If you want to increase your children's likelihood of being physically fit, improve the places in which they play, a new study shows.

Middle-school children are more likely to be physically active if they are given an attractive place to play and are supervised by adults, the study has found. Physical improvements to school grounds, such as basketball hoops and tennis courts, along with adult supervision, were associated with four times as many boys and five times as many girls being physically active during free time, according to Dr. James F. Sallis of San Diego State University and his associates.

"The results raise the possibility that making realistic improvements to school environments could increase the physical activity of students throughout the school day," he said.

However, on average fewer than 2 percent of girls and 6 percent of boys were physically active at school outside of physical education classes. At the most active schools, 5 percent of girls and 11 percent of boys were physically active, the researchers found.

In contrast, the lack of an environment that encouraged the children to play was associated with near-zero levels of physical activity, according to the study, which was published in the American Journal of Public Health.

The study involved 24 public middle schools with an average of more than 1,000 pupils each. Schoolwide estimates were based on average rates of moderate-to-vigorous physical activity across play areas, including indoor gyms and outdoor fields and courts.

The most common improvements to activity areas were basketball hoops and courts, baseball backstops, volleyball nets, tennis courts and racquetball courts, researchers said.

Although the overall message of the study was, "'If we build it, they will come -- and be active...' it is not clear how much further improvements in school environments could boost student physical activity," Sallis said.