Developed in Sweden during the 1930s, interval training was known as "fartlek" - which is Swedish for "fast play" because of the bursts of speed of various lengths and duration that are involved. German running coach Woldemar Gerschler took things a step further, focusing on heart rate recovery.
Interval training soon caught on among track stars, cross-country runners and swimmers. It is now a mainstay among professional and amateur athletic coaches.
However, intervals can be used by anyone, beginner or advanced, for almost any kind of workout.
So does it help you lose weight?
Personal trainer Lee Paines is a big fan of intervals. He says: "It burns more calories than one pace training. It challenges you, keeps your body guessing so you lose weight."
Each increase and decrease of activity corresponds to an increase and decrease in your heart rate - and that's where the extra calorie burning comes in. But it's not about how fast your heart is beating - it's how fast it recovers.
Sam Murphy is one of the UK's leading fitness experts. She's written seven books on the subject, including the best-selling Run for Life: The Real Woman's Guide to Running.
She tells BootsWebMD: "Any exercise you do can burn fat. At the end of the day, if you have burned 300 calories by interval training, or burned it just by jogging along quite steadily, it's the same result."
But with interval training you've had the same result in a quicker time, so it's good for maximising your calorie-burning.
"There is also the afterburn if you work really hard," says Sam. "You burn calories for longer after you stop exercising."
The key to weight loss is to burn more calories than you consume. Intervals contribute to weight loss because they make you fitter, which allows you to work out harder and longer and burn more fat and more calories. But don't forget that you must burn more calories than you consume to lose weight, even with intervals.
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