ABC News' Vanessa Weber reports:

CrossFit is one of the most extreme workouts on the market, but one physical therapist is raising questions about the exercise program that has an almost cult-like following.

Eric Robertson, an assistant professor of physical therapy at Regis University in Denver, wrote an article this week highlighting rhabdomyolysis, a potentially fatal condition that can be caused by several factors, including severe exertion. Skeletal muscle is damaged, rapidly releasing proteins into the blood. It results in harm to the kidneys and can lead to kidney failure.

In his article, titled "CrossFit's Dirty Little Secret," Robertson wrote: "Rhabdomyolysis isn't a common condition, yet it's so commonly encountered in CrossFit that they have a cartoon about it, nonchalantly casting humor on something that should never happen."

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Jill Kloesel, a former college athlete who is now a physical therapist, says it happened to her. Kloesel said she didn't have any warning signs that she was suffering from the condition.

"Yes, my arms felt tired, but nothing that I hadn't experienced before when I had worked out several times," Kloesel told ABC News.

Kloesel says that after experiencing swelling from her elbows to her shoulder a few days later, she went to a hospital emergency room.

"They drew my blood … And sure enough, doctors told me 'your CPK (creatine kinase) levels, which are supposed to (be) less than 100 were greater than 40k.' Like not even reading on the charts," she said.

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Russell Berger, a course supervisor and spokesperson for CrossFit, says rhabdomyolysis is not unique to CrossFit.

"There are cases of rhabdo from football players, people who run triathlons, marathon runners to military trainees to body building communities," Berger told ABC News.

Berger says one of CrossFit's main goals is to raise awareness about rhabdo.

"We have articles in CrossFit journal when we first learned about exercise induced rhabdo," Berger said. "We had articles written by physicians in our journal who did overviews of rhabdo and gave examples on how to prevent it and identify it."

Asked about the not-so pleasant cartoon - which depicts a seemingly exhausted clown connected to dialysis machine, while some of his organs and blood are on the ground - Berger said: "My response when I hear people say that is, 'Ah…but you have heard him and that's exactly the point.'"

Kloesel no longer does CrossFit, and she says she now finds it extremely difficult to perform push-ups.

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"In some instances, if this isn't taken care of, this could kill you," she said of rhabdo.

Dr. Richard Besser, chief health and medical editor for ABC News, says cases of rhabdo, which he describes as a "death of muscle cells, occur only after you ask your muscles, "to keep working after they've stopped getting any energy to get the job done."

"That's really dangerous," Besser said today on " Good Morning America," adding that the condition can be prevented.

"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," he said. "One of the warning signs is your muscles are saying, 'I need to stop now.'"

To prevent rhabdo, Besser recommends staying hydrated both before and during exercise, taking breaks and listening to your body.

"No pain, no gain is the worst approach to exercise," he said.