When I have used it on my quads, I always sit in a fairly deep chair that has its back against the wall, and just wear some shoes that grip the floor surface well. I have never had any problems with keeping my butt down, and on my knees it certainly doesn’t “feel” hard on them at all. You kind of have to “brace” at each contraction, but it isn’t bad at all. Because my legs weren’t jolting straight at every contraction, I was concerned that the contraction might not be strong enough, so out of curiosity I kept my legs free and straight for a few contractions; my legs jolted straight and cramped up so hard I could hardly do it. That at least convinced me that the intensity I was working at was still stimulating the muscle quite well, even if my legs were not straightening every time.