You’re looking in the mirror, flexing, and wondering why your arms still look a bit... flat. Even though you’ve been smashing the bicep curls. Most people think "arm" and immediately jump to the biceps brachii or the triceps. But there is a specific muscle on side of arm that acts like the structural foundation for everything else. It’s called the brachialis. Honestly, if you don't know what it is, you're basically leaving half your arm thickness on the table. It sits right underneath the bicep. Think of it like the engine under the hood of a car; you don't see it directly, but it’s doing all the heavy lifting and making the exterior look a whole lot more impressive.
The brachialis is a workhorse. Unlike the bicep, which gets all the glory and handles multiple movements like rotating your forearm, the brachialis has one job. It flexes the elbow. That’s it. But because it sits deeper than the bicep, when it grows, it literally pushes the bicep upward. This creates that "peak" everyone wants. It also fills out the space between the bicep and tricep, which is exactly what gives you that thick, powerful look from the side.
What is the Brachialis Exactly?
Anatomy can get boring, so let's keep it simple. The brachialis originates on the lower half of the humerus—that's your upper arm bone—and attaches to the ulna in your forearm. This is a big deal. Why? Because the bicep attaches to the radius, which is the bone that rotates. When you turn your palm up (supination), the bicep is at its strongest. But the brachialis doesn't care which way your palm is facing. It's always ready to pull.
Actually, the brachialis is arguably the strongest flexor of the elbow. It has a larger cross-sectional area than the bicep in many people. It's the "pure" elbow flexor. If you’ve ever felt a deep ache on the outer side of your arm after doing heavy hammer curls or pull-ups, you weren't just feeling your bicep. You were feeling this deep-tissue powerhouse.
Most lifters ignore it. They focus on "the pump" in the bicep. They do endless sets of standard curls with their palms facing the ceiling. This is fine, but it’s incomplete. To really target that muscle on side of arm, you have to change your grip. It's about leverage. By putting the bicep at a mechanical disadvantage, you force the brachialis to take over the load.
The Side-Arm Aesthetic: Brachialis vs. Brachioradialis
Wait. We need to clear something up. People often confuse the brachialis with the brachioradialis. They sound similar. They live in the same neighborhood. But they aren't the same thing.
The brachioradialis is that meaty muscle that starts on your upper arm but mostly lives on your forearm. It’s the one that pops when you’re gripping a heavy suitcase. The brachialis, however, is higher up. It’s the bridge. When you look at a bodybuilder like Dorian Yates or Phil Heath, that thick "knot" on the side of the arm between the bicep and tricep? That's the brachialis.
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It’s the difference between having "skinny-fat" arms and arms that look like they were carved out of granite.
How to Actually Target the Muscle on the Side of Your Arm
Stop doing standard curls for a second. If you want to grow this specific area, you need to master the neutral grip. Hammer curls are the gold standard here. When your palms face each other, the bicep isn't in its most powerful position. This forces the brachialis to do the brunt of the work.
But there’s a trick. Don't just swing the weights.
Slow down the eccentric phase—that’s the way down. The brachialis responds incredibly well to time under tension. Research, including studies often cited by hypertrophy experts like Dr. Brad Schoenfeld, suggests that controlled eccentric movements are key for deep muscle growth.
- Reverse Curls: Use an EZ-bar or a straight bar. Palms face down. This is the hardest way to curl, but it’s the most effective for the outer arm complex.
- Cross-Body Hammer Curls: Instead of curling the dumbbell straight up to your shoulder, bring it across your chest toward the opposite shoulder. This slight change in angle hits the brachialis from a different perspective.
- Slow-Tempo Pull-ups: If you're a calisthenics fan, use a neutral grip (palms facing each other) and take three full seconds to lower yourself. Your arms will scream.
You’ve probably seen people in the gym using those thick "Fat Gripz" on the bar. There’s a reason for that. Increasing the diameter of the handle increases motor unit recruitment in the forearms and the side-arm muscles. It makes a 20lb dumbbell feel like 40lb. It's brutal. It works.
Why Your Elbows Might Be Hurting
If you’re feeling a sharp pain on the side of your arm, it might not be the muscle at all. It could be the tendon. Tendonitis in the elbow (often called Tennis Elbow or Golfer’s Elbow, depending on the side) is a common byproduct of overtraining the brachialis without proper recovery.
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The brachialis is a "fast-twitch" dominant muscle for many, meaning it can generate a lot of power but it also gets trashed easily. If you suddenly add five sets of heavy hammer curls to every workout, your tendons are going to rebel.
Listen to your body. If the "side of your arm" feels like it's being poked with a hot needle whenever you pick up a coffee cup, you need to back off. Use ice. Work on forearm flexibility. Maybe try some eccentric-only loading with very light weight to stimulate blood flow to the tendons without further damaging the tissue.
The Genetic Factor: Can Everyone Get That "Side Pop"?
Let's be real. Some people have high bicep insertions. This means there’s a big gap between their bicep and their elbow. If you have this, you might feel like your arms look "small" from the front. But here’s the silver lining: high insertions leave more "room" for the brachialis to be visible.
If you have long muscle bellies (low insertions), your arms will look naturally thicker, but it might be harder to see the individual definition of the muscle on the side of the arm.
Regardless of your DNA, you can improve what you have. You aren't stuck. You just need to stop training like a beginner who only cares about the "show" muscles.
The Science of Hypertrophy for Deeper Muscles
Muscle growth isn't just about lifting heavy stuff. It's about metabolic stress and mechanical tension. Because the brachialis is tucked away, it can be hard to "feel" it working. This is where the mind-muscle connection comes in.
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Next time you do a hammer curl, don't think about the weight. Think about the space between your bicep and tricep. Visualize that muscle contracting. Sounds "bro-sciencey," but internal focus has been shown in various sports science studies to actually increase muscle fiber activation.
Also, variety matters. Your body is an adaptation machine. If you only do hammer curls every Monday, your brachialis will eventually stop responding. Switch to cable rope curls. Try preacher hammer curls. Change the reps. Go for 15-20 reps one week to flood the area with blood, then drop to 6-8 reps the next week to focus on sheer mechanical tension.
Common Mistakes to Avoid
- Using too much momentum. If you have to swing your hips to get the weight up, your brachialis isn't doing the work. Your lower back is.
- Neglecting the triceps. The brachialis sits next to the lateral head of the tricep. If your triceps are weak, the side of your arm will never look "complete."
- Ignoring grip strength. Your grip is the bottleneck. If your hands give out before your arms do, you’ll never reach the level of intensity needed for real growth.
Actionable Steps for Noticable Change
If you want to see that muscle on side of arm pop within the next eight weeks, here is exactly what you should do:
Start by adding "Neutral Grip Day" to your routine. Once a week, every single pulling or curling movement you do must be with a neutral (palms facing each other) or overhand grip. This means neutral grip pull-downs, hammer curls, and reverse curls.
Specifically, try the "Mechanical Drop Set." Start with a heavy set of reverse curls (palms down) until you can’t do another rep with good form. Immediately—without resting—switch to a hammer grip (palms facing each other) and go to failure again. Finally, flip your palms up and finish with standard curls. This sequence systematically fatigues the brachialis and the brachioradialis before finishing off the biceps.
Don't forget to stretch. A tight brachialis can actually limit your range of motion in other lifts. Extend your arm fully, turn your palm down, and gently pull your fingers back toward your elbow. You’ll feel it right where you need it.
Lastly, check your volume. If you’re already doing 20 sets of arms a week, adding more might just lead to injury. Swap out some of your standard bicep work for these targeted side-arm movements instead. Quality over quantity, always. You'll start noticing the thickness when you're wearing a t-shirt; the sleeve will start feeling a bit tighter in the "wrong" places. That’s when you know it’s working. Keep the tension high, the reps controlled, and the ego in check. Your arms will thank you for it.