It's a killer. I made the mistake of trying to do Lowmax this am--my first full day of going back to work after being sick with a cold all weekend--and I had to stop midway and just collapse on the floor for a while. Seriously, though, even when I'm at 100% health, Lowmax is extremely strenuous. There's no jumping, so it's kind to my knees, but if you bend low on all the moves, like Cathe tells you to, it just burns your legs up.