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In what is probably the definitive word on how little exercise we can get away with, a new study finds that a mere four seconds of intense intervals, repeated until they amount to about a minute of total exertion, lead to rapid and meaningful improvements in strength, fitness and general physical performance among middle-aged and older adults.

The study relied on a type of specialized stationary bicycle, but, even so, the results suggest that strenuous but super-abbreviated workouts can produce outsize benefits for our health and well-being. In studies, short high-intensity interval training, HIIT, workouts typically produce health gains that are equal to or more pronounced than much longer, gentler workouts.

But the ideal length of the intervals in these workouts has been unsettled. Researchers agree that the optimal interval span should stress our muscles and other bodily systems enough to jump-start potent physiological changes but not so much that we groan, give up and decline to try that workout ever again. In practice, those dueling goals have led to study intervals ranging from a protracted four minutes to a quickie 20 seconds.

But Ed Coyle, an exercise physiologist at the University of Texas in Austin, and his graduate assistant Jakob Allen suspected that even 20-second spurts, performed intensely, might exceed some exercisers' tolerance. So, he started looking for the shortest possible interval that was still effective.

And in the new study, published in Medicine & Science in Sports & Exercise, he and his colleagues settled on a blink-swift four seconds.

They arrived at that number by first working with competitive athletes, most of whom would reach their maximum power output and all-out aerobic effort after about two seconds of hard pedaling. The rest of us, Coyle and his colleagues reasoned, probably would require twice as long — or about four seconds. By that point, the researchers thought, most people should have massively stimulated their muscles and aerobic systems but not yet exhausted them.

To test that idea, the researchers initially focused on robust, young adults who repeated, if diminutive, workouts sprinkled throughout the day. But they wondered whether a more practical, single session of four-second sprints would be enough exercise to improve health and fitness in out-of-shape adults well past their college years. So, they recruited 39 of them, sedentary men and women aged 50 to 68. The volunteers began visiting the performance lab three times a week. There, they completed a brief workout of repeated four-second intervals on specialized bikes. At first, they sprinted for four seconds, with Allen calling out a second-by-second countdown, followed by 56 seconds of rest, repeating that sequence 15 times, for a total of 60 seconds of intervals.

Over two months, though, the riders' rest periods declined to 26 seconds and they increased their total number of sprints to 30 per session.

At the end of eight weeks, the scientists retested everyone and found substantial differences. On average, riders had increased their fitness by about 10%, gained considerable muscle mass and strength in their legs, reduced the stiffness of their arteries and outperformed their previous selves in activities of daily living, all from about three to six minutes a week of actual exercise.