The brachialis is not part of the biceps. It’s a separate muscle that sits underneath the biceps brachii, attached to different bones and performing a distinct mechanical role. The confusion is understandable because the two muscles are layered directly on top of each other in the front of your upper arm, and they both bend the elbow. But anatomically, they are independent muscles with separate origins, insertions, and slightly different functions.
Where the Brachialis Sits
The front of your upper arm contains three muscles: the biceps brachii, the brachialis, and the coracobrachialis. The brachialis lies deep to the biceps, meaning the biceps covers it like a blanket. If you could peel back the biceps, you’d find the brachialis pressed flat against the front surface of your upper arm bone (the humerus).
The brachialis starts on the lower half of the humerus and attaches to a bony bump on the ulna, your inner forearm bone. The biceps, by contrast, starts up at the shoulder blade and attaches to the radius, the outer forearm bone. This difference in attachment points is what makes them functionally distinct, even though both muscles cross the elbow joint.
How Their Functions Differ
Both muscles bend the elbow, but the brachialis does it more consistently. The Journal of Bone and Joint Surgery has described the brachialis as “the workhorse of the elbow joint,” noting that it fires during elbow flexion under all conditions, regardless of whether your palm is facing up, down, or sideways. It simply pulls the forearm toward the upper arm, every time.
The biceps brachii is more specialized. It’s strongest when your forearm is supinated (palm facing up) because one of its key jobs is actually rotating the forearm into that palm-up position. When your palm faces down or to the side, the biceps loses some of its mechanical advantage at the elbow, and the brachialis picks up more of the load. This is why the brachialis matters so much for real-world arm strength: it doesn’t care how your wrist is oriented.
Why It Matters for Arm Size
Because the brachialis sits directly beneath the biceps, a well-developed brachialis pushes the biceps upward and outward, making the entire upper arm look thicker. From a side view, much of your arm’s width comes from the brachialis rather than the biceps itself. If one of your arms looks noticeably bigger than the other, the difference is often due to uneven brachialis development rather than biceps size.
Genetics largely determine the shape of your biceps peak, but a larger brachialis can improve that peak by pushing the biceps up from underneath. For anyone training for arm aesthetics, ignoring the brachialis means leaving a significant amount of visual arm size on the table.
Exercises That Target the Brachialis
Standard biceps curls with your palms facing up emphasize the biceps brachii because of the supinated grip. To shift more work onto the brachialis, you want to reduce the biceps’ mechanical advantage by changing your grip.
- Hammer curls use a neutral grip (palms facing each other), which reduces the biceps’ role in supination and places greater demand on the brachialis and the brachioradialis in the forearm.
- Reverse curls with palms facing down further diminish biceps involvement, forcing the brachialis to do more of the heavy lifting.
- Preacher curls with a neutral or pronated grip isolate the elbow flexors by bracing the upper arm against a pad, limiting momentum and keeping tension on the brachialis throughout the range of motion.
The principle is straightforward: any time you move away from a palm-up grip, the biceps loses leverage and the brachialis compensates. Including one or two of these variations in your training ensures the muscle underneath the biceps grows alongside it.
Why People Confuse the Two
In casual conversation, “biceps” usually refers to the entire front of the upper arm. When someone flexes and points at the bump, they’re showing off the biceps brachii, the brachialis beneath it, and possibly the upper part of the brachioradialis in the forearm. All three contribute to that flexed-arm look, so it’s natural to lump them together.
The name doesn’t help either. “Brachialis” sounds like it should be part of “biceps brachii,” but the naming convention works differently. “Biceps brachii” means “two-headed muscle of the arm,” referring to its two attachment points at the shoulder. “Brachialis” simply means “of the arm,” describing its location on the humerus. They share Latin roots because they’re in the same region, not because they’re the same muscle.