The easiest method to improve the size of your biceps is to focus on the brachialis. The brachialis , the muscle situated underneath the biceps,it contributes to upper arm thickness more so than the biceps. Increasing its size is an easy way to make the arm bigger.
But the way most people train arms leaves the brachialis under-stimulated. People assume that training the brachialis is as simple as using a neutral/hammer grip. Not so. While grip type can have an impact, the type of contraction is a much more important factor when it comes to putting tension on the brachialis. The brachialis is a lot more active during slow speed and isometric actions.
As speed of movement increases, brachialis activation decreases – more stress is shifted on the biceps. So if you're doing very heavy dumbbell hammer curls using a fast speed (even swinging a bit to lift more weight), the brachialis won't receive much stimulation; the brachioradialis will take over. To work the brachialis effectively, do a low pulley curl with a rope attachment.
How to perform the set:
Do the reps using constant tension. This means going up fairly slowly, squeezing the muscles every inch of every rep. Imagine trying to compress your biceps between your delts and forearm.
At the top of the contraction, squeeze and hold for 2 seconds on every rep.
At the end of the set, hold the peak contraction 10-20 seconds, squeezing as hard as you can.
If you really want to prioritize brachialis development, perform all of your curls with the three guidelines above: reverse, hammer, or regular grip.