Thousands of spectators will fill the Home Depot Center’s tennis stadium in a few weeks to watch competitors do upside-down pushups, kettle-bell swings and something called muscle-ups.

OK, so the Olympics it ain’t. But it is the sixth annual CrossFit Games, a fitness competition by CrossFit Inc., the creator of a hardcore workout program that has spread rapidly across the country.

Tickets for the July 13-15 games, to be shown on ESPN2, sold out weeks ago. Organizers expect a crowd of more than 8,000, more than double the turnout of a Carson event in 2010, the first year the games were held. (The previous three years, the games were held in the Monterey Bay town of Aromas.)

Since it was founded in Santa Cruz 12 years ago, independent CrossFit affiliates have opened more than 4,000 gyms worldwide and nearly 300 in Southern California.

The conditioning program provides full-body fitness through workouts that include weightlifting, running and some novel exercises. A muscle-up, for instance, requires adherents to hang from a set of gymnast rings and then hurl their bodies upward, ending above the rings with arms fully extended.

“The games are a natural extension of what we do. Every CrossFit workout is a competition,” said Tony Budding, media director of CrossFit Inc., now based in Washington, D.C.

But can CrossFit continue its momentum or is it destined to become another Tae Bo, something that starts strong but ends up flabby?

Bobby Verdun, a senior partner at Natick, Mass., health club consulting firm Atwood Group, said he thinks CrossFit has staying power, in no small part due to the games, sponsored by Reebok International Ltd. in Canton, Mass.

“It lends credibility when you have an event that has a winner. It sets a bar,” Verdun said. “It’s really a sport now and every sport needs a Super Bowl.”