Does anyone know if the contestants on The Biggest Loser had to "work up" to doing that kind of exercise?

I mean these exercise sessions they show look grueling for anyone but especially for some of these folks with a lot of weight to lose. I realize they have all had all kinds of medical evaluations and all and have been given the okay for this exercise regimen. I just wonder if they had to work up slowly to doing those long workout sessions for hours during a day. Just seems to me they would like p out or die or something on some of these routines they put them through. Anyone know?