HIIT is good for you but don't get carried away by the hype

High Intensity Interval Training (HIIT) is "an exercise strategy alternating periods of short intense anaerobic exercise with less-intense recovery periods. The original protocol set a 2:1 ratio of work to recovery periods, for example, 30–40 seconds of hard sprinting alternated with 15–20 seconds of jogging or walking. The entire HIIT session may last between ten and thirty minutes, meaning that it is considered to be an excellent way to maximize a workout that is limited on time." The basic idea is to engage in short intense bursts of exercise instead of longer sessions of moderate exercise. HIIT has been growing in popularity as a time-saving approach to exercise.

In addition to saving time, it also improves cardiovascular and aerobic fitness (i.e. endurance). HIIT can get the heart and breathing rates going toward the maximum which is good for increasing your VO2 max, the maximum rate of oxygen consumption which depends on the efficiency of your breathing and heart pumping.

The main disadvantage is that the high intensity levels cannot be sustained, and as a result, the total number of calories burned won't be as much compared to longer more extended exercise such as jogging. It is also quite strenuous to achieve the high intensity levels of HIIT, but as they say no pain, no gain.

Along with the hype there have been several myths associated with HIIT such as the "One-Minute Workout." In an article in Vox, the writer Julia Belluz dispels some of these myths by making the following points:

1. HIIT is not one-minute workout.
2. Some claims of benefits have been exaggerated (see Figure 1).
3. It may not help with weight loss

There a couple of big-picture points to keep in mind. First, exercise has a dose-response curve "meaning you get more gains with more intensity and more time put in." The bigger the exercise "dose", the bigger the health benefits.

The second point is that exercise time and intensity are complementary. but there is also a trade-off. It is best to get both, and it is best to get as much of both as possible.

Let's illustrate the trade-off with some numbers. A typical 10-minute HIIT workout will burn roughly 12 calories/minute. By comparison, running at a 10 min/mile pace will burn about 10 calories/minute, while walking briskly at a 20 min/mile pace (3 mph) will burn about 5 calories per minute. Thus, the HIIT workout will burn a total of 120 calories (not the 1000 calories touted in the graphic in Figure 1). If you run for 2 miles (20 min), then you burn 200 calories, and if you walk for an hour then you burn 150 calories. At the end of the 10-minute HIIT, you will be exhausted because of the high intensity, whereas walking for 1 hour is easier to sustain and ultimately consumes more calories.

Of course one could try a longer HIIT program, but that would be exhausting. It would be like sprinting 400 meters, repeatedly. A 30 minute HIIT workout at full blast would consume 300 calories (equivalent to a two-hour walk), but most people probably could not complete such a demanding regimen.

In summary, the experts recommend mixing in high intensity exercise with longer lower intensity exercise:

"Gibala had a very lucid way to think about how to weave intervals into a workout: "Interval training to me are periods of more intense effort separated by periods of recovery." Pushing out of your comfort zone a few times in your workout "provides a way for people to boost their fitness in a relatively time-efficient manner."

In this manner, you can get the best of both worlds with enhanced cardiovascular fitness from the high intensity exercise while burning more calories from the extended exercise.

Figure 1. This High Intensity Interval Training (HIIT) workout does not burn 1000 calories. It is more likely a 100 calorie workout.