If you want bigger forearms, you have to build the extensors responsible for wrist extension. The flexors responsible for wrist flexion, the supinator and pronator, responsible for supination and pronation. And the break-year radialis responsible for elbow flexion. Let me show you how to build these muscles. Behind the back of wrist curls, a pro tip to increase forearm activation is to use a figure grip by attaching fat grips. dumbbell hammer curls to build the break-year radialis. dumbbell wrist extensions to build the extensors and wrist rotations.