There is a branch of my wife's family convinced that running ruined our cousin Frances (not ruined in the Southern gothic sense, of course, just hobbled in the knees and back). Frances ran pretty regularly in her day, and my mother-in-law warns me about it whenever she catches me in running shoes, most recently last month on a trip to the beach.

I guess she thought it'd be healthier to sit back and have another daiquiri.

Just in time (who doesn't love rebutting his mother-in-law?), researchers at the Stanford University School of Medicine released the latest batch of results from a long-term study comparing hundreds of older runners with others of about the same age.

Here's the gist of the research: Begun in 1984, the project assembled nearly 1,000 volunteers, about 538 runners and 423 ''healthy controls.'' They averaged about 59 years old at the time. Death and dropouts have winnowed the group, but about 440 have stayed involved.

They are now pushing into their 80s, and the research has found that the running group is living longer, with a lower incidence not just of cardiovascular disease, but of some forms of cancer as well. More to the point here, they also are suffering fewer disabilities than the non-runners.

Using a standard questionnaire that assesses eight basic life activities, such as standing, dressing and eating, the study found that non-runners, as they aged, were much more likely to reach a point where they were unable to do one or more of those tasks. James F. Fries, who began the project to look at the long-term effects of exercise on aging, calls it the ''compression of morbidity'': people who stay active as they grow older not only live longer but also go through a shorter period of infirmity before they die.

Along with faring better on the study's broad measure of disability, the runners seemed to have the same rates of osteoarthritis of the knee as the non-runners, said Eliza F. Chakravarty, an assistant professor at the Stanford medical school and one of the researchers. A rheumatologist, she found that especially interesting as she reviewed the data.

''There's a concern that with all of the pounding, [running] would wind up being detrimental to the knee,'' she said. ''But there was no significant difference between the runners and the controls.''

Why is that the case?

Chakravarty said there are plenty of possibilities.

Running itself will increase muscle and ligament strength, while the weight-bearing impact of the exercise will help bone density. The cartilage in our joints, she said, is not fed directly by any set of dedicated blood vessels or other system, and it needs the squeezing and motion of exercise to be adequately lubricated and nourished.

There may, of course, have been a self-selecting aspect to the outcome: Perhaps those still running in their 60s and 70s stuck with it because they had remained injury-free.

But Chakravarty thinks not. She said that, with nearly a quarter-century of data in hand, the research shows pretty powerfully that people can continue to exercise vigorously as they grow older and that it continues to yield benefits. Concerns about sudden cardiac events or chronic joint problems and other injuries, she said, don't appear to hold up.

The message is, ''It is never too late to adopt exercise, and we should not discourage people from exercise just because of their age,'' she said.

Although this study focused on runners, she said it is likely that biking, swimming or any strenuous activity (which the researchers defined as keeping the heart rate higher than 120 beats per minute) would have much the same result.

Of course, common sense and good training rules still apply: Warm up, cool down and stretch, and if you are just starting out on a running program, mix some walk breaks in at first as your body adapts.