High-intensity interval training (HIIT), which involves repeated bouts of high-intensity effort interspersed with recovery times, has become hugely popular in recent years. And for good reason: Research suggests HIIT improves both metabolic function and cardiorespiratory fitness, and requires considerably less workout time.

As a result, HIIT workouts can be found just about everywhere, from boutique fitness studios to large chain health clubs. The underlying principle of these workouts is nearly always the same: train hard, close to maximal capacity, rest a little, train hard, rest a little, repeat. Here’s what the research says about why HIIT is such an effective workout...

The Benefits of HIIT

Improved Cardiorespiratory Function

HIIT challenges the body to perform at the upper end of the aerobic training zone, which is called the second lactate threshold. When training at this end of the aerobic training zone, there is shift from using aerobic metabolism to anaerobic metabolism to produce energy to fuel the activity.  Training at this intensity improves cardiorespiratory function during exercise and at rest, and the body shifts from using aerobic metabolism to anaerobic mechanism to produce energy and generate force. In fact, HIIT training has been shown to benefit just about everyone, from endurance and strength athletes to recreational exercisers. That’s because it’s adaptable, meaning it can be used for aerobic training as well as muscular strength training, or a combination of the two.

Excess Post-exercise Oxygen Consumption

HIIT also increases caloric burn after an exercise bout through a process known as excess post-exercise oxygen consumption, or EPOC. Essentially, the body must consume more oxygen, which increases the amount of calories that are being burned, to return to its pre-exercising state after an intense bout of exercise. Therefore, by incorporating HIIT training into a workout regiment, body composition may improve as a result of the greater caloric burn associated with HIIT training. However, it is important to note that body composition is not altered by exercise alone; nutrition plays a key role in optimizing one’s fat-mass-to-lean-mass ratio.

Shorter Workouts

When it comes to workout efficiency, HIIT is especially attractive in that it does not require a large amount of time to reap the benefits. HIIT workouts typically last 20-30 minutes and are extremely effective as long as the intensity level is high. From both a psychological and physiological perspective, it is easier to maintain a high level intensity for a brief period of time than it is over a longer period of time, greater than 30 minutes.

With a growing body of research demonstrating that HIIT can be an effective and efficient way to exercise, this high-intensity workout is likely to remain popular for many years to come.