They are skateboarders without skateboards, urban acrobats who scale walls, hurdle mailboxes and leap between buildings in stunts that might give Spider-Man pause.

Practitioners of parkour, a daring pastime born in the streets, have long seen public spaces as their playground, and parkour as the ultimate rebel’s game, one with no rules, league, equipment or winners. It started in France (the name is derived from the French word for “course”) and has spread around the world: Gaza, Tokyo, Rome and Miami are parkour towns.

One thread binds them all: parkour takes place outdoors. Bringing it inside, purists argue, would be like asking swimmers to perfect the breaststroke on grass.

But now, parkour has grown up, traded the pavement for padding and turned into a big business.

Parkour gyms have opened across the country, from Los Angeles to Rochester, featuring juice bars, private classes and children’s birthday parties that cost $450 (cake not included). Specialized apparel companies sell tailored gloves for $34.50 and shoes for $60. An international organization offers special parkour insurance policies and charges $295 for teacher certification courses.