ride motocross... i work with a dude that does that shit and his forearms are massive like popeye and ish

 

I was going to suggest this exact thing....this is the best way to do it. My brother actually made something to do it. He took a thick wooden dowel, and drilled a hole in the middle. The he tied a rope through the hole, and covered the dowel with grip tape. Then he would tie a weight to the other end of the rope, and turn the dowel to raise it up. You need to make sure that when you are doing this exercise, your forearms are facing up, and twist with your wrists, almost like you were reving a motorcycle.