The long-term study by the National Institutes of Health evaluated the children to determine whether they achieved the minimum 60 minutes per day of moderate to vigorous physical activity (MVPA) recommended for children.
At age 9, the children averaged roughly three hours of MVPA on weekdays and weekends. By age 15, however, they averaged only 40 minutes per weekday, and 35 minutes per weekend.
"Lack of physical activity in childhood raises the risk for obesity and its attendant health problems later in life. Helping American children maintain appropriate activity levels is a major public health goal requiring immediate action," said 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 experience in various childcare 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. The children's activity was recorded with an accelerometer, a device that records movement, which the children wore on a belt. The researchers conducted follow up tracking at ages 11, 12, and 15.
The 2005 Dietary Guidelines for Americans recommends that children and adolescents engage in at least 60 minutes of physical activity on most, preferably all, days of the week.
The researchers found that, at ages 9 and 11, more than 90 percent of the children met the recommended level of 60 minutes or more of MVPA each day.
By age 15, however, only 31 percent met the recommended level on weekdays, and 17 percent met the recommended level on weekends.
The researchers estimated that physical activity declined by about 40 minutes per day each year until, by age 15, most failed to reach the daily recommended activity level.
On average, boys were more active than girls, spending 18 more minutes per weekday in MVPA than did girls, and 13 more minutes per day in MVPA on weekends.
The researchers estimated the age at which girls dropped below the recommended level of 60 minutes of MVPA as 13.1 years for weekdays, compared to boys, who dropped below the recommended level at 14.7 years. For weekends, girls dropped below the recommended level at 12.6 years, and boys at 13.4 years.
"This decline augurs poorly for levels of physical activity in American adults and potentially for health over the life-course," the study authors wrote.
"Consequently, there is need for program and policy action as early as possible at the family, community, school, health care, and governmental levels to address the problem of decreasing physical activity with increasing age," they added.
The study will appear in the July 16 Journal of the American Medical Association.