palms-down shifts the workload away from your biceps and onto muscles that standard curls tend to underwork.
The Brachioradialis: The Main Target
The brachioradialis runs from just above the outer elbow to the base of the wrist near the thumb side. It flexes the elbow and keeps the forearm in a neutral position. During a standard supinated (palms-up) curl, this muscle plays a supporting role. But when you flip to a pronated (palms-down) grip, the brachioradialis becomes the dominant elbow flexor. That’s why reverse curls are considered the single best exercise for building visible forearm thickness on the outer, upper portion of the forearm.
Electromyography (EMG) research comparing curl variations confirms this shift. During the concentric (lifting) phase specifically, the brachioradialis shows significantly higher activation during reverse curls compared to standard dumbbell curls. The difference is even more pronounced during the eccentric (lowering) phase, where reverse-grip variations produced markedly greater brachioradialis activity than conventional curls.
The Brachialis: Hidden Size Builder
Sitting underneath the biceps, the brachialis is invisible from the surface but plays a major structural role in arm size. It’s the true prime mover of elbow flexion, meaning it generates force any time you bend your arm regardless of wrist position. During reverse curls, the brachialis works hard because the pronated grip reduces how much the biceps can contribute, forcing the brachialis to pick up the slack.
A well-developed brachialis pushes the biceps outward and upward, making your upper arm appear thicker from the front and wider from the side. If you’ve been doing standard curls for months and your arms still look narrow, the brachialis is likely the missing piece.
Why the Biceps Work Less
Your biceps brachii is a powerful elbow flexor and forearm supinator. When your palm faces up, the biceps is in its strongest mechanical position. Flip your hand over and something changes: the distal biceps tendon wraps around the radius bone, and the space available for the tendon at its insertion point drops by roughly 48% compared to a fully supinated position. This mechanical disadvantage doesn’t take the biceps out of the movement entirely, but it significantly reduces the force the biceps can produce.
EMG data supports this. Biceps activation during reverse curls is consistently lower than during standard curls, particularly in the concentric phase where no statistically significant difference was found between curl types for the biceps. The biceps still fires, just not as the lead muscle. Think of reverse curls as a biceps exercise where the biceps takes a back seat while the forearm and deeper arm muscles do the heavy lifting.
Forearm Stabilizers and Wrist Extensors
One muscle group that gets overlooked in discussions of reverse curls is the wrist extensors. When you hold a bar with a pronated grip and curl it upward, gravity pulls your wrist into flexion. To keep your wrist locked in a straight, strong position throughout the rep, your wrist extensors have to fire continuously. These include several muscles running along the back of your forearm that control wrist extension and finger straightening.
This constant stabilization demand is part of why reverse curls feel so much harder on your forearms than standard curls, even at lighter loads. It’s also why the exercise is so effective at building the kind of balanced forearm development that reduces injury risk at the elbow and wrist.
Grip Strength Under Constant Tension
Reverse curls challenge your grip in a way standard curls simply don’t. With a palms-up grip, the bar naturally rests in the cradle of your fingers and palm. Flip to palms-down and you have to actively squeeze the entire time to keep the bar from rolling out of your hands. This creates constant tension through every finger flexor and forearm muscle involved in gripping.
That sustained grip demand translates directly to performance on other lifts. Deadlifts, pull-ups, rows, and farmer’s carries all depend on grip endurance. Adding reverse curls to your routine builds the forearm stamina that keeps your grip from being the weak link in those heavier compound movements.
EZ Bar vs. Straight Bar for Reverse Curls
You can perform reverse curls with a straight barbell, an EZ bar, or dumbbells, and each option changes the experience slightly. A straight bar forces your wrist into full pronation, which maximizes brachioradialis involvement but also creates more torque at the wrist joint. Some people find this uncomfortable, especially at heavier loads or with pre-existing wrist issues.
An EZ bar has angled bends that let your wrists sit in a semi-pronated position, splitting the difference between full pronation and neutral. This reduces wrist strain while still heavily recruiting the brachioradialis and forearm extensors. EMG research found that the EZ bar variant produced higher activation in both the brachioradialis and biceps compared to standard dumbbell curls across the full range of motion. For most people, the EZ bar offers the best balance of muscle activation and joint comfort.
Dumbbells allow a neutral (hammer) or semi-pronated grip and let each arm work independently, which can help correct side-to-side imbalances. However, they don’t lock you into a true pronated position the way a barbell does, so the stimulus shifts slightly toward a hammer curl pattern.
Form Details That Protect Your Joints
The reverse curl is an isolation exercise, and the only joint that should move is the elbow. Your upper arms stay pinned at your sides throughout the rep. If your elbows drift forward or your shoulders swing the weight up, you’ve shifted the load away from the target muscles and onto momentum.
Wrist position matters more here than in almost any other curl variation. Your wrists should stay straight and locked throughout the movement. If you let your wrists flex or extend under load, you lose tension in the brachioradialis and put unnecessary stress on the small bones and tendons of the wrist. Focus on squeezing the bar tightly and keeping your knuckles pointed forward as you curl.
Weight selection is another common stumbling point. You will not be able to reverse curl anywhere near what you can standard curl. Most people need to drop the load by 30 to 40 percent. Starting too heavy leads to wrist strain, body English, and poor muscle targeting. Use a weight you can control for clean reps through the full range of motion, and increase gradually as your forearm strength catches up.
Muscles Worked: Quick Summary
- Brachioradialis: Primary mover, responsible for the forearm-building effect reverse curls are known for
- Brachialis: Deep elbow flexor that adds upper arm thickness beneath the biceps
- Biceps brachii: Still active but in a mechanically weaker position, contributing less force than during standard curls
- Wrist extensors: Fire continuously to stabilize the wrist against gravity throughout each rep
- Finger flexors and grip muscles: Work overtime to maintain a secure hold on the bar with a pronated grip