Climbing onto the step machine and flogging yourself to the point of collapse may give you a glow of achievement, but it might not do much for your health. Klaas Westerterp, of Maastricht University, the Netherlands, has found that people who spend more time taking moderate exercise such as walking or cycling burn off more energy than those who exercise in short, sharp bursts, such as gymnastics and sport1.

This adds to a growing body of evidence suggesting that lung-busters make up for their exertions by increasing the amount of time they spend doing nothing. "There are several studies showing that people who do strenuous exercise do less the rest of the day," comments Kirk Cureton, head of the Department of Exercise Science at the University of Georgia, Athens.

The developed world is gripped by an epidemic of inertia. A 1999 report by the US Surgeon General described physical inactivity as "a serious nationwide problem", concluding that 60% of US adults did not get enough exercise, and that a quarter got none at all. With sloth comes obesity, and a range of health problems including heart disease and diabetes.

"Knowledge about exercise has not been effective in getting people to do it," Cureton says. This sort of finding about intensity is useful for those seeking to address the problems of inactivity, he feels.

In other words, stop reading this and go out for a walk.