CrossFit Linked to Potentially Fatal Condition Rhabdomyolysis

ABC News' Dr. Richard Besser discusses the risks of the popular extreme fitness program.
4:07 | 09/26/13

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Transcript for CrossFit Linked to Potentially Fatal Condition Rhabdomyolysis
The latest on the extreme workout, crossfit. Now linked to a potentially fatal condition. Abc's paula faris has more. Reporter: It's one of the most extreme workouts on the market. A military-style exercise that pushes bodies to the brink. But now, one expert says crossfit could cross the line when it comes to your health. With any form of severe exercise, there's risks. And crossfit falls into that extreme exercise. Reporter: Eric robertson, a professor of physical therapy wrote about a rare and potentially fatal condition that can afflict some people who do crossfit. Writing, rhabdo isn't a common condition. Let's it so commonly encountered in crossfit, they have a cartoon about it. It's a disgusting clown with organs on the floor. Reporter: When skeletal muscle is damaged, it releases proteins too quickly into the blood and can lead to kidney failure. During the workout, I didn't have warning signs. Reporter: She was in the e.R. Just days after the workout. Swelling from elbows to shoulders. This could kill you. Reporter: Russell burger is a supervisor and spokesperson. He defends the program. And says rhabdo is not unique to cro crossfit. There's cases from football players, to marathon runners, to military teenees bodybuilding communities. Reporter: One of crossfit's main goals is to raise awareness about rhabdo and how to prevent it. And when asked about that not so pleasant cartoon. My initial response is you have heard of him. And that's exactly the point. Reporter: For "good morning america," paula faris, abc news, new york. For more on this workout danger, abc's chief medical editor, dr. Richard besser. It's a term not many people know. But it's death of muscle cells. And you can see it from certain drugs. You can see it from infection. But here, you're seeing it because you're asking your muscles to keep working after they've stopped getting any energy to get the job done. That's really dangerous. It can fry your kidneys. Your muscles sort of die out when you're working out. Yes. If you're listening to your body and you're getting that burn, and you say, okay, I've reached my limit and you stop, you're never going to see this happen. I remember coaches saying, no pain, no gain. No pain, no gain. How do you know how far to push? And why don't people stop when they're in pain? Part of it is peer pressure. You're in a group class. You are next to someone with a six-pack abs. And people working there. I'm as strong as they are. And you keep going. Or a team just starting a workout. And the coach is pushing everybody. But that burn you get when you've used up your oxygen, that's fine. If you're going beyond that. And you're pushing yourself and others are pushing group. Are there warning signs? One of the warning signs is your muscles are saying, I need to stop now. And you can prevent this if you drink a lot of liquids before you workout. You drink during your workout. You take the breaks. If you're feeling pain, you stop. No pain, no gain is the worst approach to exercise. We heard that so many times. And so, those are the suggestions that you have, especially staying hydrated. Have you ever treated somebody with this condition? It wasn't exactly this. But I see a lot of kids. And I had a teen athlete who is really into basketball. He came in complaining of severe belly pain. I looked at his belly. He was ripped. He had a six-pack an eight-pack. Do you do any situps? He said, 300 or 400 a day. Are you doing exercise to stretch those out? He said, no. It's not rhabdo. But the extremes of this are dangerous. Exercise is great. But not this. Thanks, rich. Next up in "the heat index,"

This transcript has been automatically generated and may not be 100% accurate.

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