Ten-year-olds spend more time sitting on their rears and less time running around than they did at age nine, according to a new British study.

Kids mostly cut down on their physical activity during weekends, on average about 75 minutes in boys and nearly half an hour in girls.

“The extent of these decreases over 1 year would have significant implications for these children if decreases continued into adulthood,” the researchers write in the journal Pediatrics.

“Although physical activity promotion for adults is focused mostly on increasing physical activity, efforts for youths might be better focused on reducing decreases,” they add.

Earlier studies have generally found a decline in physical activity as children age. While little is known about the reasons for this decrease, an active lifestyle may have positive effects not only on weight, but also mental health, the researchers say.

Their study is the first to test physical activity over time in kids using an objective measure instead of self-reported activity levels, which may be biased.

Dr. Kirsten Corder of the Medical Research Council Epidemiology Unit in Cambridge and colleagues collected data from 844 kids at schools in Norfolk County. The kids had worn accelerometers on their waists to record their daily activity over two weeks, each spaced one year apart.

At the outset, 70 percent of kids met the recommendations for one hour of moderate or vigorous activity per day. One year later that number had slumped to 66 percent.

Nearly two-thirds of the kids increased their sedentary time, on average by nine minutes. And more than half decreased their levels of moderate or vigorous physical activity, by about three minutes.

It’s not clear if this decline will continue into adolescence. But if it does, a few minutes of extra couch time could turn into hours.

The researchers say it is unknown how many kids their findings apply to. And the reasons for the decline are still murky - it could be that kids play less outside or spend more time on homework, for instance.

However, they did find that girls, kids of wealthier parents and those with more body fat tended to show larger decreases in physical activity. Changing activities for these kids and during weekends may be the best options to counteract insidious decreases in physical activity, they conclude.

SOURCE:  Pediatrics, online September 13, 2010.