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For the fourth straight year, the American College of Sports Medicine has chosen HIIT (High Intensity Interval Training) as one of the year’s top fitness trends in its annual survey of 1,800 health and fitness professionals. It’s not difficult to see why: The workout consists of just a few short bursts of all-out effort followed by a brief recovery, and it has been shown in multiple studies to improve cardiovascular fitness by increasing the maximal amount of oxygen your body can use, or your VO2max. 

But what does a scientifically ideal HIIT workout look like? Is it a half-hour long, 20 minutes — 10? Should it be done in the water, on a track, or in the weight room? And what is the optimal number of intervals?

That, in part, is what a team of researchers at the University of Stirling in Scotland set out to find. A new meta analysis led by Professor Niels Vollaard analyzed data from 34 studies that looked at the effects of HIIT and VO2max. The results were surprising to say the least: Doing fewer intervals may actually be better for cardiovascular fitness.

The analysis showed that performing two 20-second all-out sprints was very effective in improving VO2max, but for every extra sprint completed, the improvement in fitness was reduced by five percent. Why this happens is not yet clear: “We do not currently have a biological explanation for this unexpected finding, but we are performing follow-up studies to investigate some of the mechanisms involved,” Professor Vollaard wrote on The Conversation, an academic news site.

Those two 20-second sprints come with a warm-up and cool-down, which means you’re only doing eight minutes and 40 seconds of work. Which sounds pretty good to busy folks who have long been told to put in their daily 30 minutes of exercise. 

What's more, unlike many HIIT studies, Vollaard’s research focused on people who don’t exercise regularly. “If you are looking mainly at sports performance, and to improve marathon time, 10K running time, things like that, then the 10 minutes will not be sufficient." But the short burst will help you in "reducing your risk of developing Type-2 diabetes, heart disease later in life." 

Not everyone is buying what this study is selling, however. Doug Altman, a professor of statistics in medicine at the University of Oxford, and a leading expert in health research methodology, statistics, and reporting is especially critical of this analysis. “That finding is implausible and I don't believe it,” he told us. “I'm not saying that I expected [VO2max] necessarily to go up linearly, but I definitely don't expect more training to make things worse. It doesn't have face validity.”

He points to a number of problems with this analysis. These are “terribly small studies,” he says, and they don’t have the same design: some lasted two weeks, some lasted 12; the number of sessions varied between six and 36 sessions; sprint duration varied from 10 to 30 seconds. Altman is particularly puzzled about the recovery duration: "I'm not sure why it varies so massively from 80 to 1,200 seconds. That's ludicrous." 

Altman thinks the researchers were uncritical of the studies that they reviewed, and "rather too forceful about what it all means.” The bottom line? Small studies aren’t “necessarily a bad thing, it just means you can get very uncertain results,” says Altman. “What should come out of this is a plea for people to do better research.”

Besides potential limitations to the studies, HIIT isn’t always as easy as it sounds. According to Professor Vollaard, for you to achieve your peak power output for such a short amount of time, you’re going to need to plan ahead. For one, your regular gym equipment just won’t cut it. Most stationary bikes aren’t able to go from zero to maximum output in under a second, and treadmills flat-out don’t work — you can’t go quickly from a very steep incline to a flat for the recovery duration. They're also unsafe: "If you are getting really tired during the sprints and at the end you can't get off, you'll just fall off the machine, so that will probably not be a good idea." 

Running outside is an option, but it may also put more strain on your joints, which could lead to injuries. Professor Vollaard suggests 20 seconds of uphill sprinting on a very steep hill, then slowly walking down as recovery. “Do that two times for 20 seconds, and that would mimic the same exercise protocol that we use in the bike.”

A U.K.-based company has started selling bikes that can handle super-maximal sprints to fill the machine void. High Octane Ride primarily markets to corporations such as Barclays and PWC, but it has a growing number of private homes according to High Octane founder Ratna Singh. Right now they are only available in Europe, but Singh has plans to expand to the U.S. market later this year. The bikes sell for $3,000, and for $2,000 more, you can also get a MoMA-looking privacy pod and customize it with your company's colors.