I'm in agreement with Celine J. I'm a person who goes REGULARLY, but not particularly HARD...so when I took this intro class I was in for a rude awakening (in a good, scary way- like a roller coaster!!!). I'm somewhat cardiovascularly fit...practice yoga...occasionally do circuit training, BUT this kicked my arse!!!! I went two days ago and I'm still shaking and a lil sore. Lol!!!