Spin classes have been around for decades, but in the last several years they have swept the nation by storm. Many see it as a great way to stay healthy and have fun. But not, some doctors are worried about the long term health effects of these classes.

Rhabdomyolysis is a rare but life-threatening disease that is usually caused by extreme exercise. It describes a condition where overworked muscles actually start to die; as they do, their content is leaked into their bloodstream. This forces the kidney to go into overdrive in an attempt compensate for the extra material, causing extreme pain.

Casey Daniel, an LA based actor, was diagnosed with Rhabdomyolysis, or Rhabdo as doctors often call it, after her very first class.

“The second day after the class I woke up and felt like I had been hit by a bus. I had some unusual bruising that was starting to happen above my knees,” Daniel told Inside Edition.

The pain was so severe that Daniel went to the emergency room. Once there, doctors rushed her into surgery.

“I only had 10 minutes to call my family, who is all the way across the country, and tell them I was going into surgery,” she said. “I can’t express how terrifying it is to not be able to walk and to be in that much pain.”

It took three separate surgeries to repair her thighs. Still, a physical therapist visits Daniel’s at home three times a week. They work on strengthening her legs and restoring her ability to walk unassisted.

At least 46 other people have been diagnosed with Rhabdo as a result of spin class, according to a report mentioned in the New York Times.

While these figures are concerning, they pale in comparison to the 400 cases among active duty soldiers that are diagnosed each year according to a 2012 Army Study.

So how do avoid the risk of Rhabdo? The answer is simple, according to Dr. Maureen Brogan, an associate professor of medicine at New York Medical College, know your limits and don’t push beyond them.

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

To put this in perspective, a cyclist will burn 260 calories in an hour if they are cycling at 10 mph. That means that spin classes must be going significantly faster than that.

And while the average person has between two and four million sweat glands that work to prevent your body from overheating, your muscles can be a little more delicate.

Brogan’s advice is simple, “It’s better just to err on the side of caution and take things slowly.”