Researchers in the U.S. report that as many as one-third of American children are physically unfit but boys are in better shape than girls.

The team from the University of South Carolina’s Arnold School of Public Health in Columbia assessed the physical fitness of 3,287 children, ages 12 to 19.

The children all participated in the National Health and Nutrition Examination Survey between 1999 and 2002.

In order to assess their fitness levels, the researchers interviewed the children and also had them visit a mobile examination center, where they underwent a treadmill exercise test consisting of a two-minute warm-up, two three-minute periods of exercise, and a two-minute cool-down period.

During the test, researchers measured blood pressure, heart rate and rate of perceived exertion, determined by asking the children to rate how hard they felt their bodies were working.

Heart-rate readings during the three-minute periods of exercise were used to estimate the amount of oxygen used by the body during maximum exertion.

Lead researcher Russell R. Pate, a professor of exercise science at the University says they found that boys tended to be in better shape than girls; older boys were more fit than younger boys; younger girls were more fit than older girls; and as expected heavier children were in worse shape than their slimmer counterparts.

Professor Pate says it is a concern from a public health standpoint, that a third of kids don’t meet fitness standards and the solution is for American youth to be more physically active than they are currently.

Being physically fit is an important key to maintaining healthy blood pressure and cholesterol levels and warding off many chronic diseases.

But, since the 1980s, there has been a growing trend toward overweight and even obesity among American children, with an estimated 15 percent of boys and girls between the ages of 6 and 19 now considered overweight.

Much of that weight gain has been linked to a lack of exercise.

Overall, based on standards developed by experts and used by schools and school districts nationally, only about 65 percent of the children were judged physically fit and those who watched a lot of television or played hours of video games and those who spent less time being physically active, were less likely to be fit.

Pate says children need to be involved in more quality after-school sports programs,and provided with more physical-education classes in their schools, as well as being more active in their everyday lives by riding a bike to school or walking.

Experts believe it will take a concerted effort by parents and schools to increase the amount of physical activity that children get.

The more physically fit a young person, the less likely he or she is to develop high blood pressure, High cholesterol levels or a number of other risk factors for chronic diseases later in life.

The research is published in the Archives of Pediatrics & Adolescent Medicine.