We give you the scoop on this fitness craze and tell you if the gain is worth all the pain.

What exactly is HIIT?
HIIT stands for high-intensity interval training. There are lots of incarnations—think Tabata or a more structured CrossFit workout. Basically, it's a series of intervals involving short periods of hard-core exercise (for example, 30 seconds of burpees, jumping jacks or pushups done as hard and fast as possible) followed by a period of rest.

While this type of workout has been used by athletes for decades, says Brendon Gurd, an assistant professor at the School of Kinesiology and Health Studies at Queen's University, it has recently grabbed the attention of fitness fiends, thanks to a series of studies showing that HIIT can improve your fitness level to the same extent as more time-consuming endurance training, meaning you can accomplish more faster. But don't think this is an easy way out of exercising. Read on for more about this increasingly popular fitness trend.

Feel the burn
If you're looking to lose weight, a 30- to 60-minute run will burn more calories than your average HIIT workout, says Gurd. But if you're looking to improve your overall fitness, HIIT is where it's at. Fitness gains are far more important than weight loss when it comes to preventing diseases and living a longer, healthier life, explains Gurd. "Fitness, more so than body weight, predicts health outcomes," he says.

And while you may burn fewer calories during a HIIT workout, some studies show a greater calorie burn following that workout. Data in the Journal of Translational Medicine suggests that high-intensity weight lifting increases energy expenditure for 22 hours after a training session. Researchers concluded that HIIT may be a useful tool for overall health since a shorter workout duration can reduce the barriers to exercise.

HIIT gets you fit
Studies have shown that just four minutes of interval training can improve fitness levels to the same extent as 30 minutes of jogging on the treadmill. Why? "Repeated bouts of high-intensity exercise cause the cardiovascular system to get better at delivering blood through the body, cause muscles to improve their ability to generate force and allow you to move better," says Gurd. "HIIT will help you improve in endurance sports."

No pain, no gain
But if you think these short workouts sound like a cakewalk, be warned: True HIIT programs are very intense and painful. "If you're doing HIIT properly, it's horrible—like you're-going-to-throw-up horrible," says Gurd. "If you're not feeling terrible at the end of a HIIT workout, you probably won't see a benefit." Always talk to your doctor before beginning a new fitness regimen, especially if you have a health condition such as heart disease.

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