How can I build forearm/extensor muscles with only a couple of dumbbells?

Wrist curls flex/ext. Grip dumbbells with elbows flexed to 90 degrees and arms resting at your side. With palms facing up, flex your wrist toward your body for 8-12 repetitions. This will work your wrist flexors on the palmar aspect of your forearms. For wrist extensor muscles, simply pronate your forearms so that your palms face toward the ground, perform 8-12 extension repetitions. Do 3 sets for both muscle groups.

Related Questions

How can I build forearm/extensor muscles with only a couple of dumbbells and a chinup bar?

Ask a trainer. You can build the above muscle groups using the tools you mentioned. There are several exercise options. Discuss this with a trainer so he can design an appropriate routine to fulfill and balance your goals and body needs.