(AP Photo/The Advocate-Messenger, Clay Jackson)

It has become increasingly clear that exercising into old age is one of the best -- if not the best -- thing you can do to grow old successfully. By this, researchers mean aging with the fewest number of infirmities and the least deterioration of physiological function. There's even a word for it: exercise (and other efforts such as quitting smoking) improves your "healthspan," the number of years you are able to live in good health. Which, as we know, is not keeping up with average life expectancy for much of the developed world's population.

What has proved quite difficult, however, is separating the impact of various factors on our physical decline. Do we deteriorate physically because (most of us) are increasingly sedentary as we age? Or is it simply the passage of time that takes an inevitable toll on our bodies and minds?

A clever study published this week in The Journal of Physiology sought to isolate the impact of sedentary lifestyles by looking only at 125 non-elite cyclists aged 55 to 79. Though they weren't competing in the Tour de France, these folks were no slouches: To qualify for the study, men had to be able to ride 100 kilometers in 6.5 hours and women had to be able to cycle 60 kilometers in 5.5 hours.

Alas, the research still didn't produce reliable markers of aging, highlighting its complexity. "What we found is that the relationship between what we measured and aging was not very clear," Stephen D.R. Harridge, a professor of human physiology at King's College London, said in an interview.

"Despite studying a large number and diverse range of indices, it was not possible to identify a physiological marker that could be used to reliably predict the age of a given individual," Harridge's team wrote in their paper. "Rather, we have found that the relationship between chronological age and most functions is complex...The biological ageing process, even when free from confounding factors, is thus likely to be highly individualistic."

It was, however, clear once again that physical activity is very, very good for you as you grow old. After putting these fit older people through their paces, researchers found that, overall, their results looked quite similar to those of younger people.

"They are different from people of the same age who are sedentary," Harridge said. "They are completely different animals, and in that way they are younger."

Of course it's possible that this group of people is still riding exceptional distances because they just naturally aged well, which encouraged them to continue exercising, rather than the reverse. Harridge acknowledged that his study could not rule that out. But the accumulation of evidence in many other studies seems to indicate that it's exercise that makes aging go well, not the other way around.

In this case, researchers looked at each subject's VO2 max, the maximum amount of oxygen a person can use during intense exercise, considered the gold standard of cardiovascular fitness; endurance; balance; muscle strength; nerve conduction and reflexes to name a few. In one test, they were asked to do a "Timed Up and Go," a common test of aging that asks people to stand up from a chair without using their arms, walk three meters, turn, walk back to the chair and sit down. A completion time of greater than 15 seconds "indicates a high risk of falling," according to the study, but the 125 cyclists averaged 5.6 seconds, "which is superior to that reported for the general older population and well within the norm reported for healthy young adults."

The study also looked at cognitive function and sense of well-being and found the cyclists to be "happy, high-functioning individuals."

Other research has yielded various kinds of useful information. For example, four or five cardiovascular workouts a week appears to provide the optimal defense against declining cardiac strength, according to research by Benjamin Levine, director of the Institute for Exercise and Environmental Medicine at Texas Health Presbyterian Hospital in Dallas. Even starting an exercise program in late middle age appears to have substantial benefits on the heart and blood vessels, said Levine, who focuses his research on people who report that they have exercised consistently throughout their lifetimes. But people who wait until, say, age 70 to start a fitness program have much less success, he said.

Harridge said he hopes to retest his cyclists in five or 10 years to compare their performance against the results he published this week.