Four Seconds of Exercise

 Four second sprints are the base amount of high-intensity interval training that you need, maybe even for older adults.

Gretchen Reynolds says these findings are shorter than most scientists have found:

I have often written about the potential benefits of brief, high-intensity interval training, or H.I.I.T., an approach to exercise that consists of quick spurts of draining physical effort, followed by rest, with the sequence repeated multiple times. In studies, short H.I.I.T. 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 studying H.I.I.T. 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 H.I.I.T. scientists to study intervals ranging from a protracted four minutes to a quickie 20 seconds.