20 rep squats kill me. I do them every other week and they always suck. My Quads, lungs, and mind try to defeat me. I always win. I usually collapse in the rack and see stars after the set. I have never got that from abs or bis. At least not yet lol. Calves are a real bitch too, but they are quickly becoming my best body part. They seem to really respond to the way I train them.