Brachialis

What it is

A broad, flat muscle lying directly on the front of the lower humerus, underneath biceps brachii. It runs from the bone to the ulna — the forearm bone that does not rotate.

That single fact defines it. Because the ulna does not spin during supination and pronation, this muscle's leverage on the elbow is identical whatever position your hand is in.

It is sometimes visible as a bulge on the outer side of the upper arm, between biceps and triceps, in lean and well-developed people. Most of it is hidden.

What it does

Flexes the elbow. That is its only action, and it is the strongest elbow flexor — it has a larger cross-section than biceps brachii and a better mechanical position.

It is described as the workhorse of elbow flexion because it works in every elbow bend regardless of forearm position, whereas biceps contributes much less when the palm faces down.

What loads it

Any elbow flexion against resistance. Palm-down and neutral-grip curling load it relatively more, because those positions reduce what biceps can contribute.

What is commonly got wrong about it

That biceps is the elbow flexor. It is the visible one. This is the stronger one.

That it is hard to train. It is loaded in every curling movement. What is true is that palm-down and hammer-grip variations shift more of the work onto it, and that is one of the better-founded exercise-selection claims in arm training, because it follows directly from which bone each muscle attaches to.

"Brachialis makes the arm look bigger by pushing the biceps up." It sits under and beside biceps and adds real cross-sectional area to the upper arm. The pushing-up description is a serviceable simplification rather than an anatomical account.