(CNN) When Christopher Michael Everett went to his first SoulCycle class, he gave it his all. He sat in the front of the class, cranked the resistance on his bike and started pedaling.

Within the first five to 10 minutes, his thighs began to hurt and feel abnormal. But he powered through the pain and stuck it out until the end of the class.

"They say go big or go home. I probably should have went home," said Everett, a 33-year-old actor in Los Angeles.

For the rest of the day, he felt OK, but the following night was marred by excruciating thigh pain. Unable to sleep, he searched for "Spin class and sore legs" and came across a personal account written by a woman who needed to go to the hospital after a cycling class for a condition called rhabdomyolysis.

Rhabdomyolysis leads muscle tissue to break down and release a harmful protein into the bloodstream. Everett recognized the symptoms she mentioned, such as soreness, swollen thighs, nausea and an inability to bend the knees. He had all of them except dark brown urine , which he was not about to wait for, he said.

Everett immediately went to the ER and was diagnosed with rhabdomyolysis. He stayed in the hospital for a week.

What is rhabdomyolysis?

Rhabdomyolysis quite literally means "breakdown of muscle," said Dr. Derek Fine, associate professor of medicine and interim chief of nephrology at Johns Hopkins Medicine. When the muscle breaks down, it releases myoglobin, a protein that can poison the kidneys, into the bloodstream.

The condition is caused by any type of trauma to the muscle. There are also toxins in some medications and illicit drugs that can cause it, he said. The first descriptions of the condition were among people who had been trapped under bombed buildings during World War II, Fine said.

Recently, strenuous exercise has been popping up as another common culprit, seen in people who do activities like Spinning, P90X, CrossFit, weightlifting and running.

"I've had patients come in who can't walk. They can't get out of bed. They call 911 because their muscles don't work," Fine said.

In rhabdomyolysis cases that are easier to treat, the patient is given fluids to rehydrate and released from the hospital after a few days of monitoring, Fine said.

Everett said he was hooked up to an IV and pumped with fluids for seven days to flush out the toxins in his kidneys. He was not released until his levels of CPK -- creatine phosphokinase, an enzyme that leaks into the blood when muscle tissue is damaged -- were normal.

Worst-case scenario

If the condition is really severe, the kidneys may shut down, and the patient could end up on dialysis. With kidney failure comes the risk of an overload of potassium in the body, which could lead to abnormal heart rhythms and death.

In April, the American Journal of Medicine released a study on three cases of rhabdomyolysis that were caused by Spinning classes. In one case, a 33-year-old kindergarten teacher was put on dialysis when she had kidney failure and experienced fluid overload from her hydration treatment.

"It's tough to tell somebody who's 33 years old, 'I need to start you on dialysis.' And so you treat them with a lot of fluids, and you hopefully get them to the point where their kidneys open up and they start getting rid of the toxins and they don't have to go on dialysis," said Dr. Alan Coffino, a nephrologist and chairman of medicine at Northern Westchester Hospital in New York and a co-author of the study. He treated the 33-year-old. "In her case, we had to pull the plug and put her on dialysis."

After two weeks, her kidneys opened up, and she was released from the hospital.

Another potential symptom of rhabdomyolysis is compartment syndrome, Coffino said, which is when muscles swell and are unable to expand. In some cases, surgery is needed to release pressure from the muscle.

Everett said the swelling of his thighs concerned the doctor. "He was like, 'if your thighs get any more swollen, we're going to have to cut those open,' " he said. But the swelling did not increase, and Everett was able to avoid surgery.

How to avoid rhabdomyolysis

People who work out regularly can get rhabdomyolysis, Coffino said, as well as those who don't exercise often. Those who are fit typically get it if they push themselves too hard or if they try a new exercise that works a different muscle group. The study he co-authored points out that of the 46 people documented in medical literature who got rhabdomyolysis from a Spin class, 42 developed it after their first class.

Everett worked out every two to three days and considers himself fit, but he had never taken a Spin class before.

"I just wish SoulCycle would have said at the beginning of the class, 'if you work out too hard, this can happen to you,' " Everett said. Its been one month since he was hospitalized, and his legs are still sore.

SoulCycle has not responded to requests for comment.

Although any type of extreme exercise can lead to the condition, the popularity of Spinning is what concerns Dr. Maureen Brogan, associate professor of medicine at New York Medical College, nephrologist at Westchester Medical Center and the lead author of the study.

"I mean, Spinning, you burn 600 calories in an hour, and you lose up to a liter an hour of sweat," Brogan said. "Six hundred calories is like running six miles. So if you're not conditioned, you wouldn't just run six miles."

One of her patients developed rhabdomyolysis after spending only 15 minutes on a bike.

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Rhabdomyolysis can occur at any age and in both men and women. In Brogan's Spinning study, it was more common in females, but that's because more women tend to participate in the exercise, she said.

To avoid the condition, it is important to be slow and gradual when beginning a new type of workout, she said. When going to a class, you should tell the instructor if you are new, and make sure to hydrate before, during and after exercising.

Nonsteroidal anti-inflammatory medications can be damaging to the kidneys, so they shouldn't be taken after an intense workout, Brogan said. If someone does develop rhabdomyolysis, it could worsen their condition.

"It's better just to err on the side of caution and take things slowly," Brogan said.