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Phys Ed Gretchen Reynolds on the science of fitness.

Someone can train for a marathon and simultaneously qualify as a couch potato, recent research shows, raising provocative questions about how sedentary most of us really are.

The amount of time that most of us spend sitting has increased substantially in recent decades, especially as computers and deskbound activities have come to dominate the workplace. According to one telling recent study, the average American sits for at least eight hours a day.

Such prolonged sedentariness may have health consequences, additional research shows. A study of almost 2,000 older adults published in August, for instance, found that those who spent the most hours seated every day had a greater risk of high blood pressure, elevated blood sugar, a poor cholesterol profile and body-wide inflammation than those who sat the least, no matter how much either group exercised (which, generally, was not much).

So, too, a stark numerical 2012 analysis of lifestyle, health and death statistics from a large group of Australian adults concluded that every hour that someone spent watching television — a widely accepted marker of sitting time — after the age of 25 reduced his or her life span by almost 22 minutes. More broadly, in this analysis, watching television for six hours or more per day shaved almost five years from a typical adult’s life span, compared with someone who did not watch TV. Life span was shortened even if someone met the standard medical recommendation of exercising moderately for 30 minutes or so on most days of the week.

But many highly active people, including those completing their preparations for Sunday’s upcoming New York City Marathon, probably feel immune from such concerns. After all, it seems reasonable enough to assume that multiple hours spent training must lessen the number of hours spent plopped in a chair.

Until recently, however, no studies had specifically examined whether people who are extremely active are, on the whole, also truly not sedentary.

So scientists affiliated with the School of Public Health at the University of Texas at Austin recently set out to fill that research gap. They began by contacting runners who had signed up for the local Austin marathon or half-marathon. More than 200 of the race entrants, male and female, agreed to participate.

The Texas researchers asked these volunteers to complete a questionnaire that precisely parsed how they spent their time each day. “We didn’t want to look only at certain measures” of sitting time, such as television viewing, said Geoffrey Whitfield, who devised the study as a doctoral student at the University of Texas.

Instead, the questionnaire asked about work, commuting, and telephone habits, as well as time spent watching television or playing computer games. It also asked the volunteers to enumerate how many hours they spent training each day and their anticipated race pace.

As expected, the runners, training as they were for a marathon or half-marathon, reported spending considerable time sweating. On average, they exercised vigorously for nearly seven hours per week, “which far exceeds the standard exercise recommendation,” said Dr. Whitfield, who is now an Epidemiological Intelligence Service Officer at the Centers for Disease Control and Prevention in Atlanta.

But those hours of exercise do not seem to have reduced sedentary time. On an average workday, the runners reported sitting for more than 10 hours at the office and at home, easily topping the national average. (Almost all of the participants were employed; a few were students.) On non-workdays, the runners spent about eight hours inactive.

The researchers found no correlation between running pace or training volume and sedentary time; fast runners and slow runners both sat equally often, as did those who were putting in the most or the fewest hours each week training.

In effect, the data showed that “time spent exercising does not supplant time spent sitting,” said Harold Kohl, a professor of epidemiology and kinesiology at the University of Texas and senior author of the study. “It seems that people can be simultaneously very active and very sedentary.”

The study does not necessarily intimate, however, that being a marathon runner and couch potato is in any particular way harmful, Dr. Kohl pointed out. He and his colleagues did not measure the runners’ health, he said, only their lifestyle. “It is impossible to say” based on their data, whether heavy training would ameliorate any undesirable effects of sitting or whether such effects even would occur in the supremely fit.

Still, the findings are a cautionary reminder that many of us, including the most physically active, may be more sedentary than we imagine. “The fact is that exercise, even at very high doses, does not occupy much time in most people’s days,” said Dr. Whitfield, who himself used to train for triathlons. And while the science about the health impacts of prolonged sitting may still be incomplete, he said, “it’s pretty safe to say that it would a good idea for most of us to spend more of our time up and moving.”