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Everyone has heard of HIIT training, or even participated in a HIIT class, but what exactly is it? Why do thousands of trainers and fitness enthusiasts rave about it so much? First, we need to clarify what exactly HIIT is.

What is HIIT training?

HIIT stands for High Intensity Interval Training and normally consists of short bursts of intense exercise (15 seconds – 4 minutes) followed by a short duration recovery, normally the same or shorter. To be classed as HIIT, the active element of the intervals must be performed at a high intensity, around 80-100% of VO2 or heart rate max. The exact intensity of the active period will depend on its duration.

How hard should I be training?

The most popular HIIT classes today such as Les Mills Grit and The Body Coach, use active durations between 30 seconds and 1 minute, with recovery periods the same or shorter.

As these intervals are relatively short, the active element should be performed at an intensity above 85% of VO2 or heart rate max. For example, someone with a maximum heart rate of 190 beats per minute (bpm) would need to keep their heart rate above 162 bpm for the duration of the active interval.

If you fail to work at a sufficient intensity you are not actually doing HIIT, but simply interval training. In this scenario you will not benefit from the positive adaptations associated with HIIT training.

The benefits of HIIT

The main reason many health professionals advise their clients to perform HIIT training, is to aid in weight loss. But how does performing short sets of intervals help lose more weight than a long continuous workout?

The secret behind HIIT’s ability to help you lose weight is due to its high intensity nature, and a process known as EPOC. EPOC stands for excess post-exercise oxygen consumption and occurs after performing exercise. It basically means the volume of oxygen your body is breathing in and utilising, is elevated above normal levels. This is occurring because the body is trying to regain homeostasis (the bodies happy equilibrium) and requires more energy to do so.

When performing exercise, the body can struggle to keep its equilibrium due to the rapid loss of energy, build-up of waste products, damage to tissues and increase in body temperature and blood PH.

High intensity interval training by its nature, makes this task a little harder than low or moderate intensity exercise. Research has shown higher levels EPOC up to 24 hours after HIIT compared to moderate continuous exercise. This equates to the body using more energy after HIIT. For this reason, a short high intensity exercise session can burn the same or if not more calories, than a long duration low/moderate continuous session.

There are other benefits to HIIT as well. Training at such high intensity places larger demands on muscles and connective tissues, helping to strengthen them. But probably one of the biggest benefits of HIIT, is its ability to increase cardiac capacity, the body’s ability to consume oxygen during exercise.

This occurs because HIIT pushes the heart and lungs that little bit harder than lower intensity exercise. Cardiac capacity is measured as a person’s VO2 max (maximum volume of oxygen consumed per minute) and is a key predictor of a person’s fitness level.

There are many other less well-known benefits to performing HIIT, such as increased insulin sensitivity, enhanced anaerobic and aerobic energy production and decrease in LDL cholesterol.

The limitations of HIIT

You may be thinking ‘that’s great, I’m going to do HIIT everyday’ but unfortunately there are downsides.

Due to its high intensity nature, it puts a lot of stress on the body, which means a greater degree of fatigue can result. This means trying to do this type of training more than 3/4 times per week is exceedingly difficult, and could lead to overtraining.

It would also not be the most appropriate type of training for long endurance-based activities such as marathon running. For a marathon you need good muscle endurance over many hours, which HIIT training would not train optimally. It should be noted that people new to exercise or coming back from a long break, should not jump straight into performing HIIT exercise.

Instead, they should start by performing some moderate intensity exercise for a couple of weeks to build up some base fitness.


Bang for buck, HIIT can provide a vast array of physical benefits for a relatively small exercise commitment. This makes it ideal for people on the go with limited free time to exercise. It will not only help you burn fat due to increased EPOC, but also make you fitter and stronger in the process.

However, you must work at the correct high intensity to elicit these benefits, which some HIIT classes do not. HIIT training won’t be everyone’s cup of tea as it can be very intense, and there’s no problem sticking to continuous moderate exercise, you just won’t get the same benefits.

If you would like to learn more about a HIIT programme designed to burn fat and increase fitness contact us today.

Matt Sills