Yes, bicep curls do work the brachialis, but standard supinated (palms-up) curls are not the most effective way to train it. The brachialis sits underneath the biceps brachii and is one of the strongest elbow flexors in your arm. It activates during every type of curl because it fires whenever you bend your elbow. However, the degree to which it contributes depends heavily on your grip position and the angle of the exercise.
What the Brachialis Actually Does
The brachialis originates on the front of your upper arm bone and attaches to the ulna, the non-rotating bone of your forearm. This attachment point is critical: because it connects to a bone that doesn’t rotate, the brachialis is a “pure flexor.” It bends your elbow and nothing else. It can’t help supinate (turn your palm up) or pronate (turn your palm down) the way the biceps brachii can.
This pure-flexor design makes the brachialis the strongest elbow flexor when your forearm is in a neutral or pronated position. During supinated movements, the biceps brachii gains a mechanical advantage and takes over more of the work. Research on elbow flexion torque estimates that the brachialis and brachioradialis together contribute roughly 57% of total flexion force during isometric contractions, with the biceps brachii contributing about 36%. During dynamic movements like curls, the biceps’ share increases to around 48% while the brachialis group drops to about 45%.
Why Standard Curls Undertrain It
A traditional bicep curl uses a supinated grip, palm facing up throughout the movement. This is the position where the biceps brachii has its greatest mechanical advantage. The brachialis still fires, because it has to, but the biceps is doing the lion’s share of the work. If your only arm exercise is palms-up barbell or dumbbell curls, you’re leaving brachialis development on the table.
The key principle is simple: the more you remove the biceps’ rotational advantage, the harder the brachialis has to work. When your forearm is in a semi-pronated position (thumbs up, as in a hammer curl) or fully pronated (palms down, as in a reverse curl), the biceps brachii loses leverage. Classic bodybuilding literature and biomechanics research both point to the same conclusion: all three elbow flexors work maximally when the weight is lifted with a semi-pronated forearm position.
Best Curl Variations for the Brachialis
Hammer curls are the most commonly recommended exercise for targeting the brachialis. Holding the dumbbell with a neutral grip (thumbs pointing up) reduces the biceps’ mechanical advantage while keeping the brachialis in a strong line of pull. This is well established in both training manuals and exercise science.
Reverse curls, performed with a pronated (palms-down) grip on a barbell or EZ-bar, push the emphasis even further away from the biceps. You’ll notice you need to use significantly less weight on reverse curls compared to standard curls. That drop in load reflects how much the biceps normally contributes and how much more the brachialis and brachioradialis are picking up.
Preacher curls also deserve mention, though the mechanism is different. By placing your arms over an angled pad with your shoulders flexed forward, preacher curls change the resistance curve so that the hardest part of the movement occurs when the elbow is near full extension. Research suggests that exercises placing the highest strain on the elbow flexors when the muscles are in a lengthened position may stimulate the brachialis more than the biceps. In some study participants, brachialis thickness near the elbow was twice as large as its thickness at mid-arm, confirming that this muscle dominates the lower portion of the upper arm.
Why Brachialis Size Matters for Arm Appearance
The brachialis is more prominent in the lower, outer portion of the upper arm. Because it sits underneath the biceps, developing it literally pushes the biceps up and outward, creating the appearance of a wider, thicker arm. A well-developed brachialis adds visible mass to the outside of the arm between the biceps and triceps, filling in the area that looks flat on people who only train with standard curls.
This is especially relevant for anyone who feels their arms look narrow from the front despite having decent biceps peaks. The brachialis is the muscle responsible for that “3D” look when the arm is viewed from the side or in a relaxed position. In competitive bodybuilding, the size and symmetry of the brachialis are specifically evaluated, and exercises that load the muscle in a lengthened position are considered the best option for targeting the portion closest to the elbow.
Programming for Full Elbow Flexor Development
You don’t need to abandon standard bicep curls. They still activate the brachialis to some degree, and they’re the best choice for overall biceps brachii development. The practical approach is to include at least one neutral-grip or pronated-grip exercise in your arm training. A straightforward combination might look like supinated dumbbell curls for the biceps, hammer curls for the brachialis, and reverse curls for the brachioradialis, covering all three major elbow flexors across different grip positions.
Incline curls (performed on a bench set to about 45 degrees) are another useful tool. The inclined position places the shoulder in extension, which pre-stretches the long head of the biceps and shifts some of the workload to the brachialis. Pairing incline curls with hammer curls gives you two exercises that both bias the brachialis through different mechanisms: one through shoulder angle, the other through grip position.
Because the brachialis is a pure flexor with no rotational component, it responds to straightforward progressive overload. Heavier hammer curls and controlled reverse curls over time will grow it. There’s no need for exotic isolation techniques. Just make sure your grip isn’t always supinated, and the brachialis will get the stimulus it needs.