High-intensity interval training, a type of workout that consists of very brief bouts of very strenuous exercise, has become enormously popular in recent years. A main reason is that although such workouts are draining, they can be both very effective and very short, often lasting only a few minutes.
But people take notably different approaches to this form of exercise. Some complete only one sustained, all-out, four- or five- minute bike ride or sprint — a single interval — and then are done. Others practice standard interval training, involving repeated brief bursts of almost unbearably taxing exertion, interspersed with restful minutes of gentler exercise. Some people perform such sessions two or three times per week; others almost every day.
The science of intensive interval training has, though, been lagging behind the workout’s popularity. Past studies of HIIT, as the practice is commonly known, had established that as measured by changes in cellular markers, standard short-burst HIIT training may improve aerobic fitness up to 10 times as much as moderate endurance training. But scientists had not determined whether a single sustained interval likewise improves fitness, or the ideal number of HIIT sessions per week.
So to clarify those issues, researchers at two of the laboratories most noted for HIIT science set out to learn more about the best way to do interval training.
First, for a study published this month in Experimental Physiology, scientists at McMaster University in Ontario gathered 17 healthy young men and women and divided them into groups. Ten of them were asked to exercise on two separate days. On one day they completed a standard HIIT session consisting of four 30-second bouts of all-out, tongue-lolling effort on a stationary bicycle, alternating with four minutes of recovery between. On another day they completed a single uninterrupted interval lasting for about four minutes, by which time each rider had combusted the same amount of energy as during the stop-and-go session. Separately, the remaining seven volunteers did the continuous four-minute workout three times a week for six weeks.
On the one hand, the scientists found no significant variations in how the muscles of riders in the first group responded to a single session of interval training, whether of the standard stop-and-go variety or a sole sustained effort. In both cases, the riders showed immediate, post-exercise increases in their blood levels of certain proteins associated with eventual improvements in endurance capacity.
But when the researchers checked