Stop doing endless amounts of underhand curls to build bigger arms. Do more of the exercises you are bad at and your guns could start exploding with new gains. Try this:
The Reason This Works
To get total bicep development, you cannot ignore some of your forearms and elbow flexors. Going for more curls with your palms-down (pronated) gives you the best bang for your exercise buck when attempting to reach all these with one movement.
Using a palms-down grip not only hits your neglected (and even unheard of) brachialis, it will help you pack on some muscle on your forearms. And with stronger forearms, you are less likely to be held back when it comes to biceps growth.
Any curls with a palms-down grip will be good, but some are better than others. If you have not done a lot of curling with your palms down, then you will be bad at it. It’s also pretty easy to ruin your alignment. Your elbows might start flaring and you will probably shift to a position your body is more used to.
Using dumbbells or cables (and doing just one arm) helps stop this. The one-hand palms-down curl will keep your elbows tight and in, and works your forearms and biceps in the palms-down position.
When focusing on one arm, you can focus on your forearms and bicep contraction. Your pump will be mind-blowing.
Do 8-12 reps and 3 or 4 sets as your last movement of your next arm workout. Doing this will make your arms explode!