Is your HIIT a Hit or a Miss?

If you're a regular gym goer, you will no doubt be familiar with the acronym HIIT (High Intensity Interval Training), sometimes termed 'HIIT training'. HIIT is exercise of a very high intensity performed in intervals or intermittent spurts, followed by periods of lower intensity or rest, which are repeated several times in a session. There are many HIIT protocols, but a simple example would be a 30 second all-out sprint, followed by 60 seconds of walking/rest, repeated 10 times over a period of 15 minutes. HIIT sessions usually last between 10 and 20 minutes in total, although some are longer, and some more extreme forms of interval training, such as the Tabata protocol, last only 4 or 5 minutes.


HIIT training is very popular due to its apparent effectiveness at burning body fat, and its attractively short duration. After all, who wouldn't want to spend just 10 minutes a day exercising, rather than 30 to 60 minutes slogging it out. Studies suggest that HIIT may be more effective at burning fat, decreasing insulin resistance (important in controlling blood sugar levels), and improving outright aerobic fitness when compared to steady-state, low intensity exercise performed over a 12 to 15 week period.

One of the popular reasons put forward for HIIT's effectiveness is an effect known as 'EPOC' (Elevated Post-exercise Oxygen Consumption), or 'Afterburn', as it's referred to in fitness circles. Put simply, when you train at a very high intensity, your body's metabolic rate will be elevated during the recovery period, often for 2 or 3 days as it works to replenish and repair itself. During this time, calorific expenditure is increased, so while actual calories burned during the activity may be relatively low (especially with some of the shorter HIIT protocols), calories burned during the days that immediately follow will be greater than normal resting levels. However, this increase in resting metabolic rate may account for no more than 15% of the calories burned during the HIIT routine (LaForgia J et. al. Effects of exercise intensity and duration on the excess post-exercise oxygen consumption. J Sports Sci. 2006 Dec;24(12):1247-64.), so probably not enough to fully explain its effectiveness in fat reduction, especially when you consider the overall energy burned during a shorter HIIT routine (such as a 4 minute Tabata session) could be as little as 100Kcal or less.

Another reason why HIIT may be such an effective fat torcher, is the elevated level of catecholamines experienced immediately after HIIT. Catecholamines are a group of hormones involved in the fight or flight response that our bodies experience when in immediate danger or stress, but are also active in the break down of fats used as energy. It is also suggested that HIIT is effective in suppressing appetite post-exercise, which could also encourage a greater calorie deficit and further fat loss. (ref. High Intensity Intermittent Exercise and Fat Loss by Stephen H. Boutcher).

So it would seem that HIIT training has much going for it, hence its popularity in classes and the many HIIT based fitness routines available in books, DVDs and online. Although in practice, HIIT training isn't without its pitfalls. We need to keep in mind also that the comparison between HIIT and steady-state training is usually made with fairly moderate exercise intensities in the steady-state group. A conditioned person may be able to maintain 20 to 40 minutes of steady-state exercise at a considerably higher level than those compared against HIIT, so it is reasonable to assume that the proportionate post-exercise calorie burn from EPOC may be more comparable to HIIT under these conditions, with the added potential for more calories to be burned during the activity itself.

One of the drawbacks for some doing HIIT is that, when performed correctly, it can be a very punishing workout, so motivation levels need to be high. For someone that is new to exercise, HIIT training might be a very uncomfortable start; not ideal when trying to encourage participation in more physical activity. Also, HIIT is not currently listed in the recommendations for patients referred by GP's for certain medical conditions, with lower intensity training being the preferred option.

Secondly, the delivery of 'HIIT' is sometimes too watered down to be effective. I have witnessed this in gyms, and even supervised PT sessions; evidenced by the ease with which clients continue to discuss their weekend exploits in the middle of their 'high' intensity spurts. During HIIT, the intensity level needs to be such that you are working pretty much all-out, and exercise choice has a lot to do with this. Trying to do a HIIT session with bicep curls or abdominal crunches will do little to elevate your heart rate to the levels required. A good HIIT session should use exercises that employ multiple or large muscle groups, hence why most effective HIIT routines usually involve the legs: sprinting, rowing, cycling, burpees, jumping movements etc.

Remember too, that although the short duration of HIIT is very appealing, you should factor in the need to warm up and cool down sufficiently, which could easily add another 10 to 15 minutes to your 'short' HIIT routine.

So what's the best way to approach HIIT? Well, if you're new to exercise, you should probably work on building your aerobic fitness using lower intensity exercise during the initial weeks of training. This will help you cope better with HIIT, if and when you decide to add it to your routine. If you are an experienced exerciser, and have no medical conditions that warrant you avoiding very strenuous exercise, then by all means give HIIT a go, but it's still wise to build up slowly over a few sessions so you can adapt to the new style of training.

Remember as well, that while HIIT has been shown as a very effective way to burn fat, this is not to say that lower intensity, steady-state exercise doesn't have its place. Quite the opposite in fact: current guidelines for the UK adult population include recommendations for around 150 minutes a week of moderate intensity aerobic exercise each week, and those meeting these guidelines will still benefit from the many other health benefits associated with physical activity, such as reduced risk of heart disease & certain cancers, and increased life expectancy. (ref. British Heart Foundation Physical Activity Statistics 2015)

To demonstrate HIIT, I have found a great video showing a Tabata session on a cross trainer. Tabata is a very extreme form of HIIT using 20 seconds of maximum effort followed by just 10 seconds of rest, repeated 6 to 8 times. You should consult with your doctor before increasing your level of physical activity, or if you have any doubts about the kind of exercise you should be doing.