HIIT, which stands for “high-intensity interval training,” is a hot trend in the fitness world these days. But the truth is that HIIT has been around for a long time and is still popular for a lot of good, science-backed reasons.
What Is HIIT? According to the American College of Sports Medicine, HIIT involves “repeated bouts of high intensity effort followed by varied recovery times.” The bouts of high intensity can be anything from a matter of seconds to multiple minutes in length. The same goes for the recovery periods. The entire workout, once you add the efforts plus the rest periods together, may also range in length, often between ten and thirty minutes. You may have already done HIIT sessions if you have done sprint work either on foot or on a bicycle. You typically go hard for thirty seconds, then walk or cycle at a level that you can recover your breath and recoup yourself before your next go.
Other types of HIIT workouts will include a variety of exercises that you rotate through, intermingled with rest. For example, this workout we posted recently incorporates a row/run/bike, wall balls, and burpees.
The Research and Science Behind HIIT So, now you know the practical definition of HIIT, but what has made it a fitness routine that has lasted the test of time? Why do people get so excited about it? For starters, HIIT can save you a lot of time and modern life has most of us facing very full schedules. This can make it hard to prioritize ourselves and our health. In addition, many people find HIIT to be more fun and more motivating than other, slower-paced modes of exercise. (There’s actually a study that showed this.) And the simple truth is that if you enjoy something, you’re more likely to stick with it and reap the full reward of your efforts.
But beyond those lifestyle benefits, there are a lot of research-backed reasons for the popularity and longevity of HIIT: HIIT workouts require your body to use a great deal of oxygen. Research shows your body has to work to rebuild that oxygen well after you finish your workout. This same research showed that the body’s resting metabolic rate was still elevated sixteen hours after the workout. This means you are getting some measure of “burn” from your HIIT workout long after it’s over.
-A study published in the International Journal of Obesity in 2015 reported that performing HIIT three times per week for fifteen weeks “was associated with significant reductions in total body fat, subcutaneous leg and trunk fat, and insulin resistance in young women” when compared with steady-state exercise.
-A study published in the Journal of Applied Physiology showed that seven sessions of HIIT over two weeks “induced marked increases” in fat oxidation, which means it’s a great way to burn fat and positively impact your body composition.
-A study conducted on semi-professional soccer players showed it took only one HIIT session every two weeks to maintain their in-season VO2 max (which is the maximum amount of oxygen you can use, or your maximal aerobic capacity).
-Furthermore, a meta-analysis (a study of studies on a certain topic) published in Sports Medicine in 2015 examined 28 different pieces of research and concluded, “Endurance training and HIIT both elicit large improvements in the VO2 max of healthy, young to middle-aged adults, with the gains in VO2 max being greater following HIIT when compared with endurance training.” Research done to compare sprint interval training with endurance training showed that both methods reduced aortic stiffness and increased insulin sensitivity. Meaning, you’re not missing out on those benefits by choosing to save time with HIIT and you’ll still be reducing your risk of things like stroke, heart attack, and diabetes.
Similarly, a 2016 study showed that resistance-based intervals (like those wall balls we mentioned) could be a “time-efficient and effective exercise method to acutely improve endothelial function.” Endothelial dysfunction can put you at greater risk for cardiovascular disease, particularly if you already suffer from type 2 diabetes. Said another way, resistance-based intervals are good for your vascular health.
Numerous studies, like this one from 2016 and this one from 2017 also support that the HIIT approach can be of great benefit to those who are pre-diabetic or diabetic. And for those who are healthy it still offers benefits to cardiac function and reduction of liver fat. In the 2017 study, pre-diabetic and diabetic men reached regular blood sugar levels after just six HIIT sessions.
And finally, both a 1992 study and a 2014 study showed a positive impact on human growth hormone levels after HIIT sessions. The 1992 study found, “A minimum duration of 10 min, high intensity exercise consistently increased circulating GH in adult males.” And among the benefits of growth hormone are stronger bones, improved lean muscle mass, better exercise performance, and increased energy levels.
The Benefits of HIIT Workouts:
Strengthen against diseases like stroke, heart attack, and diabetes.
Help restore healthy cardiovascular function in those already suffering from these diseases.
Help burn fat both during and after a workout.
Provide more fun and motivation than long, steady-state training sessions. Raise growth hormone levels needed for muscle building and bone strengthening.
Boost your aerobic capacity.
Take less time.
So, what will your next HIIT workout be?