Half began a supervised exercise program that reiterated the Norwegian researchers’ former routine. After briefly warming up, these volunteers ran on a treadmill at 90 percent of their maximal heart rate — a tiring pace, says Dr. Tjonna, at which “you cannot talk in full sentences, but can use single words” — for four four-minute intervals, with three minutes of slow walking between, followed by a brief cool-down. The entire session was repeated three times a week for 10 weeks.
The second group, however, completed only one four-minute strenuous run. They, too, exercised three times a week for 10 weeks.
At the end of the program, the men had increased their maximal oxygen uptake, or endurance capacity, by an average of 10 percent or more, with no significant differences in the gains between the two groups.
Four minutes. The answer is four minutes.