National Fitness Partners, a midstate operator of Planet Fitness franchises, is doubling its footprint after buying a dozen gyms in the Carolinas.

Private-equity firm Argonne Capital Group LLC, the parent of National Fitness Partners, announced Thursday that it acquired the clubs in the Charlotte metropolitan area from GNT Holdings LLC.

National Fitness Partners, which is based in the Harrisburg area, had 11 franchise locations in its portfolio prior to the deal with GNT. Its most recent local addition was a Planet Fitness in East Pennsboro Township

Georgia-based Argonne formed National Fitness Partners when it teamed up last April with the Kindler family, the local Planet Fitness franchisee, and marketing executive Julie Morrison. The Kindler family opened a Planet Fitness gym in 2005 in Central Pennsylvania and eventually grew to 10 locations.

Frank Kindler, president of National Fitness Partners, said he's hoping the Carolina deal is the beginning of more growth for the company.

"With these newly acquired clubs and the continued financial support of Argonne, we look forward to further expanding the Planet Fitness brand in the Carolinas and beyond," he said.

The company's 23 clubs are spread between Pennsylvania, Delaware, North Carolina and South Carolina.

National Fitness Partners plans to retain its local headquarters, officials said. After the acquisition, the company plans to maintain an office in Charlotte to support the new clubs.