HIIT has been one of the most popular and, ironically, long-lasting fitness trends of the past few years, underpinning pretty much every overpriced boutique studio class and home workout program promising an six-pack in an improbable time span. (“Listen to this: Seven. Minute. Abs.”) But what exactly is HIIT? Should you be doing it? And if so, how?

What is HIIT?

For the uninitiated, HIIT stands for High-Intensity Interval Training. What that means in practice can vary wildly, but according to the American College of Sports Medicine, which really knows its HIIT (and publishes an annual survey of worldwide fitness trends), “high” corresponds to 80 to 95 percent of your maximum heart rate.

“In other words, taking your heart rate to an elevated level, getting sweaty and generally working close to your limit,” says Oliver Lee, a Tread instructor for Peloton, the cult Netflix-style streaming workout service known for its two-grand, touchscreen-equipped bikes used by the likes of David Beckham and Hugh Jackman, currently only available in the US. (And you don’t need to fork out to access workouts via the Peloton Digital app – just a £20 monthly subscription, which is the same as one IRL class at many of those boutique studios.)

An “interval” meanwhile can last anywhere from five seconds to eight minutes. Intensity and interval duration tend though to be inversely proportional: the shorter the bouts, the sharper, and vice versa. And crucially, they’re interspersed with periods of complete rest or active recovery (40 to 50 percent of your maximum heart rate). These can be longer than the work intervals, the same duration or, if you’re feeling particularly masochistic, shorter.

Why do HIIT?

How does “results in less time” sound? Like infomercial charlatanism, to put it politely. But that’s precisely what HIIT is scientifically proven to do.