Kids should exercise at least one hour a day, according to the Centers for Disease Control and Prevention. But in the U.S., less than half of six- to 11-year-olds and only 8 percent of 12- to 19-year-olds meet that target. Schools help to promote physical activity, with recess accounting for up to 40 percent of a child's daily exercise needs. So how exactly do kids spend that welcome break from their desks?

Over in Denmark, a longitudinal study of how schoolchildren move through urban environments provided an opportunity to find out by outfitting hundreds of students with accelerometers and GPS trackers during their waking hours. Henriette Bondo Andersen, a research assistant in the department of sports science and clinical biomechanics at the University of Southern Denmark, used some of the collected data to analyze how children spent their recess time and whether various school yards—including those made up of grassy areas, playgrounds or asphalt lots—influenced activity levels.

The researchers found that children were significantly more active when playing on grassy areas and at sites featuring playground equipment. Concrete lots elicited the least energy expenditure, and in all five areas studied, girls spent more time being sedentary than boys. The findings (below) were published this month in Landscape and Urban Planning.

Insights into which schoolyard elements work best to promote physical activity could help developers around the world create more exercise-friendly spaces. “We're working with seven schools that are renovating their playgrounds by adding dancing, climbing, skating and trampoline areas,” Andersen says. “The goal is to make it so kids can more easily choose to be active.”