I just recently purchased a stability ball. I normally just follow the substitions in Cathe's workouts when she uses one. So, I was excited to use my ball when I got it. Specifically, I was doing the ball portion of PLB and the first exercise for hamstrings where you have one leg in the air and one on the ball was almost impossible. The ball was moving and my leg was shaking all over the place. I could do other exercises where two legs were involved, etc. although they weren't easy. Does Cathe and the gang just make it look easy and it's really something I need to practice with or is it me? Your input is appreciated.