Brachialis

armsMusculus brachialis

The brachialis lies underneath the biceps and does one thing: bend the elbow. It is the strongest elbow flexor you have, and most people have never trained it deliberately.

The brachialis sits directly beneath the biceps on the lower half of the upper arm, running from the front of the arm bone down across the elbow to the ulna — the forearm bone on the little-finger side. You cannot see most of it, but you can see the edge of it: on a developed arm it pushes out between the biceps and the triceps, giving the outside of the upper arm its width.

It is a pure elbow flexor, and it is the only muscle in the body whose sole action that is. The reason is where it attaches. Because it ends on the ulna, which does not rotate, it cannot turn the palm — so unlike the biceps, its contribution to bending the elbow is identical whether your palm faces up, down or sideways.

That makes it the more reliable of the two, and by most measures the stronger. It has a larger cross-section than the biceps and a mechanical position better suited to flexion, and several standard references describe it rather than the biceps as the main elbow flexor. The biceps gets the attention because it is on top and visible.

Its nerve supply is a small oddity worth knowing: it is supplied by the musculocutaneous nerve like the biceps, but usually also receives a small branch from the radial nerve, which supplies the muscles on the back of the arm. It is one of the few muscles that draws from both.

Training notes

Neutral and pronated grips are what train it. A hammer curl, with the palms facing each other, and a reverse curl, with the palms facing down, both put the biceps at a mechanical disadvantage and leave the brachialis to do the work. A standard supinated curl trains it too, but with the biceps taking a larger share.

This is a genuine and cheap opportunity. Because the brachialis sits underneath the biceps, developing it pushes the biceps up and makes the whole arm measurably thicker — and most people have never done a set aimed at it. Adding hammer curls to an arm routine that only has supinated curls is one of the highest-return small changes available.

It works hard in every pulling movement too, especially rows and chin-ups performed with a neutral grip, since it flexes the elbow regardless of hand position.

It responds well to slower, controlled work with a full range. There is no supination to add and no shoulder position to manipulate — the only variable that matters is how completely the elbow bends and straightens.

Anatomy

Origin
  • Distal half of the anterior surface of the humerus
Insertion
  • Tuberosity and coronoid process of the ulna
Actions
  • Flexes the elbow, in every position of the forearm
Innervation
Musculocutaneous nerve (C5–C6), with a small contribution from the radial nerve (C7)

Brachialis arises from the front of the lower half of the humerus and from the intermuscular septa either side of it, passes in front of the elbow deep to the biceps, and inserts on the tuberosity and coronoid process of the ulna by a short thick tendon.

Its insertion determines its function completely. The ulna is the fixed bone of the forearm — the radius rotates around it during pronation and supination — so a muscle ending on the ulna cannot contribute to either. Brachialis therefore flexes the elbow and does nothing else, and it does so equally well in every forearm position.

It has a larger physiological cross-sectional area than biceps brachii and a shorter moment arm, and standard references commonly identify it rather than biceps as the principal flexor of the elbow. Biceps contributes most when the forearm is supinated and progressively less as the forearm pronates; brachialis does not vary.

Its dual innervation is a recognised feature rather than a variation: musculocutaneous supply to the main body, with a branch of the radial nerve typically supplying the lateral part. The radial contribution is generally described as sensory or as supplying a small lateral portion, and its exact role is not uniformly reported.

Under load

It is loaded by elbow flexion at any grip, and its relative share rises as the forearm moves out of supination — which is what makes hammer and reverse curls its most direct loading.

It also contributes to every pulling movement that bends the elbow, independent of hand position, so it is rarely completely untrained even without direct work.

Hypertrophy here displaces the overlying biceps and increases upper arm circumference disproportionately to the muscle's own size, because of where it sits rather than how large it is.

Exercises

No exercise in the library names Brachialis yet. Nothing has been invented to fill the gap — a false reference costs more than an absent one.

Antagonists

Synergists