HIT - The new way to exercise

    HIT image

    No matter how you dress it up; health, diet, exercise and lifestyle advice always seems to boil down to the same thing; eat less and exercise more. This mantra has been batted around as a simplistic summary of just about every reputable government recommendation and weight loss programme to come into the public eye. The problem is, whilst it’s difficult to argue against this idea, it also appears to be difficult to adhere to. Recently, a new mantra, something that could be summarised as “eat the same and exercise less”, has hit the headlines as the latest way to combat the obesity crisis.

    HIT (high-intensity interval training) is the new way to exercise. “Improve your health and fitness with only 3 minutes of exercise” the headlines shout. This seems much more appealing than disruptive, time consuming lifestyle changes and something that might motivate more people to train. However, given that the current guidelines advise doing 30 minutes of exercise a day, I was initially sceptical that one tenth of the activity would have the same effect. However, being a sports physiologist I thought it best to investigate the science behind the headlines more thoroughly before passing judgement.

    What is HIT?

    HIT, the kind most commonly described in the headlines involves 4-6 bouts of vigorous exercise performed at an intensity that requires maximal exertion for 30s, interspersed with 4 minutes of rest. Most studies have used the cycling Wingate test to create this exercise. This test that involves cycling as fast as you possibly can against a heavy resistance equal to 7.5% of your body weight (that’s more than it sounds). The Wingate test was originally designed to assess peak power and power endurance of elite athletes. To put it in perspective, moderate intensity exercise; activity that makes you breathe heavily, tends to be performed at ~70% of your maximum heart rate (130-140bpm). Done properly, HIT repeatedly pushes you to your maximum (170-190bpm), a bit like doing the 100m sprint at full pelt 4-6 times.

    Interval training is nothing new. Athletes have incorporated elements of HIT for the last one hundred years, with great success. The reason being that repeatedly exercising at high intensity for short periods and resting; long enough to be able to repeat the effort but not quite long enough to recover fully, causes the body to adapt to the higher intensity and one is eventually able to sustain that level of exercise for longer, thus increasing fitness. While this can also be achieved with continuous moderate exercise, the attractive characteristic of HIT is that the time it takes to increase fitness is much shorter.

    That HIT can improve fitness quickly is important because just about every survey investigating barriers to performing physical activity reports “lack of time” as the number one reason people don’t exercise enough (which, as it happens, is an estimated 95% of the population).

    Why exercise at all?

    The main benefit of exercise is that it improves fitness. But good fitness means more than just sports performance. Being “fit” means that your heart is healthy and can pump efficiently, reducing the risk of heart attack. Physical activity keeps your muscles strong and flexible, reducing the risk of arthritis and keeping you independent as you grow older. Exercise not only trains your muscles, but also your blood vessels, reducing the risk of atherosclerosis (fatty deposits in the arteries), hypertension (high blood pressure) and stroke. High fitness means high insulin sensitivity i.e. optimal glucose regulation and a reduced risk of type 2 diabetes. In total there are over 20 chronic diseases that exercise helps prevent, mostly due to its halo effect whereby one action results in a cascade of benefits.

    The duration:intensity trade-off

    Minute for minute; higher intensity exercise is thought to enhance health to a greater extent. Thus, it might seem obvious that training at high intensity training would allow an individual to exercise for less time with the same benefit. Indeed, the recommended activity time for vigorous exercise (when you are out of breath and can hardly speak) is only 75 minutes per week, half the time for moderate exercise. Advocators of HIT go one step further suggesting that just 15 minutes of maximal exercise per week can produce the same benefits. That works out as about 20% of the energy expenditure you would use for 30 minutes of moderate exercise.

    The duration:intensity trade-off has been a long debated issue in sports science. No one could argue that there is not an interaction between them; the evidence demonstrates that the higher the intensity, the less time required to stimulate benefits, and vice versa. On the other hand, some researchers suggest that for some aspects of health, duration is the most important factor, and that there may be a threshold below which health adaptations do not occur.

    The evidence

    Two independent reviews of “Wingate-style” HIT (see diagram) conclude that aerobic fitness is improved by HIT to an extent that is superior to the effect of traditional exercise. These reviews pooled the results of 22 studies, which collectively demonstrate that maximal oxygen uptake (VO2max; an objective measure of fitness) increases by a clinically significant amount when subjects perform HIT. Having read most of the original articles included, this is enough to convince me that when it comes to improving fitness, HIT is time-efficient and effective. If HIT can improve fitness, it follows that HIT should improve all aspects of health…

    There is a general consensus, although not a finding that has been regularly reported, that HIT does not reduce body fat. Fortunately, while excess weight is best avoided, fitness appears to be more important than fatness when it comes to health.

    There are a number of other forms of interval training such as; 4 repetitions of 4 minutes of high-intensity exercise with 4 minutes off in between, or 10 reps of 60s on, 60s off as well as a whole host of other variations. The difference between these types of interval training and Wingate-style HIT is that the “high-intensity” of Wingate HIT is much higher than that of the “aerobic” interval training just described. When all types of interval training are assessed together (Wingate and aerobic), interval training appears to be equal to continuous training in improving heart function, blood pressure, cholesterol and glucose regulation. The benefits of Wingate-style HIT alone over and above fitness have not been reviewed.

    HIT at home

    Two concerns people have with HIT are its safety and how easy it is to do outside the gym environment. Health professionals may be cautious about recommending very high intensity exercise to anyone other than well-trained individuals and certainly not before a rigorous screening process available in exercise laboratories or by a GP. That Andrew Marr attributed his stroke to HIT training did nothing for its reputation as a safe activity. Given Marr’s medical history, I question a direct link though. In hindsight Marr was at high risk for stroke, which could have been triggered by any increase in blood pressure by one of many potential causes. I also resent the omission of the idea that Marr’s successful recovery could partly be attributed to his good health and high fitness. We need to consider that exercise prevents far more cardiovascular incidents and deaths than it causes. This event does, however, stress the need for safety checks, something I can only assume would deter people further by adding another barrier to exercise.

    Taking HIT out of the exercise lab may also be difficult. In reality most people do not have access to exercise bikes where a high load can be added and removed very quickly. A few studies have addressed this issue with some success where HIT has been performed in a supervised group setting using running as exercise. While sprint training is easier (it requires less specialist equipment), there is a necessity for individuals to have to motivate themselves to perform repeated “maximal” efforts. The determination of “maximal” would be subjective instead of prescribed so the question of “how high is high enough?” arises. In addition, running is associated with relatively high rates of muscular-skeletal injury in unfit individuals. Taken together, these factors make the routine use of HIT by most people somewhat limited.

    Finally, one detail that the media fails to report is that when warm-up, recovery periods and cool-down are taken into account, HIT sessions actually take at least 25minutes (see diagram). Whether 5 minutes is a meaningful time saving is up to the individual, but what I will reiterate is that the national recommendation for continuous vigorous exercise is 75 minutes per week, or 15 minutes 5 x per week.

    hit blog diagram

    The Verdict

    My research has led me to conclude that so far, HIT has been demonstrated to be as effective as traditional exercise training in improving fitness and to some extent health as described earlier. Despite requiring more effort, some find HIT less boring than continuous aerobic exercise such as jogging, and are therefore more motivated to perform it. Therefore, if safe for the individual and performed properly (i.e. at a high enough intensity), in my opinion, HIT is an appropriate alternative form of exercise to continuous training. However, there are still many questions as to whether HIT is truly time efficient or feasible in daily routines.

    Charlotte Jelleyman MSc.


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