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Phys Ed Gretchen Reynolds on the science of fitness.

One reason so many American women are overweight may be that we are vacuuming and doing laundry less often, according to a new study that, while scrupulously even-handed, is likely to stir controversy and emotions.

The study, published this month in PLoS One, is a follow-up to an influential 2011 report which used data from the U.S. Bureau of Labor Statistics to determine that, during the past 50 years, most American workers began sitting down on the job. Physical activity at work, such as walking or lifting, almost vanished, according to the data, with workers now spending most of their time seated before a computer or talking on the phone. Consequently, the authors found, the average American worker was burning almost 150 fewer calories daily at work than his or her employed parents had, a change that had materially contributed to the rise in obesity during the same time frame, especially among men, the authors concluded.

But that study, while fascinating, was narrow, focusing only on people with formal jobs. It overlooked a large segment of the population, namely a lot of women.

“Fifty years ago, a majority of women did not work outside of the home,” said Edward Archer, a research fellow with the Arnold School of Public Health at the University of South Carolina in Columbia, and lead author of the new study.

So, in collaboration with many of the authors of the earlier study of occupational physical activity, Dr. Archer set out to find data about how women had once spent their hours at home and whether and how their patterns of movement had changed over the years.

He found the information he needed in the American Heritage Time Use Study, a remarkable archive of “time-use diaries” provided by thousands of women beginning in 1965. Because Dr. Archer wished to examine how women in a variety of circumstances spent their time around the house, he gathered diaries from both working and non-employed women, starting with those in 1965 and extending through 2010.

He and his colleagues then pulled data from the diaries about how many hours the women were spending in various activities, how many calories they likely were expending in each of those tasks, and how the activities and associated energy expenditures changed over the years.

As it turned out, their findings broadly echoed those of the occupational time-use study. Women, they found, once had been quite physically active around the house, spending, in 1965, an average of 25.7 hours a week cleaning, cooking and doing laundry. Those activities, whatever their social freight, required the expenditure of considerable energy. (The authors did not include child care time in their calculations, since the women’s diary entries related to child care were inconsistent and often overlapped those of other activities.) In general at that time, working women devoted somewhat fewer hours to housework, while those not employed outside the home spent more.

Forty-five years later, in 2010, things had changed dramatically. By then, the time-use diaries showed, women were spending an average of 13.3 hours per week on housework.

More striking, the diary entries showed, women at home were now spending far more hours sitting in front of a screen. In 1965, women typically had spent about eight hours a week sitting and watching television. (Home computers weren’t invented yet.)

By 2010, those hours had more than doubled, to 16.5 hours per week. In essence, women had exchanged time spent in active pursuits, like vacuuming, for time spent being sedentary.

In the process, they had also greatly reduced the number of calories that they typically expended during their hours at home. According to the authors’ calculations, American women not employed outside the home were burning about 360 fewer calories every day in 2010 than they had in 1965, with working women burning about 132 fewer calories at home each day in 2010 than in 1965.

“Those are large reductions in energy expenditure,” Dr. Archer said, and would result, over the years, in significant weight gain without reductions in caloric intake.

What his study suggests, Dr. Archer continued, is that “we need to start finding ways to incorporate movement back into” the hours spent at home.

This does not mean, he said, that women — or men — should be doing more housework. For one thing, the effort involved is such activities today is less than it once was. Using modern, gliding vacuum cleaners is less taxing than struggling with the clunky, heavy machines once available, and thank goodness for that.

Nor is more time spent helping around the house a guarantee of more activity, over all. A telling 2012 study of television viewing habits found that when men increased the number of hours they spent on housework, they also greatly increased the hours they spent sitting in front of the TV, presumably because it was there and beckoning.

Instead, Dr. Archer said, we should start consciously tracking what we do when we are at home and try to reduce the amount of time spent sitting. “Walk to the mailbox,” he said. Chop vegetables in the kitchen. Play ball with your, or a neighbor’s, dog. Chivvy your spouse into helping you fold sheets. “The data clearly shows,” Dr. Archer said, that even at home, we need to be in motion.