Functional Anatomy of the Arm Flexors: Biceps Brachii, Brachialis, Brachioradialis

Arm flexors anatomy comes down to three muscles. The biceps brachii, the brachialis and the brachioradialis all bend your elbow, but they do it from different positions, with different levers, and they react differently when you turn your wrist.

Most lifters train “biceps” and stop there. That leaves two thirds of the elbow flexor team to chance. This guide maps the muscles one by one, shows what the research says about grip, and ends with practical curl choices. It sits inside our complete arm and forearm training guide, which covers the triceps and forearms too.

What are the arm flexors and where do they sit?

Elbow flexion has three main drivers. Here is the short map.

Biceps brachii. The showy one. It has a long head and a short head, both of which cross the shoulder and the elbow. It attaches low on the radius, one of the two forearm bones. Because the radius rotates when you turn your palm, the biceps does two jobs: it bends the elbow and it supinates the forearm (palm up). A 2004 review of human electrophysiology describes the biceps as acting not only as a flexor but also as a forearm supinator, which makes its activity “much different” from the other two flexors (Naito, 2004).

Brachialis. It lies deep, directly under the biceps, on the front of the upper arm bone. A StatPearls anatomy entry describes it as providing elbow flexion at all physiologic positions and as a “pure flexor” of the forearm at the elbow (Plantz et al., 2023). It attaches to the ulna, the forearm bone that stays put when you rotate your palm. Which is why grip cannot take it out of the picture.

Brachioradialis. The forearm one. Its belly sits on the thumb side of your forearm and attaches near the wrist end of the radius. Textbooks have long called it a forearm supinator, but a 2018 EMG study set out to test whether it is also one of the main elbow flexors. We will get to that below.

One note on “forearm flexors”. People searching that phrase usually mean the muscles on the palm side of the forearm that curl your wrist and fingers. Those are a different group. The brachioradialis is the odd one out: it lives in the forearm but bends the elbow. Wrist and grip work gets its own treatment in forearm flexor and extensor hypertrophy.

Why does the biceps work differently from the other two flexors?

Because it crosses two joints and rotates the forearm.

Think of lifting a heavy bag with your palm facing the ceiling. The biceps is perfectly set up: it bends the elbow and holds the forearm in supination at the same time. Now turn the palm to face the floor and try the same lift. You are weaker, and the biceps loses its supination advantage.

A small study measured this. In 11 young men, elbow flexion in supinated and neutral forearm positions was about 50% stronger than in the pronated position, and the supinated and neutral positions were about 60% steadier (Smart et al., 2018). It is a small sample of young men in a lab, so treat the exact figures loosely. The direction matches what you feel in the gym.

How does grip change which flexor works hardest?

It changes the share of the work, not who shows up.

A 2018 study of 15 young men recorded biceps, brachialis and brachioradialis EMG during maximal isokinetic elbow flexions with the forearm neutral, fully pronated and fully supinated. All three muscles were very active throughout resisted flexion in all three positions (Caufriez et al., 2018). Brachioradialis activity peaked at the end of flexion in about 73% of subjects and was slightly higher in supination. The authors conclude the brachioradialis is one of the main elbow flexors, especially in supination.

That sits awkwardly next to the older Naito data, where brachialis and brachioradialis contraction levels were higher with the forearm prone and the biceps’ were higher supine. So what should you take from the disagreement? The studies used different tasks (slow forearm rotation with or without a light load vs maximal isokinetic flexion), different joint angles and tiny samples. Surface EMG also picks up crosstalk between neighbouring muscles, so “which one is highest” is shakier than “all of them are working”.

A 2025 cross-sectional study of 36 healthy men used 1 kg loaded isometric holds at five elbow angles. Biceps surface EMG was higher in supination than neutral and higher in neutral than pronation across 30-90 degrees of elbow flexion, and the brachioradialis responded to both forearm rotation and elbow angle (Chen et al., 2025). Light load, static holds, no training outcome. Useful for the biceps trend, not for programming.

What about trained lifters? Ten men with at least a year of lifting experience did preacher-bench curls at 80% of their maximum in supinated and pronated positions. Activation of the brachialis, both biceps heads and the brachioradialis was similar, with no significant differences between muscles (Bankoff et al., 2007). The authors read this as all the flexors working together to resist the load. Small sample again, and it did not look at growth.

Does the brachialis really “push the biceps up”?

This is the popular claim, so here is what we can and cannot say.

What is anatomically solid: the brachialis sits directly beneath the biceps. A larger deep muscle takes up more room, and the muscle above it has to sit on top of it. That is why many coaches say a developed brachialis adds “height” and thickness to the upper arm.

What is not proven: nobody in the studies reviewed here measured whether growing the brachialis makes the biceps visibly higher or the arm measurably thicker. The claim is a reasonable inference, not a tested result. Treat it as a plausible bonus, not a promise.

What we do know is that the brachialis grows when you curl. In a 2025 randomised trial, 15 young men trained one arm with a preacher-style cable curl and the other with a Bayesian cable curl (shoulder extended) for 10 weeks. Brachialis thickness rose by roughly 10% and 8% respectively, with no significant difference between the two curl types (Attarieh et al., 2025).

Same study, biceps thickness: 6-7% for the preacher arm and 9% for the Bayesian arm across the proximal, mid and distal regions, again with no significant difference. Fifteen young men is a small sample, and the program was only 10 weeks, so do not read this as the final word on curl variation. It does tell us that curling grows both muscles, and that a shoulder position difference did not matter much when resistance profiles were matched. For the shoulder position question in more depth, see long head vs short head biceps.

What does this mean for the curls you pick?

Short version: use more than one grip.

A neutral grip is the standard way to ask more of the brachialis and brachioradialis. Our guide to hammer curls for upper arm thickness covers technique and loading.

A supinated preacher curl is the classic biceps builder, and a bench supports the upper arm so momentum has fewer places to hide. If the long-length position is what you want, incline dumbbell curls load the biceps with the shoulder behind the body.

A simple template, not a prescription:

Grip	Leans on	Example exercises
Palm up (supinated)	Biceps brachii	Preacher curl, incline curl, barbell curl
Neutral (thumbs up)	Brachialis, brachioradialis, biceps	Hammer curl, rope curl
Palm down (pronated)	Brachioradialis, brachialis	Reverse curl

Pick one exercise from the supinated row and one from the neutral row most weeks. Add the pronated one if your forearms lag or your grip fails first on pulls. Rows and pulldowns already hit the elbow flexors, so check the weekly total with the training volume calculator before you pile on curls, and read direct vs indirect arm volume for the sets question. If you want a ready-made layout, our free pull day plan already includes arm work.

What about speed and range of motion?

Two quick pointers, both with limits.

A 2022 trial in untrained young adults trained the elbow flexors at extended elbow angles (0-50 degrees) for 5 weeks. Biceps and brachialis thickness rose about 10% with eccentric-only or combined concentric-eccentric work, but not significantly after concentric-only training (Sato et al., 2022). That is a short study in non-trainees, so it is not a reason to skip lifting the weight. It is a reason to lower the dumbbell with control rather than dropping it.

On range of motion, a 2020 systematic review found full range clearly helped leg hypertrophy, but upper-limb evidence was limited and conflicting (Schoenfeld and Grgic, 2020). Practically, go from nearly straight arm to a firm squeeze, and do not cut the stretched position short just because it feels hard.

When should you see a doctor?

Gym soreness is a dull, widespread ache that fades in a few days. Front-of-elbow pain that is sharp, sudden or stays for weeks is different. Distal biceps tendon problems can progress or rupture, and radiology reviews note that brachialis injuries are uncommon and usually follow forced elbow hyperextension (Willaume and Bierry, 2021). If you felt a pop, see a visible bulge in the upper arm, lose strength suddenly or have persistent pain, get it assessed in person. Our guide on distal biceps tendonitis and golfer’s elbow covers common overuse issues, but it is no substitute for an examination.