LOS ANGELES, July 17 (Xinhua) -- Physical activity among American children has dropped significantly between age 9 and 15, causing childhood obesity, a new study indicated.

    The study was published in the Journal of the American Medical Association in July.

    It found moderate-to-vigorous physical activity (MVPA) of the selected children decreased by more than 70 percent on weekdays and 80 percent at weekends from age 9 to 15.

    "While we all knew children and youth were moving less, the magnitude of the rate of decline calls for renewed action to increase activity levels in the population," said Philip R. Nader, leading author of the article.

    "Declining physical activity will only complicate the obesity epidemic," he added.

    The study selected more than 1,000 children aged nine in 10 cities, measuring their physical activities from 2000 to 2006.

    The researchers found that the nine-year-old children engaged in about three hours of MVPA per day both on weekdays and at weekends.

    By age 15 the MVPA time dropped to only 49 minutes per weekday and 35 minutes per weekend day, much less than the recommended 60 minutes per day.

    The research also indicated that boys were more active than girls, spending 18 more minutes in MVPA per weekday and 13 more minutes per weekend day.

    "Program and policy action are needed immediately at the family, community, school, healthcare, and governmental levels to find ways to encourage children to remain active as they get older," said co-author Renate Houts.

    "Adults will have to become role models for children and youth, stressing activity and exercise for enjoyment and fun rather than drudgery or punishment," said Nader.