Being a world-class distance runner in your youth does not guarantee that you will be fit and healthy in retirement. But it helps, according to a new study that followed a group of elite American runners for 45 years.

The study’s findings raise interesting questions about how we can and should age and the role that youthful activity might play in our health later in life.

Aging is one of the great mysteries of life and science. Its chronology is clear: With each passing year, we are a year older. But the biology of the process is murky. Scientists remain uncertain about how and why our bodies change as we age and to what extent such changes are inevitable or mutable.

In other words, we do not know whether aging as most of us now experience it is normal for the human species or not.

That issue is at the heart of the new study, which was published this month in Medicine & Science in Sports & Exercise. It began almost 50 years ago, with a spate of coaching and testing that took place just before the 1968 Summer Olympic track and field trials in the United States. At that time, Jack Daniels, an exercise physiologist and running coach, began working with some of America’s top distance running prospects. He tested 26 of the athletes extensively, determining their aerobic capacity, or VO2 max, and many other measures of health and performance capability.

All of the runners, who were in their early to mid-20s, were in exceptional shape, with aerobic capacities at or above the 98th percentile for men of their age. Several won medals at the 1968 Summer Games.

Twenty-five years later, in 1993, curious to see how the athletes’ bodies had changed in the intervening years, Daniels, a professor of kinesiology at A.T. Still University in Mesa, Arizona, assembled the same group at a human performance lab and tested them again. Then in 2012, he mentioned this trove of unpublished data to his colleague Sarah Everman, an assistant professor of kinesiology at the university.

Intrigued, Everman suggested that they bring the athletes back to the lab again. Twenty-two of the men, who by this time were in their late 60s or early 70s, agreed to participate when she contacted them the next year, by then 45 years after their original testing. (Three had died since 1993 and one declined to be tested again.)

At the lab, the researchers ran the men through the same tests as before and asked about exercise routines.

Their answers revealed that, although the men remained physically active, none were competitive athletes at this point. In general, they exercised for a few hours each week by walking, jogging or cycling.

But their fitness remained relatively outsized, she found. Each man’s VO2 max had declined significantly since 1968, when he was in his 20s and competing, and also since the second testing in 1993. But their 2013 VO2 max numbers still placed them in the top 10% or so of older American men, based on tables developed in recent years using cardiovascular testing data from thousands of aging people.

These findings might indicate that the former athletes were genetically gifted, Everman says. They might be physiological outliers whose lucky cardiovascular quirks lingered into old age and allowed them to remain unusually fit in comparison to other older people.

But she is skeptical of that reading. Numerically, the men’s VO2 max levels declined more during the 45 years of the study, she says, in terms of the percentage of the capacity they lost per decade, than would be considered normal, based on data from nonathletes.

But they were declining from such a height of fitness that, even as their capacities contracted, the men’s fitness stayed above average, she says.

Such data suggest that squirreling away fitness when we are young with sustained, frequent exercise might help to blunt some of the losses later, she says.

But the broader message of the study, she says, could be that we may need to rethink what normal fitness is or should be in older people. The tables that doctors and other experts currently use to determine “normal” fitness have been constructed with data gathered from typical older people today, many of whom have been sedentary for years.

These men’s lives suggest that greater fitness is possible in old age, even for those of us who were not previously Olympians, Everman says. “These guys were not training hard” by the time they became septuagenarians, she says, and most already had eased back considerably in their workout routines by the 1993 testing, she says, when they were middle-aged.

But they never stopped exercising altogether, except during periods of illness or injury.

If the rest of us followed a similar workout trajectory during our lives, she says, we might wind up with a higher VO2 max than otherwise in our old age, resetting both our expectations about age-related fitness and the existing tables.

© 2017 The New York Times Company