Dr. Martin Gibala is a professor and Chair of the Department of Kinesiology at McMaster University.
High intensity interval training (HIT) is hot. In fact, it was recently crowned the top fitness trend for 2014, based on an annual worldwide survey conducted by the influential health and fitness organization, the American College of Sports Medicine (ACSM).
High intensity interval training refers to the basic concept of alternating brief periods of vigorous exercise with short periods of recovery. Serious endurance athletes have long appreciated its powerful performance benefits, but what is the scientific evidence to support claims that HIT will improve your health?
When describing the current state of interval training research, I often use an analogy borrowed from the pharmaceutical industry. Current physical activity guidelines, which generally call for at least 150 minutes of moderate– to vigorous-intensity aerobic physical activity per week, represent the well-established drug of choice and are based on a rich body of scientific evidence.
In its most recent "position stand" on exercise prescription, the ACSM adopted an evidence-based medicine approach, which involves the careful and systematic analysis of published literature on a given topic. It concluded that research on the health benefits associated with traditional aerobic training constituted Category A evidence, the highest rating. In contrast, HIT can be viewed as an emerging new drug on the market. It is showing considerable promise in early feasibility studies, but there is nowhere near the evidence required to suggest this type of training elicits all of the health benefits associated with traditional aerobic training.
What we know
In evaluating the relative merits of HIT, one can make an 'apples to apples' comparison, whereby a given amount of training is compared against an equivalent volume of moderate-intensity continuous exercise. Using this approach, there is good evidence that HIT can provide superior physiological benefits, at least over the short term. This was the conclusion of a recent systematic review that was published in the British Journal of Sports Medicine. (Similar to evidence-based medicine, a systematic review attempts to synthesize the results from a large number of studies on a particular topic.) The authors concluded, "(HIT) is superior to moderate-intensity continuous training in improving cardiorespiratory fitness in lifestyle-induced cardiometabolic diseases" such as coronary artery disease and metabolic syndrome. In fact, the improvement was almost double that of moderate-intensity continuous training.
An 'apples to oranges' comparison reveals that a surprisingly small dose of HIT can provide benefits similar to a much greater amount of continuous-moderate intensity training, again over the short term. For example, a study from our laboratory compared two groups of young healthy subjects who trained for six weeks. One group performed up to an hour per day of continuous cycling at a moderate pace, five days per week. The other group trained using 30 sec all out bursts of cycling, repeated 4-6 times with a few minutes of rest in between three times per week. Both groups showed similar improvements cardiorespiratory fitness and various cardiovascular and metabolic indices of health after training, despite the fact that the HIT group performed 90 per cent less total exercise, and the total training time commitment was only a third of the other group. These findings are potentially significant from a public health perspective, given that "lack of time" remains the number one cited barrier to regular physical activity.
What we don't
The vast majority of HIT studies have lasted only a few weeks to up to several months, and have involved relatively small numbers of subjects. Research on the potential benefits of low-volume HIT, in which a very small total amount of very intense exercise is performed, are particularly sparse. Much of this work has examined physiological changes in young healthy individuals, and only a handful of studies have applied low-volume HIT protocols to non-healthy individuals including those with cardiometabolic diseases such as type-2 diabetes. While generally found to be safe, perceived as enjoyable and well tolerated in laboratory studies, only a few studies have examined the feasibility of implementing HIT in real-world scenarios, where people have to perform the training on their own. It also remains unknown whether adherence to this form of training is any better than for continuous moderate-intensity exercise; that is, will people stick with a HIT protocol over the long-term?
To return to the pharmacological analogy, large-scale, randomized clinical trials are needed in order to systematically compare HIT versus traditional aerobic training in different groups of people over the long-term, and employing gold-standard clinical markers of health status such as insulin sensitivity and blood pressure, as well as psychosocial determinants of well-being.