You know what's the best part of spending a solid hour trotting on the treadmill? Nothing. Not only does that kind of slow-and-steady cardio take forever each time out, but like an expense report, it pays off slowly and doesn't really get you ahead. You want to be a toned and fit beach god? Come join the cult of high-intensity interval training.

The concept behind HIIT is simple: Ercise as hard as you can for a few minutes, recover just long enough to stop sucking wind, and repeat. This spikes your metabolism and builds muscle quickly. And unlike those leisurely treadmill sessions, it burns calories both during a workout and (here's the beautiful part) for up to 24 hours afterward. The regimen is brutal but quick. You'll be done with the whole thing in thirty sweat-filled (and swear-filled) minutes.

As a guy who's been on a HIIT kick, I can say that the hardest part is finding the motivation to volunteer for redline-speed wind sprints or set after set of jump squats. You can't half-ass these ercises. I've learned that eating a banana and downing two glasses of water pre-workout—rather than just hydrating during—helps avoid wicked cramps and soreness. And man, the results are swift: Each session is mentally easier than the last, and I began waving good-bye to my love handles around week five.

At right are three HIIT-ified versions of common workouts. They're flexible, and you can be, too. Replace one gym visit each week with a HIIT routine, or power through three a week. When you're no longer gasping for oxygen after a set, you know your pain is paying off. This is your moment of triumph—and your signal to be even more ferocious next time out.

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HIIT the Ground Running, Cycling, and Training

Unless you want to be extra sore tomorrow, warm up and cool down for each of these. Oh, and "recover" means "keep moving," not "Snickers break."

Make a Run for It

This speedcentric interval from Nike Master Trainer Joselynne Boschen yo-yos your heart rate before and after a short run.

Step 1: Sprint (45 seconds)

Step 2: Recover (15 sec)

Step 3: Repeat steps 1–2 three times

Step 4: 20-minute run at half speed

Step 5: Repeat steps 1–2 four times at your fastest heart-exploding, lung-busting speed.

Total time: 28 minutes

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Start a Vicious Cycle

Whether you're on a stationary bike or a real one, this stamina-building interval from Flywheel's Holly Rilinger will have you walking like Yosemite Sam when you're through.

Step 1: Sprint at low resistance (30 sec)

Step 2: Recover (60 sec)

Step 3: Sprint at high resistance (30 sec)

Step 4: Recover at low resistance (60 sec)

Step 5: Repeat steps 1–4 nine times

Total time: 30 minutes

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Fight the Flab

MMA fighters look like Greek sculptures, not lumpy 'roid freaks, because they use body-weight strength-training routines like this one from Noah Neiman of Barry's Bootcamp.

Step 1: Squats (45 sec)

Step 2: Squat jumps (15 sec)

Step 3: Push-ups (60 sec)

Step 4: Plank (60 sec)

Step 5: Jump rope (60 sec)

Step 6: Recover (60 sec)

Step 7: Repeat steps 1–6 five times

Total time: 30 minutes

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