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The truth about high-intensity interval training

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In our time-crunched era, there's a seemingly unlimited appetite for time-saving shortcuts, hacks, and more efficient ways to do mundane activities. So it's no surprise that really, really short workouts have broad appeal.

When exercise scientists tell us — as they have lately in a number of new studies — that we don't have to run for an hour, or even half an hour, to get big health benefits, of course we listen up. These studies are catnip for health journalists, too.

Some of the media coverage of late might lead you to believe that tiny bursts of exercise for as little as one minute are a quick route to better fitness.

But there are some important nuances about high-intensity interval training, or HIIT, that have gotten lost in all the excitement. The proven benefits of these workouts relate to very particular health measures. In the studies, the interval routines that work best require more than just a few minutes of activity. And, as with any discussion about health, goals and context matter hugely — and intense interval training may not be for everyone.

Tiny workout mania is generating increasingly bold claims about its benefits

A recent headline in the New York Times stated, "1 Minute of All-Out Exercise May Have Benefits of 45 Minutes of Moderate Exertion." Another, from the Toronto Star: "Just 60 seconds of intense exercise can boost your fitness level."

The headlines were based on a recent study published in PLOS One. But according to one of the authors, Martin Gibala, chair of the Department of Kinesiology at McMaster University, the broad takeaway from the research should not be that you can spend one minute exercising and get fit. "That's just not what the evidence shows," he said.

In the new study, the researchers followed two groups of participants for 12 weeks: One group worked out for 10 minutes (including intense intervals that added up to a minute), and the other for 45 minutes (at a moderate, even pace). Note: Each group worked out for longer than one minute.

The most remarkable finding in the study was that the two groups of exercisers saw similar improvements in health — the researchers were tracking insulin resistance and endurance — despite their varying time commitments.

Unfortunately, however, Gibala noted that most of the proven high-intensity interval workouts take 25 or 30 minutes to complete. "And if you do that three times a week, you're in the lower end of the public health guidelines of 75 minutes of vigorous exercise."

In this meta-analysis, the researchers evaluated the effects of high-intensity interval training studies, separating out nine studies that showed the largest improvements in VO2 max (a measure of endurance that calculates the maximum volume of oxygen the body can use) and nine studies that reported the smallest gains.

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.

Another popular and effective workout includes four bursts of exercise that last four minutes. Between each burst is several minutes of lower-intensity work — a routine that would take at least 20 minutes to perform if you don't include a warmup or cool-down.

The health benefits of intense interval training are very specific — and don't include weight loss

Many pursue high-intensity interval training in hopes that it will lead to a sculpted body and weight loss. But as we noted in a recent feature, exercise — including this newly beloved kind — generally only has very modest effects on weight loss, if any at all.

Kathryn Weston, an exercise scientist at Teesside University in Middlesbrough, England, told Science News, "The only thing [HIIT] definitely doesn’t do is weight loss." The main reason, she said, is that you don't burn enough calories in the short workouts to create the kind of energy deficit that would lead to slimming.

But there are particular health benefits from this type of training on two measures. One is cardio-respiratory endurance (again, that's measured by VO2 max). A number of meta-analyses have shown that HIIT routines lead to greater gains in VO2 max compared with other forms of training.

For this reason, athletes have long used the interval technique to up their game, said Mayo Clinic exercise researcher Michael Joyner. "There's observational data in athletes going back almost 100 years showing the benefits of a few bouts of really high-intensity exercise in people." He added: "If you want to get people to their biological maximum, they need to be doing four to five times of three- to five-minute intervals."

The other benefit researchers have discovered: Short bursts of intensity in a workout may also reduce the risk of Type 2 diabetes by improving the body's response to insulin and blood glucose. In a meta-analysis on the metabolic effects of HIIT, published in 2015, researchers found the interval workouts had a greater impact on reducing insulin resistance compared with non-interval training or no exercise.

When should you try tiny, intense workouts?

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In the "one-minute" study, (which was actually a "10-minute" study), the participants were very sedentary, but otherwise healthy, individuals. The benefits they got suggested that when people go from doing nothing to doing something they can see health benefits — even in as little as 10 minutes.

Gibala warned that not everyone who picks up this routine will necessarily experience the same benefits. "People are coming from all kinds of different perspectives — highly trained athletes, deconditioned cardiac patients — and the [10-minute] program won't work for them." The athletes would be too fit to gain anything, and the cardiac patients not fit enough to endure the bouts of intense activity.

In Joyner's view, the best way to approach interval training — or any workout routine — is to consider your starting point and your goals. "Is the purpose of your workout to go out on a bike ride with your wife? Break three hours in a marathon? Or just be a little bit healthier?"

He noted that "the dose-response curve [for exercise] is pretty robust" — meaning you get more gains with more intensity and more time put in. For people who are trying to improve their endurance, adding intervals into their workout routine is a great approach, he said.

For the rest of us, Gibala had a very lucid way to think about how to weave intervals into a workout: "Interval training to me are periods of more intense effort separated by periods of recovery." Pushing out of your comfort zone a few times in your workout "provides a way for people to boost their fitness in a relatively time-efficient manner."

And for those less inclined to start up cycling or a running route, it's completely fine to try it with short bursts of faster walking.


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