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Understanding High Intensity Interval Training (HIIT) and what it can do for you

Anyone who has ever done a HIIT session will understand when I say, “if you do it right, you’ll probably never want to do it again!”. To achieve the benefits of high-intensity training you will have to leave your “workout comfort zone”. But don’t despair; the health benefits are well worth the effort.

What is HIIT?

A HIIT session involves repeated short intervals of exercises at maximal “all out” effort, complimented by low intensity exercise or rest. For example, 20 seconds of burpees at your quickest pace, followed by 40 seconds of rest, repeated 3-5 times.

Why is HIIT so popular?

HIIT is increasing in popularity because it is time-efficient, requires little if any equipment and it can be adapted easily for different fitness levels. Not only that combined with the right nutrition it gets results fast!

HIIT training can applied across all exercise modules including cycling, running, swimming, weight training circuits and group exercise classes.

The proven benefits of HIIT training include improved:

  • body composition – a decrease in percentage body fat whilst maintaining muscle mass

  • aerobic and anaerobic fitness including maximal aerobic capacity

  • insulin sensitivity to assist in the management of type 2 diabetes

  • overall cardiovascular health

  • blood pressure and cholesterol levels

When you push your body to its limit, a number of energy systems are stimulated. Whilst working up to 90% MHR (an effort of 90%), your body produces some energy via your anaerobic systems - the ATP-PC (phosphagen) system and anaerobic gylcolytic, (lactic acid system).

This occurs because your body requires ATP (your body’s energy currency) at a faster rate than can be produced via aerobic energy metabolism alone. Aerobic energy production relies on the presence of oxygen to produce ATP. When your body’s demand for oxygen exceeds the oxygen available, the anaerobic energy systems start producing ATP.

What makes HIIT so effective is the fact that you can utilize all three energy systems in exercise session. Obviously, the type, duration, intensity and rest periods of each exercise will determine how much of each energy system is used. Thus, it can be easily adapted to help you reach your goals.

For example, 30 seconds intervals of “all out” maximal cycling with 3 repetitions would use overall approx 20% of total energy from aerobic metabolism and 80% from anaerobic metabolism. Nonetheless, the 3rd repetition (with 4 minutes of recovery between each interval) would result in the majority of energy coming via aerobic metabolism.

The “after-burn” affect of HIIT - Burn more calories at rest

HIIT workouts provide similar fitness benefits as continuous endurance training but in shorter space of time. Since you have burnt so many calories in a short time period you end up with an “after-burn” affect, also known as EPOC (Excess Post – Exercise Oxygen Consumption).

During this time your body will burn calories to regain homeostasis (get back to a normal balanced state). Therefore, allowing you to burn calories after your exercise session. The amount of calories you burn will depend on the intensity of your HIIT session but it is estimated it adds an 6-15% more calories to the overall energy expenditure of your workout (Kravitz, 2014).

The mechanisms thought to contribute to EPOC include:

  • Increased fat oxidation in the removal of lactic acid and resynthesise of glycogen.

  • Re oxygenating blood and regaining adequate hormone levels.

  • Re-establishing optimum core body temperature, heart rate and ventilation.

There is a growing body of evidence to support the benefits of HIIT in many different areas of health and fitness. Nevertheless, to optimize your results from high intensity training you need to consider other lifestyle factors including adequate sleep (between 6-8 hours a night) and proper nutrition.

If you would like more information on tailoring your diet to fit your exercise regime please contact us at

Article based on the following papers:

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