Anyone who has ever done an 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.

HIIT training_2

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

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 6-15% more calories to the overall energy expenditure of your workout (ACSM, 2007).

The mechanisms thought to contribute to EPOC include:

– Increased fat oxidation in the removal of lactic acid and resynthesize 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.

We would love to hear about how you use HIIT to get the most out of your fitness regime. Feel free to contact us with any queries about how to optimize your diet whilst training.

Connect here with Expert Charlotte Umney.

Article based on the following papers:
Boutcher S.H (2011). High Intensity Intermittent Exercise and Fat Loss. J Obes
Kravitz, L (2014). High Intensity Interval Training. ACSM
Gibala, M.J (2007). High-Intensity Interval Training: New Insights.


WatchFit Experts change lives!

And they can do the same for you.


Pollyanna Hale Health and Lifestyle coaches
Lost 13 Kg in Total
Mel, 32y Location: London, United Kingdom Working with Pollyanna changed everything. I lost 13kg, got toned and have more energy than ever! Get same results!


Chriz Zaremba Fitness Consultant
Lost 45 Kg in Total
Chris, 50y Location: London, United Kingdom Lost 45kg after the age of 50 and now competes and wins physique competitions and runs marathons Check our weight loss plans