To those of you who have the Butts and Gutts workout, how do you find the floor work on your lower back? I find that the kneeling on all fours type butt exercises and too much kickbox-type kicking (eg in Power Half Hour) give me a sore lower back. I also experienced the same in a Legs Bums and Tums class at the gym. Would skipping these types of exericses leave me with enough to warrant buying this workout?