Jillian Michaels, the trainer from NBC's The Biggest Loser and self-proclaimed "TV's toughest fitness guru," runs through circuit training like a drill sergeant on caffeine. Her theory is to keep moving in between exercises to burn twice as many calories, so she cycles her team through sumo squats, mountain climbers, jump ropes, shoulder raises and bicycle moves without taking a breath. If she doesn't think one of her comrades is picking up his knees high enough, she makes everyone do 30 more, with a sadistic grin. As a result, you can almost literally watch the muscle burn. Michaels' Shape-Up Front DVD works out the chest, shoulders, triceps and quads.