You’ve probably spent way too much time staring at your biceps in the mirror. Most people do. We obsess over the "peak" of the arm, but we completely ignore the muscle side of arm—that lateral view that actually defines how your upper body looks and functions. It's the outer terrain. Honestly, if you aren't paying attention to the brachialis or the lateral head of the triceps, you’re basically leaving half your strength on the table. Your arm isn't just a front-and-back hinge; it's a complex stack of fibers that wrap around the humerus, and the stuff on the side is often what determines if you'll end up with "tennis elbow" or shoulder impingement later down the road.
Think about the way you lift a heavy grocery bag. It’s not just the bicep. Your brachioradialis—that meaty muscle on the top of your forearm that extends up the side of the elbow—is doing the heavy lifting. When we talk about the lateral side, we are looking at a cross-section of stability.
The Brachialis: The Secret "Side" Muscle You Can't See
Most people think the bicep is the only muscle on the front/side of the arm. It isn't. Deep underneath the biceps brachii lies the brachialis. It's the true workhorse of elbow flexion. If the bicep is the show pony, the brachialis is the plow horse. Interestingly, the brachialis doesn't attach to the radius (the bone that rotates); it attaches to the ulna. This means its only job is to bend the arm, and it does it with more raw force than the bicep ever could.
When this muscle grows, it literally pushes the bicep up. It makes the arm look wider from the side. If you’ve ever seen a bodybuilder with a "split" look between their bicep and tricep, you’re looking at a well-developed brachialis. It's the literal muscle side of arm anchor. To hit it, you have to change your grip. Neutral grip—like hammer curls—takes the bicep out of the equation slightly because of the mechanical disadvantage of the supinator muscles, forcing the brachialis to take the brunt of the load.
Experts like Dr. Mike Israetel often point out that the brachialis is one of the most underrated muscles for overall arm thickness. It's not just about aesthetics, though. A weak brachialis means your elbow joint is taking more stress during heavy rows or pull-ups. If you've got nagging pain right in the "pit" of your outer elbow, you might just have a chronically tight or weak brachialis.
Anatomy of the Lateral Triceps Head
Turn your arm to the side. That "horseshoe" shape? The outer part of that shoe is the lateral head of the triceps. This is arguably the most visible muscle side of arm component when you're wearing a t-shirt. While the long head of the triceps provides the bulk on the back of the arm, the lateral head provides the "pop."
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It’s a thick, powerful slab of muscle. It originates on the posterior surface of the humerus and is primarily responsible for terminal extension—that last bit of "snap" when you straighten your arm. If you’re a powerlifter, this is your lockout muscle. Without a strong lateral head, your bench press is going to stall about four inches off your chest.
Why Your Triceps Side Might Be Lagging
- You're doing too many overhead extensions (which favor the long head).
- Your grip is too wide on pressing movements.
- You aren't reaching full lockout on your reps.
- The mind-muscle connection just isn't there because you can't see the muscle while you're working it.
To really target this area, you need to think about pushdowns with a straight bar or a V-bar. Using a rope attachment is great for a range of motion, but sometimes the instability of the rope prevents you from using the kind of heavy weight that the lateral head needs to grow.
The Brachioradialis and the Forearm Connection
We can’t talk about the side of the arm without moving down to the elbow. The brachioradialis is that thick muscle that starts on the lower third of the humerus (the upper arm bone) and runs down to the wrist. It’s unique. It's technically a forearm muscle, but it lives on the muscle side of arm transition zone.
It’s the muscle that makes your arms look powerful when you're wearing a watch. More importantly, it bridges the gap between your grip strength and your upper arm strength. If you have "skinny" elbows, it's usually because the brachioradialis is underdeveloped.
This muscle is most active when your palm is facing down (pronation) or in a neutral position. Think about how you pick up a box. Your palms aren't usually facing the sky; they're facing each other. That’s the brachioradialis working. If you only ever do standard palm-up curls, you are completely neglecting the lateral stability of your elbow. This leads to imbalances. If the muscles on the inside of your arm (the flexors) are way stronger than the muscles on the side and back, you’re basically begging for medial epicondylitis. Not fun.
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Functional Reality: It’s Not Just About Looking Good
Let's get real for a second. Most people want "tone." But the lateral side of the arm is about survival for your joints. The lateral collateral ligament (LCL) of the elbow is supported by the surrounding musculature. When you have strong side-arm muscles, your elbow is less likely to hyperextend or suffer from valgus stress.
Athletes who throw—pitchers, quarterbacks, even javelin throwers—rely on the decelerating power of the lateral arm. When you whip your arm forward, something has to stop it from flying out of the socket. That tension is distributed across the triceps and the brachialis. If those muscles are weak, the connective tissue takes the hit. That's how you end up in physical therapy.
Actually, a lot of "shoulder" pain is actually referred pain from the lateral triceps. If you have a trigger point in the lateral head of the triceps, you’ll often feel it as a dull ache in the back of your shoulder. You might think you have a rotator cuff issue, but you actually just need to roll out the side of your arm with a lacrosse ball.
Common Misconceptions About Side-Arm Training
People think that to get a wide arm, you just need more biceps. Wrong. The bicep is a relatively thin muscle when viewed from the side. To get "width," you need to develop the depth of the arm. This means focusing on the brachialis to push the bicep out and the lateral tricep to create the outer curve.
Another mistake is overtraining the "top" of the arm and forgetting the "side-bottom." The way the triceps and the brachialis interact creates a "V" shape on the side of the humerus. If you only train one side of that V, the arm looks lopsided. It looks "flat."
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- The "High Rep" Myth: People think small muscles need 20+ reps. No. The lateral head of the tricep has a high percentage of fast-twitch fibers. It likes heavy weight. It likes to be pushed.
- The "Isolation" Trap: You don't need 5 different types of side-arm raises. You need heavy, neutral-grip pulls and heavy, close-grip presses.
- The "Wrist" Confusion: Some think turning the wrist during a curl changes the bicep. It actually changes which muscle side of arm is being prioritized (brachialis vs. brachioradialis).
Fixing Your Lateral Arm Deficit
If you want to actually fix this, you have to be intentional. You can't just "show up" and do some curls. You need to look at your programming. Are you doing any neutral grip work? If the answer is no, your side arm is probably weak.
Try this. Next time you do pull-ups, use the parallel bars (palms facing each other). Feel that strain on the outside of your elbow? That's the brachioradialis and brachialis screaming because they aren't used to the load. That's the gap in your armor.
- Switch to Hammer Griz: Do your curls with a neutral grip. Don't twist your wrist at the top. Keep it locked.
- Close-Grip Bench Press: This is the king of lateral tricep development. Keep your elbows tucked to your ribs.
- Cross-Body Hammer Curls: Instead of curling straight up to your shoulder, curl the dumbbell toward your opposite chest. This puts a massive stretch on the lateral fibers.
- Reverse Curls: Use an EZ-bar. Palms facing down. It’ll feel weird. Your weights will drop by 40%. That’s okay. It’s targeting the exact muscles we’re talking about.
Practical Steps for Long-Term Health
Stop ignoring the side of your arm. It's the bridge between your shoulder and your hand.
First, check your elbow mobility. If you can’t fully straighten your arm without pain, your lateral tricep or brachialis might be chronically shortened. Spend five minutes a day just hanging from a pull-up bar to let those tissues stretch under load.
Second, incorporate "iso-holds." At the top of a hammer curl, hold the weight for 3 seconds. The brachialis responds incredibly well to time under tension.
Third, massage the area. Take your thumb and press it into the space between your bicep and tricep on the outside of your arm. Find a sore spot? Hold it. Breathe. This is called myofascial release, and for the muscle side of arm, it’s a game-changer for elbow health.
Lastly, stop overthinking the "perfect" workout. Just add one neutral-grip movement and one overhand-grip movement to your current routine. Consistency over intensity. You’ll see the thickness change in about six weeks. Your elbows will feel "sturdier," and your shirts will fit differently. That's the reality of training the lateral side. It's not flashy, but it’s the foundation.