America's Hardest Workout Classes

Plyometric Athletic Circuit

Try It At: Easton Gym, Santa Monica, California

The Benefit: By the time you catch your breath, you'll burn off the calories in more than half a pint of Haagen-Dazs chocolate ice cream -- and sculpt sexy muscles.

Calories Burned: 600 to 700

Every Saturday morning at 7:45, 20 gung-ho exercisers warm up on stationary bikes and treadmills. They're getting ready for an all-out cardio kick-butt workout led by former Olympic swimmer and strength coach Tom Williams. Eight 90-second cardio stations call for plyometric hops over a step, alternating with strength exercises that use resistance. Each 90-second cardio interval ends with full-on sprints. After 60 minutes of cardio-sculpt stations, Williams sends the staggering class back to the treadmills and stationary bikes for another 20 minutes of aerobic work.

Signature Move: Squat Jump

Stand with legs shoulder-width apart, straddling a step. Bend knees into a 90-degree squat. Hold for 30 seconds. Jump up onto the step, then jump back into the squat. Do 12 reps. Finish by holding a 90-degree squat for 30 seconds. Rest for 15 seconds, then complete 3 more sets.

At Home: Try Cathe Friedrich's Imax 3 DVD, $26.95 at collagevideo.com.

Originally published in Fitness magazine, May 2006.