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"CrossFit is real attractive because it's new and sexy," said Thompson, who heads the survey and has been studying fitness trends for 32 years. "But my suspicion is the rate of injuring is going to be so high it's going to fall off."

Thompson also said he does not believe CrossFit is "sustainable" because it's trying to appeal to everyone. "Once these companies like CrossFit and Insanity start messing with their core, not their core muscles but their core marketing strategies, and adapt to older people and people with chronic disease and maybe disabilities, it becomes a hybrid program. You may have the CrossFit brand, but it's no longer the CrossFit program."

Reebok, one of the world's largest makers of sports footwear and apparel, disagrees with that short shelf-life assessment.

"I've seen all [the fitness fads] come and go, but CrossFit is here to stay," said Chris Froio, vice president of fitness and training for Reebok.

The global company, with $3 billion in sales, is banking on CrossFit to help the company with it's new direction -- turning to its fitness roots, a reaction to the loss of its 10-year contract with the National Football League and its purchase six years ago by Adidas. In 2010, Reebok signed a 10-year deal with CrossFit that includes title sponsorship of the CrossFit Games, which a Forbes blogger called one of the fastest-growing sports.

The sponsorship was not a snappy decision. Reebok spent a couple of years exploring many types of fitness activities. "Along the way, we stumbled onto CrossFit," Froio said, discovering that some employees were trying the exercises on the back field of Reebok's Massachusetts headquarters. "Basically, it spurred about 20 of us to try it. We instantly fell in love with the whole physical part, and the social, community and team environment it provides."

But, Froio conceded, "I was as skeptical as anybody when I first started researching it."

Froning said he thinks Reebok's sponsorship makes the fitness workout with the macho image more appealing to the masses. "It may be kind of scary at first. But put a name like Reebok and there is a sense of security. Maybe I can try it. And the beauty of CrossFit is that it's not just for competitors like me. You can scale it back."

Reebok owns two CrossFit gyms for its employees near Boston. It also is affiliated with about 50 CrossFit gyms in 20 countries, but only three are in the United States. One of them is Reebok Miami Beach CrossFit.

Reebok backing

"The Miami Beach box is in close proximity to athletes, celebrities and employees," Froio said. "We want to be anywhere there is an epicenter of the Reebok brand. But we do not want the independent affiliates to think Reebok is coming in and opening monster gyms to put them out of business. There is nothing further from the truth."

Reebok wants CrossFit to thrive, so CrossFitters will buy zillions of dollars worth of their shoes and apparel designed especially for the strength and conditioning program in which people can sprint, lift weights and jump in the same workout. CrossFit's motto is "our specialty is not specializing." Reebok launches its new CrossFit line on June 1.YouTube had played a big role in bringing the CrossFit community together. On one YouTube posting, Glassman explained how he came to name the infamous Fran WOD. "Anything that left you flat on the back, looking up at the sky and asking what the [expletive] happened to me deserved a female's name ... right?" Glassman said. "They're not old girlfriends. They're just like storms. If hurricanes that wreak havoc on the whole town can be Fran, so can a workout."