WHILE many gymgoers complain that they might not survive a tough workout, Brian Anderson can speak from experience. For his first CrossFit session, he swung a 44-pound steel ball with a handle over his head and between his legs. The aim was to do 50 quick repetitions, rest and repeat. After 30 minutes, Mr. Anderson, a 38-year-old member of the special weapons and tactics team in the sheriff's office in Tacoma, Wash., left the gym with his muscles sapped and back pain so excruciating that he had to lie in the driveway to collect himself.
Jim Wilson/The New York Times
That night he went to the emergency room, where doctors told him he had rhabdomyolysis, which is caused when muscle fiber breaks down and is released into the bloodstream, poisoning the kidneys. He spent six days in intensive care.
Yet six months later Mr. Anderson, a former Army Ranger, was back in the gym, performing the very exercises that nearly killed him. "I see pushing my body to the point where the muscles destroy themselves as a huge benefit of CrossFit," he said.
In the last year this controversial exercise program has attracted a growing following of thousands nationwide, who log on to CrossFit.com for a daily workout, said its founder, Greg Glassman. Participants skip StairMasters and weight machines. Instead they do high-intensity workouts that mix gymnastics, track and field skills and bodybuilding, resting very little between movements.
The emphasis is on speed and weight hoisted, not technique. And the importance placed on quantifiable results has attracted hard-charging people like hedge fund managers, former Olympians and scientists. But some exercise experts are troubled by the lack of guidance for beginners, who may dive into stressful workouts as Mr. Anderson did. (He had not worked out regularly for two years.) "There's no way inexperienced people doing this are not going to hurt themselves," said Wayne Winnick, a sports medicine specialist in private practice in Manhattan, who also works for the New York City Marathon.
Other critics say that even fit people risk injury if they exercise strenuously and too quickly to give form its due, as CrossFit participants often do. For people who like to push the limits of fitness and strength - there are many police officers, firefighters and military personnel in the ranks of CrossFit athletes - the risks are worth it, because they consider it the most challenging workout around.
The short grueling sessions aren't for the weekend gym warrior. The three-days-on, one-day-rest schedule includes workouts like "Cindy": 20 minutes of as many repetitions as you can of 5 pull-ups, 10 push-ups, 15 squats. "Fight Gone Bad" entails rotating through five exercises, including throwing a 20-pound ball at a target 10 feet away. And only veteran CrossFit devotees even attempt, and few complete, "Murph," a timed mile run, 100 pull-ups, 200 push-ups, 300 squats and then a second mile run. (A weighted vest is optional.)
Mr. Glassman, CrossFit's founder, does not discount his regimen's risks, even to those who are in shape and take the time to warm up their bodies before a session.
"It can kill you," he said. "I've always been completely honest about that."
But CrossFitters revel in the challenge. A common axiom among practitioners is "I met Pukey," meaning they worked out so hard they vomited. Some even own T-shirts emblazoned with a clown, Pukey. CrossFit's other mascot is Uncle Rhabdo, another clown, whose kidneys have spilled onto the floor presumably due to rhabdomyolysis.
Mr. Glassman, 49, a former gymnast from Santa Cruz, Calif., walks with a slight limp because of a knee injury, and at 5-foot-7 and 185 pounds admits he should lose weight. He began developing CrossFit more than two decades ago, but he says that he spends so much time running the business now that he no longer regularly does the routines. At first his program was a hard sell to clients who weren't keen to climb ropes or grapple with gymnastic rings.
Then in 2001 he launched CrossFit.com and began publishing a monthly journal and holding seminars at his California gym. People from around the world have come to learn Mr. Glassman's techniques. Today CrossFit has more than 50 affiliates in 21 states and 5 countries, Mr. Glassman said. And CrossFit.com has 25,000 unique visitors a week, according to WebSideStory, a Web analytics company in Seattle.
Mr. Glassman's followers call him Coach and share a cultlike devotion to his theories.
"We are all drinking the Kool-Aid," said Eugene Allen, another Tacoma SWAT team member who introduced Mr. Anderson to CrossFit last summer. "It's hard not to catch Coach's enthusiasm."
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