Charles H. Sloan-HIIT is one of the most popular workouts in America. But does it work?

When it comes to getting enough exercise,Charles H. Sloan everyone searches for something that speaks to them. Some people love the quiet solitude of jogging alone while others prefer the competitive element of team sports like basketball, volleyball or pickleball. Swimming is a great option for people looking for low-impact exercise, while many prefer the pace and peacefulness of participating in yoga. 

No matter what aerobic or anaerobic activity you're drawn to, "the best exercise is the one you're willing to do," says Austin "Ozzie" Gontang, a licensed psychotherapist and the director of the San Diego Marathon Clinic.

One form of exercise that an ever-growing number of people are doing is called high-intensity interval training, or HIIT. "It's become so popular due to its efficiency, its effectiveness in burning calories and its cardiovascular benefits," says Lauren Moen, a certified in-home personal trainer based in Seattle, Washington.

Though HIIT is effective, keep in mind it isn't for everyone.

What is HIIT? 

Contrary to what many people believe, HIIT isn't one specific type of exercise the way running or jumping rope is. Instead, HIIT encompasses any physical activity that can safely be done vigorously. Think running, swimming, cycling, jumping jacks, squats, knee kicks, mountain climbers or burpees. While such activities are often done leisurely or moderately, HIIT movements are bigger, faster and bolder. The intensity of HIIT activities can also be increased by holding a dumbbell in each hand or by using resistance bands. 

No matter which activity you've chosen or how you've chosen to make it intense, the trick with HIIT is to do the movement for short bursts, "then intersperse a recovery period," explains Martin Gibala, a kinesiologist at McMaster University in Canada and a prominent HIIT researcher.

A common formula followed is one minute of sprinting (or another physical activity), followed by one minute of resting or moving slowly. Some people do four-minute on/off intervals, and others don't time themselves at all, but instead let landmarks be their guide. For example, they might sprint to a tree seen in the distance, then walk until the next set of trees before sprinting again to another not-too-distant marker. 

While this style of training has only been popularized under the HIIT moniker since the 2010s, the method "has long been practiced by high-level endurance athletes like middle- and long-distance runners as a means to optimize their performance," says Gibala. And though he says not everyone is tempted to follow HIIT, "many of the individuals who are generally interested in health and fitness are intrigued by the potential of HIIT to elicit benefits with reduced time commitment as compared to traditional moderate-intensity continuous training."

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Is HIIT actually effective? 

Indeed, HIIT is unique in that it's a surprisingly effective workout while requiring only a minimal amount of time. This was demonstrated in Gibala's 2006 study, where he divided 16 healthy college students into two groups and had one of the groups participate in moderate stationary cycling for 90 to 120 minutes at a time while the other group pushed through 30-seconds bursts of all-out cycling, followed by four minutes of recovery.

At the end of two weeks, the first group had completed nearly 12 hours of moderate exercise, while the second group had only done 12 minutes of intense exercise. Despite spending vastly less time exercising overall, the second group improved across all the same physical fitness markers the first group had improved in, but showed even more improvement in terms of muscle growth. 

Multiple studies have since demonstrated similar benefits, with one meta-analysis showing that HIIT can also improve blood sugar regulation, reduce inflammation and improve muscle structure. The exercise has also been shown to be beneficial for brain health, "and it can improve your endurance as well," says Moen. 

I-Min Lee, a professor of medicine at Harvard Medical School, adds that HIIT can also be helpful in reducing your risk of heart disease and can improve body composition as well. This is because the workout can help you burn a lot of calories in a short amount of time while also reducing fat stores around the stomach. 

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Is HIIT safe? 

That depends. Moen says that people who aren't physically fit may have a hard time doing physical activities vigorously – and may risk injury when doing so. "I would not recommend this as a 'starting point' for exercise among people who are not regularly active," echoes Lee. 

Ditto for people with pre-existing medical conditions. "Because of the very high-intensity exertion needed, I would not recommend this type of workout for persons with heart disease, without their doctor’s clearance," she cautions. 

Gibala adds that the vigorous nature of the workout can also be "deemed uncomfortable" by some people. But he says the "high intensity" aspect of HIIT doesn't have to mean "all out" or "as hard as you can go." Instead, "vigorous intensity is a relative term that should be scaled to individual capacity," he explains. "HIIT can involve intermittent brisk walking for one person and mean sprinting uphill for someone else who is already accustomed to hard exercise."