Want your kids to get more exercise when they go to the park? Don't go with them.

A study published in the September issue of the American Journal of Preventive Medicine looks at the way 2,712 kids, most them 12 and younger, used parks for physical activity.

According to the study, girls were seen to be less active than boys, and the highest levels of activity were observed in parks featuring basketball courts. Kids also tended to be most active when surrounded by lots of other active kids.

The headline-grabbing finding, though, was that the single most potent factor associated with how much physical activity children got was the presence of a parent: Kids with a parent in the park engaged in far less physical activity than those whose parents weren't there. (Nonparental caregivers had a similar but lesser influence.)

The study's authors say this is in keeping with other research showing that parents' worries about their kids' safety hampers outdoor play. The dwindling amount of outdoor play among U.S. children is thought to contribute to childhood obesity.

Creating safe access to parks and playgrounds could help turn that problem around. The authors suggest that landscaping and design for parks could create play spaces that would leave room for children to run free while allowing their parents to keep an eye on them without hovering.