Well, this doesn't really answer your question, but I remember way back when I used to take aerobics classes. I had one favourite instructor, she was fantastic and I would come staggering out of her classes soaking wet and limp from fatigue, so sore from DOMS I could barely be touched 2 days later... Sometimes there were a few other "regulars" there I wondered about. I would see them taking other classes as well, or talking about the back-to-back classes they were doing, pretty teenage girls in light makeup and cute workout gear - who never ever broke a sweat. They were there plenty of hours, but they just weren't working very hard.