High Intensity Interval Training (HIIT) has been around for a while but has become particularly popular in recent years. Offering better results than any other form of exercise and in only a fraction of the time, it’s not hard to see why so many people have been attracted to the idea of it. HIIT works by shortening the duration of an exercise whilst massively increasing the intensity. Resting and then repeating for a certain number of intervals.
For example, instead of cycling on an exercise bike at a slow and steady pace for 40 minutes, you would cycle for as fast as you can for 30 seconds, then slow down for 90 seconds and peddle at a leisurely rate whilst getting your breath back. Then you would cycle at maximal intensity again, repeating this eight times (or however many you sets you choose to perform).
You can use different time frames, different forms of exercise and shorter/longer rest periods but the idea is performing short bursts of very high intensity exercise, before resting and then repeating. Many personal trainers love HIIT and fill their sessions with them, exercise classes have been created that are just HIIT and even HIIT gyms now exist.
But how effective is HIIT really? And is it more effective than weight training or low intensity cardio? Below we look into the supposed benefits of HIIT and assess whether it is fit for purpose, or just another fad.
While there are a lot of purported benefits for HIIT how many of them are actually backed up by science? The following benefits are all evidence-based benefits that HIIT can cause.
Benefit #1. Increase in VO2 max
A 2015 study found that HIIT was more effective at increasing VO2 max (the maximum rate that the heart and lungs can send oxygen round the body) than endurance exercise. This means that if you wanted to improve your cardiovascular system, then HIIT would be more effective .
Benefit #2. HIIT allows you to train for longer at your VO2 max
Whilst benefit number one showed us that HIIT was effective at increasing your VO2 max, this second benefit shows that HIIT can allow you to train at your VO2 max for longer. In layman’s terms this means you can train at maximum effort for a slightly longer duration. A study by Billat et al (2000) found that HIIT that intermittent runs were very effective at increasing distance run at VO2 max .
Benefit #3. HIIT can lead to higher EPOC
EPOC stands for excess post-exercise oxygen consumption, after an exercise session you may find yourself in an oxygen deficit, and as a result your body will intake more oxygen to redress the balance. During this period of time your body begins to rebalance after strenuous exercise, your hormone levels normalise, your ATP levels are replenished and stored energy (particularly fat) is burned for fuel.
Having a higher EPOC means that you will oxidise (burn) more fat, and HIIT has been shown to produce higher EPOC than endurance exercise . In fact, the fitter you are (due to more HIIT) the higher the EPOC . This is mostly due to the fact that a well-trained individual can train at a higher intensity (see benefits 1 and 2).
This means that the fitter you are, the more fat is oxidised too . So getting very good at HIIT is fantastic for increasing fat loss.
Benefit #4. HIIT can lower the risk of Type II Diabetes
A study by Osawa et al (2014) looking into the effects of a 16 week HIIT program found that it could help people regulate insulin better helping to lower the risk of them getting Type II Diabetes *.
Now that we have seen the many benefits of high intensity interval training we should spend some time looking at some of the drawbacks. You see the thing about HIIT is that it is... well... intense! This means that it can be very difficult to perform properly. The sad fact is that a lot of the people who most need the benefits are incapable of exercising at that intensity.
Telling an obese person to sprint as fast as they can on a treadmill is going to be a costly mistake, not only are they going to suffer from a cardiorespiratory point of view, they won’t have the coordination to perform the exercise correctly. Think about it this way if someone walked into the weights room for the first time would you stick them under a 100kg bar and tell them to squat?
Obviously not, but by asking them to sprint as fast as they can you are putting them at the same risk. In a study by Levinger et al (2015) titled “What doesn’t kill you makes you fitter”  the point is made that whilst HIIT has many benefits for sufferers of cardiovascular and metabolic disease sufferers it is also a danger to them.
But even without the injury risk, the fact that someone is actually incapable of training at that level of intensity means that they won’t be getting the benefits of HIIT anyway. If you can’t cycle fast enough to reach your VO2 max then you won’t get the increased EPOC which means less fat loss.
In the case of obese or unfit persons good old fashioned low intensity steady state (LISS) would be much more effective. Walking around the block a few more times per week may not be cool, but it will be as effective at burning calories in a person who hasn’t exercised in 15 years. Treating HIIT as a one size fits all for all fitness levels is a mistake.
If you are planning on utilising HIIT in your session, or alternatively you are thinking of adding it into a client’s session then make sure that they are 1) Fit enough to survive it and 2) Coordinated enough to perform the exercise without risking injury.
*Interestingly the study also found that HIIT increased muscle hypertrophy in both the upper and lower body
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 MilanoviĆ, Z., SporiŚ, G., Weston, M. 2015. Effectiveness of high-intensity interval training (HIIT) and continuous endurance training for VO2max improvements: A systematic review and meta-analysis of controlled trials. Sports Medicine 45(10): 1469-1481
 Billat, V., Slawinski, J., Bocquet, V., Demarle, A., Lafitte, L., Chassaing, P., Koralsztein, JP. 2000. Intermittent runs at the velocity associated with maximal oxygen uptake enables subjects to remain at maximal oxygen uptake for a longer time than intense but submaximal runs. European Journal of Applied Physiology 81: 188-196
 Laforgia, J., Withers, R., Shipp, N., Gore, C. 1997. Comparison of energy expenditure elevations after submaximal and supramaximal running. Journal of Applied Physiology 82(2): 661-6
 Frey, G., Byrnes, W., Mazzeo, R. 1993. Factors influencing excess post exercise oxygen consumption in trained and untrained women. Metabolism 42(7): 822-828
 Hetlelid, K., Plews, D., Herold, E., Laursen, P., Seiler, S. 2015. Rethinking the role of fat oxidation: substrate utilisation during high-intensity interval training in well-trained and recreationally trained runners. BMJ Open Sport & Exercise Medicine 1: e000047. doi: 10.1136/bmjsem-2015-000047
 Osawa, Y., Azuma, K., Tabata, S., Katsukawa, F., Ishida, H., Oguma, Y., Kawai, T., Itoh, H., Okuda, S., Matsumoto, H. 2014. Effects of 16-week high-intensity interval training using upper and lower body ergometers on aerobic fitness and morphological changes in healthy men. Open Access Journal of Sports Medicine 5: 257-265
 Levinger, I., Shaw, C., Stepto, N., Cassar, S., McAinch, A., Cheetham, C., Maiorana, A. 2015. What doesn’t kill you makes you fitter: A systematic review of high-intensity interval exercise for patients with cardiovascular and metabolic diseases. Clinical Medicine Insights: Cardiology 9: 53-63