When doing box jumps, you should only be jumping onto a step that is a comfortable height. You should land on the box with a soft sound, and it should be low enough that you are coming down from the jump and absorbing the shock through your knees. If the step/box is too high, you are not coming down into it, and will jolt your knees. I have seen many people trying to jump onto a ridiculously high step, which causes improper mechanics, and can lead to injury. It is better to use a 6 inch hurdle, progress to a 12 in hurdle, the eventually an 18 inch. With hurdles, you are always absorbing the shock on the way down. PM me your email, and I will send you Sk8strong's plyometric download.