Some people start their day with a cup of coffee or, if they're ambitious, a run. Others head to a gym to perform a series of power cleans, burpees, box jumps, tire flips and other taxing exercises -- all in quick succession with little or no rest, while fellow exercisers raucously cheer them on. These diehard individuals are CrossFitters, and their numbers are growing every day. With more than 5,000 affiliate gyms worldwide and a loyal following that borders on the obsessed, CrossFit has exploded in popularity over the past few years -- yet its safety has been and continues to be a hotly debated topic.

The Rumor: Because of the intensity of its workouts, CrossFit can be seriously dangerous

Founded in 1995 by former gymnast Greg Glassman to "prepare the body for any physical contingency -- not only the unknown, but also the unknowable," CrossFit eschews the idea that a person must do one form of exercise at a time. Instead, it combines varied functional movements like plyometrics (think hardcore jumping exercises), Olympic-style weight lifting, resistance drills, gymnastics and cardio, among others, into a single high-intensity Workout of the Day (WOD). The WOD is posted on the CrossFit website and is designed to take CrossFitters anywhere from five to 20 minutes to complete. That may not sound tough, but the circuits can be so intense that they often cause CrossFitters to vomit -- something they brag about by saying, "I met Pukey." (Some even wear T-shirts adorned with an upchucking clown -- named Pukey -- to emphasize this desired result.)

But vomiting is far from CrossFit's biggest risk. The American College of Sports Medicine, physical fitness experts and even Glassman himself warn that the program -- if not done properly -- poses the chance of serious injury and even death. 

The Verdict: There are big-time dangers in CrossFitting, but being smart about your regimen can greatly reduce the risks

"The possibility of injury is an increased risk with participation in any high-intense fitness regimen like CrossFit, especially if you are new to Olympic-style weight lifting and plyometric workouts, or have a previous injury," says WebMD contributor Michael R. Esco, Ph.D. "Not only are the exercises themselves risky, but performing them under a fatigued state, such as during an intense circuit, increases the risk of injury even further."

Esco cautions that the greatest danger with CrossFit is rhabdomyolysis (or "rhabdo" for short), a condition in which damaged skeletal muscle quickly breaks down, releasing fibers into the bloodstream that can cause kidney failure and, if not treated properly and in a timely fashion, death. 

A handful of CrossFitters have experienced rhabdo, though so far none have died. In 2004, former Army Ranger Brian Patterson went to the gym for his first CrossFit session and, following a workout involving a 44-pound steel ball, ended up in the hospital diagnosed with rhabdo. He remained in intensive care for six days. In 2005, CrossFitter Makimba Mimms was doing an "orientation workout" with an uncertified trainer at a Manassas, Virginia, CrossFit gym when he also suffered rhabdo. He successfully sued the gym -- though not CrossFit -- for $300,000 in damages.

But CrossFit’s founder doesn't deny that his program is dangerous; he matter-of-factly acknowledges it. "It can kill you," Glassman told The New York Times. "I've always been completely honest about that."

CrossFit has even published several articles in its online journal warning about the dangers of rhabdo for those who overextend themselves. In a 2005 entry following a spate of five CrossFit-related rhabdo diagnoses, Glassman admitted the dangers of his program, but pointed out that all who had suffered from rhabdo were new to CrossFit and "had typically previously experienced only low-power-output, low-intensity workouts." He went on, "From our perspective, it seems that these folks were exposed to too much work in too short a time."

There is no empirical data to support Glassman's claim, and Esco urges all CrossFitters to be wary of overdoing it. "To prevent rhabdomyolysis, make sure you start slow and gradually increase the intensity of each workout," he says. "Drink plenty of water, and avoid exercising in a hot and humid environment."

Despite the admitted dangers of CrossFit, its practitioners seem largely unfazed.

"I've been doing CrossFit for three years and have gotten injured playing sports more than anything that has ever happened to me at CrossFit," says Suzy Sanchez-Galarraga of Miami. "I would say that the biggest problem I've seen is people trying to do too much too soon. I can't tell you how many times I have seen new guys come into the gym and go heavier on the weights than they should because they don't want to be doing less weight than a girl. Their form suffers, and injuries can certainly occur." 

"Like any workout, you can always get hurt if you aren't careful," says Ashley Endlich, a Wood Ridge, New Jersey, resident who has been attending CrossFit classes six days a week for the past three years. "But CrossFit is never boring, and the people I do it with aren't just friends -- they're family. We live and breathe CrossFit, and no one gets that but us."