Researchers tracked children beginning at age 9 and then again at ages 11, 12, and 15. At age 9, the kids were doing well, getting in three hours a day of moderate to vigorous physical activity, well above the 60-minute minimum suggested by most experts.
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By age 15, they fell far below the recommendation, getting only 49 minutes on average on weekdays.
"We basically knew that kids were not as active as they used to be," says researcher Philip R. Nader, MD, professor emeritus of pediatrics at the University of California San Diego School of Medicine, La Jolla. "But I think the thing that surprises you was the degree and the speed at which physical activity declined."
Nader and his colleagues collected physical activity data on more than 1,000 children who participated in the long-running National Institute of Child Health and Human Development Study of Early Child Care and Youth Development. The children wore accelerometers, devices attached to the belt that can record movement minute by minute, for one week at each of four recording periods -- at age 9, in 2000, and at ages 11 and 12, and again at age 15, in 2006.
"This study was started many years ago [in 1991] to follow kids' development over time," Nader tells WebMD. Besides physical activity, it looked at a host of other topics, such as the influence of nonmaternal care as more mothers returned to the work force.
Declining Physical Activity
At age 9, the kids were doing well. But by age 15, the teens only got 49 minutes of moderate to vigorous activity per weekday and just 35 minutes on Saturday and Sunday on average.
Although more than 90% of the kids met the recommended level of 60 minutes at age 9 and 11, by age 15 only 31% did on weekdays and only 17% on weekends. At the age 15 data collection point, more than half, or 604, had valid data from the accelerometer.