Australasian Science: Australia's authority on science since 1938

Go Hard or Go Home

Is high intensity interval training the latest exercise fad or is there a physiological basis to it?

Exercise trends come and go, and thank goodness for that in most cases. Those unfortunate enough to be my age may recall when lycra midriff tops were popular among men at gyms (admittedly in the 1980s, the decade that style forgot) and when blokes totally lacking coordination like myself were obliged to attempt impossibly complex dance steps in aerobics classes. I’m hoping that core stability, a ridiculous form of exercise based on grown men prancing across the gym floor while holding kettle weights in either hand, will soon also be issued with a “never to be revived” notice.

But occasionally something more substantial turns up. In gyms around Australia today the hot whisper is HIIT: high-intensity interval training. HIIT involves repeated short bouts of very high intensity exercise, alternating with low intensity exercise or rest. The length of both the all-out and rest periods can vary from a few seconds to a few minutes, with typical exercise:recovery ratios 1:1 or 1:2. You can use a stationary bike, a treadmill, a rower, or any other mode where you can reliably estimate intensity.

HIIT has been touted as being more effective than the traditional long moderate-intensity steady state exercise at reducing body fat, increasing aerobic power, reducing insulin resistance, controlling blood pressure, improving blood fat levels and making exercise more fun and more time-efficient. It has been used successfully for both those who are healthy and chronically ill, lean and fat, young and old.

What happens physiologically when we do very high-intensity exercise? First, we get a massive hormone hit. Levels of cortisol, epinephrine (adrenalin) and norepinephrine (noradrenalin) skyrocket (all hormones release fat from body fat stores), and human growth hormone levels shoot up (this creates a good muscle-building environment). There is a rapid release of blood glucose (which may help diabetics who risk hypo­glycaemia during moderate steady-state exercise). All of these changes are a kind of emergency response to what the body sees as an urgent demand for energy.

There is evidence that following a HIIT session, the sustained increase in metabolic rate lasts longer and burns more calories. This “metabolic afterburn”, known as EPOC (excess post-exercise oxygen consumption), may be one of the mechanisms whereby HIIT may be so effective in reducing body fat. HIIT also appears to suppress appetite much more than steady-state exercise.

HIIT improves endothelial function. Endothelial cells are the cells lining the walls of blood vessels. They constrict or dilate blood vessels, and thus regulate blood pressure.

Finally, while most exercise reduces inflammatory responses (which is a good thing, as chronic low-grade inflammation is increasingly emerging as a common factor underlying diseases from depression to diabetes), high-intensity exercise may be particularly effective.

There’s some epidemiological evidence to back up the benefits of high-intensity exercise, although long-term studies on HIIT are not yet available. Vigorous exercise is associated with a reduction in all-cause mortality, which is about twice as great as moderate exercise of the same duration ( The evidence from laboratory trials is impressive. With programs lasting 2–24 weeks, body fat has been reduced by 7–18%, maximal aerobic power increased by 6-41%, and insulin sensitivity (the effectiveness of insulin in storing blood glucose) improved by 19–58% (

Some exercisers do much better than others, with about 50% of the variance in training response being genetically determined. When HIIT and steady-state exercise are compared matched for time, energy expenditure or both, HIIT generally performs better.

Perhaps the most attractive aspects of HIIT are that it requires no dance skills, can be performed without a lycra top, and doesn’t involve making an idiot of yourself in the gym. Those benefits alone should make it worth a try.

Professor Tim Olds leads the Health and Use of Time Group at the Sansom Institute for Health Research, University of South Australia.