Although women in the study appeared to be inactive for a good portion of the day, they frequently moved about in short bursts of activity, an average of nine times an hour.
"This is the first part of an ongoing study, and the first paper to look at the patterns of activity and sedentary behaviors," said lead author Eric Shiroma, a researcher at Brigham and Women's Hospital, in Boston.
"Some research says that sitting for long periods is harmful and the recommendation is that we should get up every 30 minutes, but there's little hard data available on how much we're sitting and how often we get up and how measures such as these affect our health risks," Shiroma explained.
Results of the study are published as a letter in the Dec. 18 issue of the Journal of the American Medical Association.
Previous studies have suggested that the more people sit each day, the greater their risk for chronic health problems, such as heart disease, diabetes and cancer.
The current study included more than 7,000 women whose average age was 71 years. For almost seven days, the women wore devices called accelerometers that measure movement. However, the device can't tell if someone is standing or sitting, only if they're still or moving. The women wore the devices during their waking hours, which averaged close to 15 hours a day.
A break in sedentary (inactive) behavior had to include at least one minute of movement, according to the study.
On average, the women were physically inactive for 65.5 percent of their day, or about 9.7 hours. The average number of sedentary periods during the day was 86, according to the study. Women moved an average of nine times an hour, even if only for a short burst of activity.
Older women and heavier women had longer bouts of sedentary behavior with fewer breaks in those inactive periods, the study reported.
Shiroma said the researchers don't know what activities the women participated in when they were moving. They only know whether the women were moving or not.
"I was kind of surprised. I thought the women would be sitting more, for longer periods," he said. "Now we need to know if it matters. Does sitting for five, 10 or 30 minutes mean something different for your health than sitting longer?"
One expert said she wasn't surprised by the study findings.
"It's what I see in the geriatric world," said Dr. Yonette Davis, chief of geriatrics at the Brooklyn Hospital Center, in New York City. "The routine of working and taking care of the kids has changed. They don't have that nine-to-five routine any more, and their lifestyle just isn't as rigorous," she explained.
Davis said it also wasn't surprising to see that as people got older, or as their weight increased that they were more sedentary. "You have less reserves for those short energy bursts as you get older or heavier," she said.
Davis recommended having a plan as you get older. "You have to mentally transition yourself when you get to the end of taking care of kids or working. You have to change and find other activities," she advised. "Tell yourself, 'This is a different point in my life. I need to look for other outlets of interest now that my kids no longer need me and I'm finished with my job.' Go out with friends, volunteer, get involved with your church, go back to school. Don't wind yourself all the way down."
Study author Shiroma said that the researchers didn't know for this first phase of the study whether or not the women were still working. And, he said, it's not clear if these findings would be similar for older men.
Learn more about staying active from the U.S. National Institute on Aging.