Washington — The activity level of a large group of American children dropped sharply between the ages of 9 and 15, when most failed to reach the daily recommended activity level, according to findings from a long-term study by the National Institutes of Health (NIH).

Researchers evaluated the children to determine whether they achieved the recommended minimum 60 minutes per day of moderate to vigorous physical activity (MVPA).

At age 9, the children averaged roughly three hours of MVPA on weekdays and weekends. By age 15, however, they averaged only 49 minutes per weekday, and 35 minutes per weekend.

Girls' level of physical activity dropped below the recommended level at 13 years old during the week and 12 ½ years old on the weekends. Boys fell below the recommendation at 14 ½ years old on weekdays and 13 ½ on weekends.

“Lack of physical activity in childhood raises the risk for obesity and its attendant health problems later in life,” says Duane Alexander, M.D., director of NIH's Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD).

The analysis was conducted on data collected for the NICHD Study of Early Child Care and Youth Development, a long-term study of more than 1,000 children from ethnically and economically diverse backgrounds. The study collected information on various other aspects of children's health and development. It was geared toward gathering information on children's experiences in various child care arrangements but did not constitute a nationally representative sample of the United States as a whole.

Beginning at age 9, the researchers recorded the activity levels of more than 800 children for four to seven days using an accelerometer.