I agree that a lot of it is genetics. I have a small bone structure, and my attachments are about midway down, but now really low. Since I started concentrating on them again though, I see a definite improvement in size and vascularity. As well as doing wrist curls and reverse curls, I like doing wrist rollups with a small bar and a rope attached. A few sets rolling both ways, and your forearms will be pumped and burning!