High Intensity Interval Training (HIIT) has again become a popular trend in the fitness industry, although we have to wonder whether it actually ever left?

While it's come back into the spotlight from books such as Fast Exercise, by the BBC's Dr Michael Mosley, leading fitness experts have touted the benefits of HIIT for the past 20 years, and popular programs including Body For Life have utilised the concept of hard, short periods of exercise to improve cardiovascular health and increase metabolic rate for up to 24 hours afterwards.

It was originally developed by athletic coaches to train runners, but it has crossed over to the fitness industry due to its fat-burning benefits confirmed many times over in scientific studies.

In fact, one of the first studies to discover that HIIT was more effective for fat loss was done in 1994 study by researchers at Laval University (Ste-Foy, Quebec, Canada). They reported that young men and women who followed a 15-week HIIT program lost significantly more body fat than those following a 20-week continuous steady-state endurance program. This, despite the fact that the steady-state program burned about 15,000 calories more than the HIIT program.

The Laval University study that found a decrease in body fat with HIIT also discovered that the HIIT subjects' muscle fibres had significantly higher markers for fat oxidation (fat burning) than those in the continuous steady-state exercise group.

According to the American College of Sports Medicine, a mere two weeks of HIIT training can improve aerobic capacity as much as six to eight weeks of endurance training.

Another wonderful benefit of interval training is that unlike endurance training, which often leads to muscle loss, HIIT training allows you to retain that hard-earned muscle.

And – to top it all off – it stimulates the production of human growth hormone in the body, which slows down the ageing process.

For those of you who aren't familiar with HIIT, it involves intervals of high-intensity exercise (such as running at 90% of your max heart rate) followed by low intensity (walking at a moderate pace) or complete rest. This is in sharp contrast to the typical steady-state cardio most people do at a moderate intensity, such as walking on a treadmill at 60–70% of their max heart rate.

HIIT sessions can go for as little as 5 minutes, incorporating bouts of 15 – 60 second intervals of intensity.