By Gretchen Reynolds, New York Times

Intense exercise changes the body and muscles at a molecular level in ways that milder physical activity doesn’t match, according to an enlightening study.

Though the study was conducted in mice, the findings add to growing scientific evidence that to realize the greatest benefits from workouts, we probably need to push ourselves.

For some time, scientists and exercise experts have debated the merits of intensity in exercise. Everyone agrees, of course, that any exercise is more healthful than none. But beyond that baseline, is strenuous exercise somehow better, from a physiological standpoint, than a relative stroll?

There have been hints that it may be. Epidemiological studies of walkers, for instance, have found that those whose usual pace is brisk tend to live longer than those who move at a more leisurely rate, even if their overall energy expenditure is similar.

But how intense exercise might uniquely affect the body, especially below the surface at the cellular level, had remained unclear. That’s where scientists at the Scripps Research Institute in Florida stepped in.

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