CrossFit workouts mix weightlifting, gymnastics and cardio activities such as jump rope and rowing, into short, intense combinations that change daily. A typical "workout of the day" (WOD) might consist of running 400 metres, then doing 21 kettlebell swings and 12 pull-ups before repeating this sequence twice more. Participants may have 20 minutes to complete as many rounds as possible of 10 push-ups, 20 pull-ups and 30 lunges. Besides the intensity factor, what distinguishes CrossFit from other workouts is its emphasis on moves - bending and squatting, for example - that are functional in everyday life, says Russell Berger, a head trainer for CrossFit Inc.

Rather than training people to become very good at a few exercises, CrossFit prompts them to do things people do in real life, such as getting up and raising things off the ground, Berger says. "We're just asking people to do them a little faster with a little more weight to help them get better at it," he says.

The intensity of workouts and a culture that encourages participants to push themselves to their limits has earned CrossFit a reputation for extremism, says Eric Robertson, a Denver physical therapist. "People tend to brag about the injuries that they got. They post pictures of hands torn from lifting weights, and it's like a badge of honour." Exercise-induced vomiting is so common it has a mascot - Pukie the Clown.

Not everyone sees the macho humour in this. It's one thing to push yourself; it's another to go beyond the point where your body is telling you to stop, Robertson says. He recalls a young, fit physical therapist who developed severe swelling in her arms and exertional rhabdomyolysis, a potentially life-threatening condition, after pushing herself through hundreds of push-ups and overhead presses in a CrossFit class.

Strength training normally increases muscle power by creating microscopic damage to muscle fibres, which the body then repairs, making them stronger. But if the damage is great, it can flood the bloodstream with myoglobin, a large protein that can overwhelm the kidneys, leading to "rhabdo" and possible kidney failure, Robertson says.

The symptoms of rhabdo don't turn up until the damage is done. Prevention requires listening to your body and avoiding sudden and severe increases in repetitions. "Don't exercise beyond exhaustion, and if you find yourself losing form, stop," Robertson says. He says it's important to take the time to master proper technique because many of the exercises used in CrossFit are unfamiliar to the typical gym goer.

Neal Henderson, an exercise physiologist, says intermittent high-intensity exercise is a great way to get fit, but only when the body can recover. "Tolerating high intensity is different from adapting to it and benefiting from it," he says. Studies suggest constantly exercising at high intensity won't lead to optimal results and may instead provoke overtraining - a drop in energy and performance that happens when the body can't fully recover from workouts. Robertson advises people to start with individual training before beginning group classes and look for an instructor who emphasises form and safety.

The Washington Post