Hi Cathe and everyone else--I have a question about the PUB workout. In the stability ball segment, there's a glute exercise where you lay down and put your heel on the ball w/the other leg extended up in the air. (it's in other workouts too but uses a step) For some reason, I CANNOT do this exercise using the ball!! I always have to use a step and I'm just wondering if this is an advanced version that I will be able to work up to as I get stronger or if I'm just doing it wrong and need to change my form. Thanks for any input!!