"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.... 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 decided to start looking for the shortest possible interval that was still effective.... [Volunteers aged 50 to 68] sprinted for four seconds, with Dr. 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 percent, 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."

From "Can 4 Seconds of Exercise Make a Difference?" (NYT).