Fancy playground equipment is fine for improving coordination and sharpening minds, but if you really want preschoolers to play hard, give them a ball, jump-rope or hula hoop, concludes a new study.

"We were surprised but encouraged to find that inexpensive equipment could add to kids' opportunities to be outside," says senior author Dianne Ward of the University of North Carolina at Chapel Hill's School of Public Health.

Nearly 60 percent of U.S. children between the ages of 3 and 5 attended a child-care center in 2005, Ward and her co-authors write in next month's American Journal of Preventive Medicine. Increasing numbers of young children weigh more than they should, underscoring the importance of physical activity in preschools, they write.

Previous research has shown that children's physical activity in preschool varies greatly from center to center. But, the authors write, research into what causes those variations is lacking.

Ward, head of intervention and policy in the nutrition department, and her co-authors assessed environmental factors thought to be linked to physical activity at 20 North Carolina preschools.

While "the easiest and least expensive means of increasing physical activity may be as simple as providing more active play time," the authors write, they identified two other factors that appeared to increase physical activity. One was providing hula hoops and other portable play equipment. The other was training staff in how to encourage more physical activity.