The Hardest 20-Minute Workout of Your Life

Are you ready for this butt-kicking, muscle-burning, heart-pumping cardio challenge?

Sprints are a great way to blast fat and send your heart rate through the roof. But if you want to make them even more intense, try sprinting on a Woodway. The Woodway Curve is a motor-free machine that's powered by the runner, and requires him to expend 30 percent more energy than a traditional treadmill, according to a study from the University of Wisconsin-Milwaukee.

"Unlike a motorized treadmill, the Woodway's belt doesn't pull your feet along at a certain pace," says Bruce Mack, co-founder of Thrive Training Systems.  "Your feet must move it, so the faster you go, the faster it moves." Plus, the curved base makes you feel as if you're running uphill the entire time, forcing your lower-body muscles to work harder with each stride.

Sound tough? Then just wait until you try this nasty workout from Mack. You'll perform a 15-second sprint as fast as you can, and then rest for 15 seconds. Keep your chest high and your core braced the entire time you're running. That's 1 round. Do 40. The entire workout takes 20 minutes to complete, but you'll spend 10 of those minutes in a vigorous all-out sprint. By the final round, you'll be gasping for air and drenched from head to toe.

Don't have one of these high-tech toys? Try the workout on a standard treadmill with a 15-percent incline, suggests Mack. You can do it on a short, steep hill outside, too.

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