I've been doing Cardio Kicks for a few months now, and I keep trying to do the power hops landing on my heels like you say we should. What is the reason for landing on the heels? I've tried and tried, but every time I land on my heels I feel it jarring all through my back and into my neck. I can do them landing on the ball of my foot and that doesn't bother me at all. Will that cause problems over time?