Teenagers get less exercise than almost any other age group, a new study shows.Credit

Teenagers can be a notoriously sedentary group. Now a new study showed that school may be a big part of the problem.

The study, which used GPS devices to track when and where teenagers were getting physical activity, found that, on average, they were physically active only 23 minutes a day while at school. Meager as that figure is, it made up over half the 39.4 minutes of physical activity the average teenager got every day.

Teenagers are “one of the tougher groups to get active,” said Jordan A. Carlson, a research assistant professor at Children’s Mercy Hospital in Kansas City, Mo., and one of the authors of the newstudy, which was published in Pediatrics. By some estimates, fewer than one in 10 adolescents get the 60 minutes of physical activity a day recommended by the Centers for Disease Control and Prevention.

The researchers undertook their study to find out where teenagers were getting most of their physical activity — whether at or near home, school or elsewhere. Their hope was that the information could lead to ways to maximize opportunities for activity and make adolescents’ environments more conducive to exercise.

They used data from the Teen Environment and Neighborhood study, which initially recruited 928 adolescents between the ages of 12 and 16 who lived in a mix of urban and suburban neighborhoods in Baltimore, the District of Columbia/Maryland area and Seattle between 2009 and 2011. The participants were split fairly evenly between girls and boys; just under a third were nonwhite.

The teenagers were asked to wear an accelerometer and a GPS tracker on a belt around their waist throughout the day, with the accelerometer measuring physical activity and the GPS noting their location every 30 seconds. Making use of tools like GPS helps “reduce the error in our data and give us much more precision” than many earlier studies that rely on self-reporting, which tends to be inaccurate, Dr. Carlson said.

Participants had to wear the belt for at least one whole school day and one weekend day for their data to be included. The current analysis included data from 549 participants who wore the device for a mean number of seven days; many teenagers were not included because they failed to follow instructions or didn’t provide information that was needed, like their school location.

What the researchers found surprised them.

Even though teenagers spend 42 percent of their waking time at school and get about half their day’s physical activity there, Dr. Carlson said, “They’re getting a lot less activity at school than we thought. We were surprised that they only spent about 4.8 percent of their time at school actually physically active.”

Teenagers tended to be more active when they were near their school or in the neighborhood around their home — but spent very little time being active when they were at home indoors. Girls were less active in most places than boys.

The researchers concluded that teenagers would be more active if there were more opportunity for physical activity at school, and if they spent less time indoors at home and more time in the vicinity of the home and school.