I once ran on a treadmill and the person I was with was tired and I was tired also. They asked me if the kickboxing I do was more difficult then that. I responded with a immediate yes! Don't get me wrong, running was challenging, especially because it was on a incline. But I tried to explain why the kickboxing was so much more intense. The reason I came up with was because it uses many muscle groups all at once. You are working your legs, abs, obliques, lower back, upper back, chest, and arms all at the same time. And when you do a punch or a kick you are putting all of your effort into it. It is kind of like sprinting on the inline. It is a lot of fun and a killer workout.