Less than 40 per cent of Australians achieve the minimum amount of physical activity recommended by government and professional organisations. This contributes to the fact 60 per cent of Australians are overweight or obese. Exercise and Sports Science Australia (and similar organisations overseas) recommend at least 30 minutes of moderate intensity exercise most days of the week, adding up to 150 minutes a week. The most commonly reported barrier to reaching these targets is a perceived lack of time. So it's important to maximise the benefits of exercise in what little spare time we have. But how can we get the most out of our exercise sessions? And is there a way to get enough exercise to stay healthy without spending 150 minutes a week?
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Over the past few years, a number of laboratories have shown that High Intensity Interval Training (commonly known as HIIT) can provide similar — if not greater — health and fitness benefits as traditional exercise approaches. HIIT incorporates repeated bursts of intense exercise interspersed with periods of rest.
But it should be pointed out that, while HIIT is likely to improve overall metabolism, when it comes to fat burning and blood glucose control with minimal weight loss, calorie restriction through dieting is still the best way to lose body weight.
Most HIIT studies are performed on a bike, although some of the aerobic studies have been performed on a treadmill. Essentially, from the exerciser's point of view, it shouldn't matter, as long as the intensity is achieved.
Research has so far focused on three HIIT variations:
Low-volume supra-maximal HIIT
Low-volume supra-maximal HIIT involves four to six repetitions of 30 seconds of all-out exercise (approximately four times greater than maximum aerobic exercise intensity) with recovery periods of around four minutes.
The advantage of this approach is the small time commitment required. Clear benefits in terms of aerobic fitness and insulin sensitivity have been observed with just nine minutes of exercise a week (or 90 minutes total time commitment when you include rest).
But this method of training typically involves specialised equipment (such as electromagnetically braked stationary lab bikes) and due to the very high exertion and motivation required it may not be feasible for the general population.
Aerobic HIIT sessions on the other hand typically involve four to six four-minute repetitions of exercise performed at 80-90 per cent of the maximum aerobic exercise intensity. These efforts are interspersed with one to three minutes of recovery with a single session taking up to 38 minutes.
As this is a lower intensity than the supra-maximal HIIT, it is deemed more achievable and has repeatedly been used successfully to treat patients with metabolic syndrome, hypertension and coronary artery disease.
But as the aerobic HIIT sessions are somewhat prolonged it doesn't offer the same time-saving advantages as supra-maximal HIIT.
Low-volume maximal HIIT
Low-volume maximal HIIT sessions may provide a compromise between the previous two protocols.
This strategy involves eight to ten one-minute bouts performed at maximal aerobic exercise capacity, interspersed with 60-75 seconds of light recovery, therefore offering significant time advantages, with a single session taking around 20 minutes.
Therefore its lower intensity (compared to supra-maximal HIIT) and shorter session duration (compared to aerobic HIIT) may make it suitable for sedentary or obese people, and those with existing metabolic conditions.
How low can you go?
We still don't really know the minimum amount of exercise required to induce significant health and fitness benefits. But a recent study has cut down the exercise time even further, showing that just six ten-second all-out sprints, spread throughout a week can improve aerobic fitness and blood-sugar control.
Evidence is building that the HIIT approach to exercise can substantially improve aerobic fitness while providing a range of positive health outcomes, including better blood-sugar control, blood pressure and blood vessel function in a range of conditions such as obesity, metabolic syndrome, diabetes and heart disease.
These studies have challenged current thinking regarding how much exercise is needed to produce substantial health benefits. This has important implications for those who decide a 20-minute lunch break is insufficient for worthwhile exercise, or for those looking to make the most of their precious gym time.
Meanwhile, exercise physiologists and healthcare providers are keenly awaiting much-needed, large scale randomised control trials on HIIT to be funded and carried out.
Those studies will give us the evidence to challenge current exercise recommendations and allow the incorporation of HIIT into the standard recommendations needed to treat and prevent chronic metabolic and cardiovascular diseases in the future.
While exercise preferences no doubt vary among individuals, and HIIT may not appeal to everyone, HIIT offers a solution for those with little time to be active.
Nigel Stepto is Senior Lecturer, Exercise Physiology at Victoria University. He receives funding from NHMRC and the Australian Governments Collaborative Research Network Scheme. He is affiliated with Exercise and Sport Science Australia and PCOS Alliance of Australia.
Chris Shaw is Research Fellow, Institute of Sport, Exercise and Active Living at Victoria University. He has received funding from BUPA Foundation UK and the Insulin Dependent Diabetes Trust. He is a member of the PCOS alliance, European College of Sports Science and the American Physiological Society.