Many studies show that low-volume, high-intensity interval training (HIIT) can improve aerobic capacity and cardio metabolic health. Indeed there is increasing evidence to suggest that shorter and more intense exercise may be just as good as, if not better than, longer and less intense workouts such as traditional continuous aerobic sessions. Lets take a closer look.
Research published in the Journal of Physiology in 2008, found that six weeks of “sprint” interval training (SIT) produced metabolic changes in muscles comparable to those seen with a traditional aerobic training, but in much shorter time. There were two groups of untrained subjects. One group performed workouts of 40 – 60 minutes of continuous cycling 5 days per week. The sprint interval training consisted of four to six repeats of a 30 second ‘all out’ cycling session with 4.5 minutes recovery between repeats. This was completed 3 times per week.(1)
Given the markedly lower training volume in the SIT group, the data suggests that high-intensity interval training is a time-efficient strategy to increase oxidative capacity in the muscles and induce specific metabolic adaptations during exercise that are comparable to traditional aerobic training. Oxidative capacity is a muscles maximal capacity to use oxygen and basically reflects your aerobic performance. Exercise induced oxidative capacity is strongly correlated with health and longevity.
A study published in the European Journal of Applied Physiology in 2014, found that healthy but sedentary men who did high-intensity interval training three times a week for eight weeks showed greater improvements in aerobic capacity than those who did traditional continuous workouts. (2)
In addition to these studies, there has been a lot of interest in the metabolism of fat for fuel during high intensity interval training. A study published in Applied Physiology, Nutrition and Metabolism in 2008 showed that fat oxidation, or fat burning was significantly higher and carbohydrate oxidation, significantly lower after 6 weeks of interval training.(3) Similarly, a study published in the Journal of Applied Physiology in 2007 reported that young females who performed seven HIIT workouts over a two-week period experienced a 36% increase in fat oxidation. (4)
Another benefit of HIIT training is the increase in post exercise energy expenditure referred to as Excess Post-exercise Oxygen Consumption (EPOC). Following a high intensity workout, oxygen consumption remains at an elevated level to restore physiological and metabolic factors to pre-exercise levels. This results in a higher caloric expenditure following the workout. It’s a bit like a post-exercise after burn that expends more calories. The intensity of an aerobic exercise session has the greatest impact on EPOC. As exercise intensity increases, the greater the magnitude and duration of EPOC and therefore the greater the calorie burn.(5)
Lets turn our attention to the benefits of interval training on cardio-metabolic health. A study published in April 2016, investigated whether sprint interval training (SIT) could improve insulin sensitivity and other measures of cardio-metabolic health to the same extent as traditional moderate-intensity continuous training (MICT). SIT involved 1 minute of intense exercise within a 10-minute time commitment, whereas MICT involved 50 minutes of continuous exercise per session. (6)
In this randomized controlled trial, sedentary men performed three weekly sessions of sprint interval training or moderate intensity training for 12 weeks or served as non-training controls. The sprint interval training involved 3 x 20 second ‘all-out’ cycle sprints interspersed with 2 minutes of cycling at a lower level. The moderate intensity training involved 45 minutes of continuous cycling at 70% maximal heart rate. Both protocols involved a 2-minute warm-up and 3-minute cool-down period. The results showed that twelve weeks of brief intense interval exercise improved indices of cardio-metabolic health to the same extent as traditional endurance training in sedentary men, despite a five-fold lower exercise volume and time commitment.
A study published in Diabetologia in October 2014 showed that people with Type 2 diabetes benefited more from interval walking, showing more controlled blood sugar in comparison to people who walked continuously.(7) Individuals with type 2 diabetes were randomised to 3 groups. The interval walking training group (IWT), the continuous walking training group (CWT) and the control group. The training groups performed five sessions per week with 60 minutes per session. The results showed that blood sugar control was only evident in the IWT group. The researchers said this was likely to be caused by interval walking training (IWT) induced increases in insulin sensitivity and increased peripheral glucose disposal which indicates improved glucose metabolism No changes occurred in the continuous walking training (CWT) group or the control group. These results suggest that training with alternating intensity, and not just training volume and mean intensity, is a key determinant of changes in whole body glucose disposal in individuals with type 2 diabetes. Estimated glucose disposal rate (eGDR) is a validated clinical tool for estimating insulin sensitivity in diabetes.
Short burst or interval training means exercising intensely for a short period, 30 to 60 seconds for example, and then resting for 1 – 2 minutes. Basically, we are alternating between high and low intensity exercise or rest over a 20 minute period. There are a number of variations to this with names such as burst cardio, high intensity interval training, peak fitness and so on. They are all based on the same idea of short intense bursts of exercise, followed by a short rest or lower intensity exercise.
Here is one variation of this type of training. The objective is to elevate the heart rate to anaerobic thresholds for 30 seconds and recover for 90 seconds. This cycle is repeated 8 times. You’ll need a couple of minutes to warm up preceeding the exercise and a few minutes to cool down afterward.
The exercise could be on a treadmill, elliptical trainer or recumbent bike for example. The intensity is dependant on the individual and for those starting out, it could simply be alternating fast walking with slow walking. The important thing is to elevate the heart rate into the training zone for 30 seconds and then rest. You might think that 30 seconds is not very long but if you are working hard enough, it will be enough. Think of the 100 metre sprint. Olympic athletes can run the 100 metres in less than 10 seconds these days. This is extreme high intensity. These athletes spend all their time training for this 10 seconds so all of their training is very high intensity in short bursts. However, you don’t need to be an Olympic athlete to apply the principles of short burst/interval training. It can be tailored to any starting level of fitness so if you are very fit, then you might be sprinting at the track for example. If you are just starting out, then you could be out in the countryside walking. It really doesn’t matter as the benefits are the same and your fitness will increase as you progress.
These workouts are much shorter than traditional hour long cardio sessions and much more effective. In total they last 20 minutes and in that time you will have done only 4 minutes of intense exercise. However, you will certainly feel like you have worked hard, therefore, break into this routine gradually as you don’t want to overdo it when you first start out.
Here is another variation of burst training that you can apply to your favourite exercise.
Warm up for 3 minutes. Then start your exercise as intensely as you can for 60 seconds. This should be intense enough so that it is difficult to talk while you are exercising. Next, reduce the intensity to at least half. You may even need to rest to catch your breath and prepare for the next repetition. After 60 seconds, repeat this cycle. After you’ve completed this 4 times, you may find you’ve had enough and its time to cool down with a few minutes of low intensity exercise.
It should be noted that you need to allow more rest between these types of training sessions. It wouldn’t be wise to do them every day as you need more recovery time, therefore, it is recommended to take a day off between these sessions or do some low intensity cardio exercise. High intensity doesn’t necessarily mean military style bootcamp workouts. High intensity is an individual concept and will be different for someone just starting out than for a seasoned athlete. It doesn’t matter what your level of fitness currently is, the benefits are there for everyone.
The evidence clearly shows that high intensity interval training has many benefits. It’s time effective and can induce similar performance adaptations as traditional, moderate intensity, longer duration aerobic workouts. However, it might not be for everyone. There is the psychological aspect of this training that can potentially put people off from starting such an exercise routine or sustaining such a routine. These workouts can be tough because of the intensity, even though they are of short duration. This takes sustained motivation to keep coming back for more and if something is very hard work and you simply don’t enjoy it, then, quite naturally, it will probably fall by the wayside. Consistency is the key to any exercise program and achieving your goals. That is why it is important to choose a type of exercise that suits you and one that you find enjoyable. It is also important to have compelling reasons to keep you motivated. Without enough compelling reasons, the motivation simply doesn’t last. Compelling reasons can include a goal to lose a certain amount of weight, to be fitter and healthier, to look better in clothes, to have more energy, to reduce the risk of chronic inflammatory diseases, or even to get back to good health from a chronic illness. The more reasons, the better. If you enjoy a particular type of exercise and have a number of compelling reasons, then you have the ingredients of a sustainable exercise routine. Incorporating high intensity interval training into your health and fitness strategy will certainly pay dividends but does this mean we should stop doing traditional moderate intensity, longer duration aerobic exercise? Absolutely not. I believe any exercise is better than none and many people will enjoy the traditional aerobic exercise more than high intensity interval training, regardless of the increased time commitment. Your exercise routine has to suit you, your preferences and our lifestyle and ultimately, this leads to exercise being an integral part of your life.
For those already engaged in intense exercise such as heavy weight training in the gym 3 or 4 times a week, doing another 3 workouts of high intensity interval training sessions on top of this, would probably be too much and lead to over-training. We’ll talk about overtraining in another article, but for now, lets say that doing too much exercise can certainly put too much stress on the body, particularly the central nervous system and put a stop to any progress. I personally cycle my training so that I do a combination of high intensity training and moderate intensity training. If I’m in the gym 3 times a week with intense weight training workouts, I’ll do lower intensity cardio sessions in between which I find promotes active recovery. If I am in a lower intensity, maintenance phase with my weight training workouts, I will be able to switch to high intensity interval training outside of the gym training. If I was to do both high intensity interval training and high intensity weightlifting workouts, I would overload my recovery ability, overtrain and burn out.
So there we have it. The evidence shows that high intensity interval training has a multitude of health benefits including increases in both aerobic and anaerobic fitness, increased metabolism following exercise, increased fat oxidation during and after exercise, significantly lowers insulin resistance and results in increases in skeletal muscle capacity for fatty acid oxidation. With all these benefits as well as a lower time commitment, it is a useful tool to have in our overall health and fitness strategy. Since both HIIT and continuous aerobic exercise programs improve all of the important physiologic and metabolic functions,, it makes sense to incorporate both approaches into our routine. Its up to you, dear reader, to make an informed decision on how to incorporate this into your life and to what extent, based on your preferences, current capabilities and lifestyle.
It also goes without saying that if you have a heart condition, or perhaps some other chronic illness, then it is best to consult with your doctor before embarking on any exercise program. Likewise, if you have been sedentary for a while and looking to start an exercise program, it is wise to break into the training gradually.
1/ Similar metabolic adaptations during exercise after low volume sprint interval and traditional endurance training in humans.
J Physiol. 2008 Jan 1; 586(Pt 1): 151–160.
2/ Low-volume, high-intensity, aerobic interval exercise for sedentary adults: VO2 max, cardiac mass, and heart rate recovery.
Eur J Appl Physiol. 2014 Sep;114(9):1963-72.
3/ High intensity aerobic interval training increases fat and carbohydrate metabolic capacities in human skeletal muscle. Applied Physiology, Nutrition and Metabolism, 33(6) 1112-1123
4/ Two weeks of high-intensity aerobic interval training increases the capacity for fat oxidation during exercise in women. Journal of Applied Physiology 102(4):1,439-1,447, 2007.
5/ Effect of exercise intensity, duration and mode on post-exercise oxygen consumption. Sports Medicine, 33(14) 1037-1060.
6/ Twelve Weeks of Sprint Interval Training Improves Indices of Cardiometabolic Health Similar to Traditional Endurance Training despite a Five-Fold Lower Exercise Volume and Time Commitment. PLoS ONE 11(4): e0154075.
7/ Mechanisms behind the superior effects of interval vs continuous training on glycaemic control in individuals with type 2 diabetes: a randomised controlled trial. Diabetologia October 2014, Volume 57, Issue 10, pp 2081–2093
Sports & fitness nutritionist, researcher and author on a mission to improve the human condition. Focusing on evidence-based and outcome-based nutrition, training, mindset & environment