''We wanted to examine skin that had not been frequently exposed to the sun,'' said Mark Tarnopolsky, a professor of pediatrics and exercise science at McMaster who oversaw the study, which was presented at the annual meeting of the American Medical Society for Sports Medicine in New Orleans this month.

To test that possibility, the scientists first gathered 29 local male and female volunteers ages 20 to 84. About half of the participants were active, performing at least three hours of moderate or vigorous physical activity every week, while the others were resolutely sedentary, exercising for less than an hour per week.

Of course, we humans long ago swapped our fur for naked skin. But if exercise could keep animals' outer layer from changing with age, it might, the researchers speculated, do the same for our skin.

But recently, researchers at McMaster University in Ontario began to wonder if such alterations were inevitable. Earlier studies at McMaster involving mice that were bred to age prematurely had shown that a steady regimen of exercise could stave off or even undo the signs of early ageing in these animals. When members of this breed of mice remained sedentary, they rapidly grew wisened, frail, ill, demented, and graying or bald. But if they were given access to running wheels, they maintained healthy brains, hearts, muscles, reproductive organs and fur far longer than their sedentary labmates. Their fur never even turned grey.

The scientists biopsied skin samples from each volunteer and examined them microscopically. When compared strictly by age, the skin samples overall aligned with what would be expected. Older volunteers generally had thicker outer layers of skin and significantly thinner inner layers.

But those results shifted noticeably when the researchers further subdivided their samples by exercise habits. They found that after age 40, the men and women who exercised frequently had markedly thinner, healthier stratum corneums and thicker dermis layers in their skin. Their skin was much closer in composition to that of the 20- and 30-year-olds than to that of others of their age, even if they were past age 65.But as the researchers realized, other factors, including diet, genes and lifestyle, might have influenced the differences in skin condition between the exercising and sedentary groups. It was impossible to know whether exercise by itself had affected people's skin or been incidental to lucky genetics and healthy lives.

So the researchers next set a group of sedentary volunteers to exercising, after first obtaining skin samples from their buttocks. The volunteers were aged at 65 or older and, at the study's start, had normal skin for their age. They began a fairly straightforward endurance training program, working out twice a week by jogging or cycling at a moderately strenuous pace, equivalent to at least 65 percent of their maximum aerobic capacity for 30 minutes. This continued for three months. At the end of that time, the researchers again biopsied the volunteers' skin.

But now the samples looked quite different, with outer and inner layers that looked very similar to those of 20- to 40-year-olds. ''I don't want to overhype the results, but, really, it was pretty remarkable to see,'' said Tarnopolsky, himself a middle-aged exerciser. Under a microscope, the volunteers' skin ''looked like that of a much younger person, and all that they had done differently was exercise''.

How exercise changes skin composition is not completely clear, but in a separate portion of the study, the researchers checked for alterations in the levels of certain substances created by working muscles. Called myokines, these substances are known to enter the bloodstream and jump-start changes in cells far from the muscles themselves. In this case, the scientists found greatly augmented levels of a myokine called IL-15 in the skin samples of volunteers after exercise. Their skin samples contained almost 50 per cent more IL-15 after they had been exercising than at the start of the study.