Brachialis

The brachialis sits beneath the biceps, between the humerus and the ulna. You don't see it directly, but when it's well developed it 'pushes' the biceps upward, giving more height to the arm and filling the space between biceps and triceps in the side view. It's a muscle that makes the arm look thicker than the biceps measurements alone would suggest.

Unlike the biceps, the brachialis is not a supinator. It activates maximally when the forearm is in neutral or pronated position, that is, when the biceps is mechanically disadvantaged. The dumbbell hammer curl is the main exercise for the brachialis: neutral grip, elbow flexion, no supination. The reverse curl (pronated grip) and the rope curl on the low cable also activate it well.

If you want bigger arms and the biceps seems to have stopped growing, the brachialis is probably the missing piece. Adding 2-3 sets of hammer curls or reverse curls to your routine can make a visible difference in a few weeks. The brachialis is also fundamental for elbow flexion strength: if your curl stalls, strengthening the brachialis with neutral grip can unlock even the supinated curl.