What Is HIIT?

How to get the most out of your exercise time, according to science

Modern life has a way of making us feel time-crunched and pressured to find the most efficient ways of using the precious hours when we’re not sleeping. The trendy fitness regimen high-intensity interval training, or HIIT, epitomizes this feeling.

HIIT promises the best workout in the least amount of time. Runners have used interval training for more than 100 years, alternating between sprints and jogging to improve their endurance. But HIIT didn’t really go mainstream until about a decade ago, when exercise physiologists started to come out with study after study demonstrating that intervals could deliver the biggest health improvement for your exercise time.

Recently, fitness professionals voted HIIT one of the top fitness trends for 2020 in a survey by the American College of Sports Medicine. And interval-based workouts are now popping up seemingly everywhere.

But there are some important nuances scientists have learned about HIIT that have gotten lost in the hype. The proven benefits of these workouts relate to a very particular type of interval training, and they’ve got nothing to do with weight loss. Here are six basic questions about HIIT, answered.

First things first: What is HIIT?

HIIT workouts generally combine short bursts of intense exercise with periods of rest or lower-intensity exercise. At fitness studios and online, these workouts often mix aerobic and resistance training.

To be clear, most of the interval workouts researchers have studied focus solely on aerobic exercise. Which means the scientific understanding of interval training is based on a more specific routine than what’s appearing in most gyms, videos, and magazines. And the researchers’ definition matters because when we’re talking about the evidence of benefits, we need to be specific about the kinds of workouts that science was based on.

When researchers talk about HIIT, they’re referring to workouts that alternate hard-charging intervals, during which a person’s heart rate reaches at least 80 percent of its maximum capacity usually for one to five minutes, with periods of rest or less intense exercise. (It’s not easy to know that you’re working at 80 percent, but a Fitbit or heart rate monitor can help.)

What does a HIIT routine look like?

What differentiates HIIT (or SIT) from the steady-state, continuous types of exercise — jogging at an even pace or walking, for example — is the intervals, those periods of heart-pounding intensity. If you want to try it, you can simply take a HIIT class, or run or even walk in a way that involves higher-speed and higher-incline bursts.

If you want a routine that’s been lab-tested, there’s the 4-by-4 from Norway. It involves a warmup, followed by four four-minute intervals (again, where your heart rate reaches past 80 percent of its maximum capacity), each interspersed with a three-minute recovery period, and finished off with a cool-down.

So, for example, you’d jog for 10 minutes to warm up, then do four four-minute intervals of faster running, with three three-minute intervals of moderate jogging or brisk walking in between, and a five-minute cool down at the end. And you can substitute jogging with other aerobic exercises, such as biking or swimming. The whole routine should take 40 minutes.

A shorter, and also heavily studied, example of an interval routine is the 10-by-1, which involves 10 one-minute bursts of exercise each followed by one minute of recovery.

Many bootcamp style workouts involve versions of HIIT where they combine cardiovascular exercise with strength training.

What are the benefits of interval training?

The single most well-established benefit of interval training has to do with heart health. Intervals can boost cardio-respiratory health with a smaller time investment compared to continuous forms of exercise. So we’re not talking about superior fat-burning capacity (more on that later) or bigger muscles. We’re talking about improved VO2 max, a measure of endurance that calculates the maximum volume of oxygen the body can use.

“Scientists have found that [VO2 max] is one of the best predictors of overall health” according to the recent interval training book The One Minute Workout. “The more aerobically fit you are, the better your heart can pump blood, the longer it takes you to get out of breath, and the farther and faster you’re able to bike or run or swim.” And that, in turn, can help prevent heart disease.

Consider this 2016 SIT study, in which Gibala and his co-authors followed two groups of participants for 12 weeks: One group worked out for 10 minutes (including several intervals that added up to one minute), and the other for 50 minutes (at a continuous pace).

The most remarkable finding in the study was that the two groups of exercisers saw the same improvement in their oxygen uptake, despite their varying time commitments.

Of course, the more you put into a HIIT workout, the more heart health benefits you get out. The findings were telling: Less intense training programs with shorter intervals carried the least health benefits, while interval training studies reporting the greatest increases typically used longer (three- to five-minute) intervals.

Why does HIIT improve cardio health?

Researchers still haven’t completely figured out exactly why HIIT works to improve aerobic fitness more than continuous types of exercise. But one key hypothesis, Gibala explained, has to do with the heart’s ability to pump blood.

One measure for blood pumping is something called stroke volume, or the volume of blood that comes out when the heart contracts. And a major determinant of VO2 max is stroke volume.

Is HIIT the best exercise regimen for weight loss?

There’s no doubt that interval training can be a time-efficient way to burn calories. Researchers have repeatedly shown that people can burn comparable amounts of calories in HIIT routines lasting, say, 20 minutes, compared to longer continuous exercise routines lasting, say, 50 minutes. The reason for that, is that higher-intensity exercise, like intervals, results in a greater disturbance of the body’s homeostasis.

But the question is whether that calorie burn translates into weight loss, and that’s where HIIT falls short. A 2019 systematic review of the trials comparing HIIT and SIT with moderate-intensity continuous training found all workouts performed about the same on fat loss.

Many people overstate the potential for interval training to cause you to lose weight. But that’s a problem with exercise in general, not HIIT specifically. Always remember, it’s much easier to lose weight by cutting calories in your diet than trying to burn excess calories. So a combination of fitness and nutrition is our recommendation.

That’s especially true if your workout is only 20 minutes long, said Jeffrey Horowitz, a kinesiology professor at the University of Michigan. To burn a lot of calories, “you need to exercise [for] a more prolonged period of time. HIIT routines, by definition, tend to be shorter. So if your goal is weight loss, you might consider a longer interval routine, and you definitely want to look at your diet.”

Most bootcamp style workouts involve many aspects of HIIT and usually range from 30-60 minutes in duration.