With the agility of a squirrel, Zoe Antczak-Chung, 8, bounded across the monkey bars at Palisades Recreation Center in the District. She slipped her lanky legs through the bars, released her hands and dangled upside down with glee."Did you catch that?" Zoe asked before jumping down and heading to the pull-up rings. In a matter of seconds, she leapt up, grabbed the rings and completed a set of pull-ups that would put most grown-ups to shame."She's gotten quite good at those," said Agni Chung, Zoe's mom. "Her twin brother likes the climbing wall, goes up and down like 25 times. It's a great outlet to release some energy."Palisades has one of the area's several fitness clusters, a type of playground designed for kids to improve their balance, agility and strength. These are not your average swing-and-slide sets; they're more like mini-military training grounds in crayon colors. Clusters vary in size and complexity but often feature at least six components, including balance beams, parallel bars, sit-up benches and chin-up bars. Unlike the raw wood and steel structures of my 1980s childhood, clusters often come with protective coating to reduce injuries. "Part of the recommendations for physical activity for kids is to not only get aerobic activity, but muscle and strength activity," said Dawn Podulka Coe, an associate professor in the Department of Kinesiology, Recreation and Sports Studies at the University of Tennessee. "So monkey bars, where they're holding their bodyweight .â.â. anything where they're loading the muscle and the bones is going to help strengthen them."As much as Zoe enjoyed careering down the slide at Palisades, she explained, "It's a lot of fun to climb up [the ropes] and swing from the monkey bars." Years of gymnastics made her a bit of a daredevil, her mom said, but the playground seemed to bring that out in kids of all ages. That day at the park, there were mostly little ones scampering about, save for a few children around Zoe's age making their way through the cluster."It's good that they have this space," Chung said. "It's combines many physical activities and motor skills for every age."Playgrounds have had some iteration of fitness clusters for decades. Lynne Vanderveer of PlayPower said the company introduced its "Miracle Junior Challenge Course," a 13-station obstacle course, in the 1970s. Landscape Structures out of Minnesota rolled out its first cluster in 1991 and has since sold hundreds of units across the country, according to product manager Jill Dunning-Harris. Coe of the University of Tennessee said clusters have become especially popular across the country as communities have grown concerned about childhood obesity. Nearly 1 in 3 children in America is overweight or obese, triple the proportion of 30 years ago, according to the Centers for Disease Control and Prevention. During that time, schools have cut back on gym classes and after-school sports, making public parks and recreation centers a critical resource for physical activity.Although playgrounds have always been a good place for such activity, Coe said, today there is more of a focus on using the equipment to improve core strength and balance, which is key to fundamental motor skills such as running and jumping."These types of playgrounds allow kids to engage in different activities to work on all components of fitness, including aerobic fitness, muscular strength and muscular endurance," she said. It's been two years since the D.C. Department of Parks and Recreation renovated the playground in Palisades as a part of its Play D.C. initiative. The $35million program has allowed the city to spruce up 45 play spaces in that time. "While we wanted to make sure the equipment was fun, we absolutely wanted to create a space that would increase the health and wellness of our residents," explained John Stokes, a spokesman for the department. He said the city plans to renovate one to two playgrounds a year from now on. Sparks at Play, which supplies playground equipment, installed fitness clusters at four of the play spaces renovated by the city, including the one at Palisades. The Owings Mill, Md., company works with Landscape Structures, which has created eight publicly accessible playgrounds in the Washington region, including Little Paint Branch Park in Beltsville."The District has been very good at integrating a variety of features into their parks," said Stephanie Sparks, who handles project development for the company. "There's been a really big push among many of our clients to create spaces that keep kids, parents and grandparents engaged."A cluster, she said, can run anywhere from $1,200 to $6,000, depending on the size and features. While most of the demand comes from regional parks, she said, a growing number of health-care facilities have expressed interest in using clusters for rehabilitation therapy.Although most clusters are designed for children younger than 12, Sparks said many are perfect for older kids to train for team sports. The clusters at Harry Thomas Sr. Recreation Center and Raymond Recreation Center in the District are separated from the play spaces for small children, leaving enough room for teenagers and adults to get in a workout."The city thought it was really important to take a holistic approach to play spaces," Stokes said. "While there's equipment for the youngsters, there is also equipment that adults can use to stay in shape."