I've not done Insanity, but I've heard it is quite a workout! However, I have been working out 2x/week with a personal trainer who likes to make me to horrible ab things (planks and sit ups and all sorts of things). I think my stomach muscles are firming up, but only in the last 10 lbs has my stomach lost some of its bulge. For me, it is all fat. And the muscle under the fat, I'm sure, is really toned, but until I worked off the fat, I still had a belly.