Rhabdomyolysis, a rare but potentially life-threatening condition that can be a side effect of extreme exercise, is not a new discovery. Hundreds of documented cases appear among those in demanding professions, such as firefighters and the armed forces. Occasionally, college athletes are diagnosed after a grueling workout.

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But, as fitness culture permeates American society and weekend exercisers are bombarded with the challenge to “push past you limits,” doctors are seeing more cases of rhabdomyolysis crop up in people who are simply participating in an unfamiliar exercise, particularly in beginners doing high-intensity workouts.

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Three years ago, Christina D’Ambrosio, a kindergarten teacher from Pleasantville, N.Y., took her first indoor cycling class, and found that the workout was more than she had bargained for.

After the high-intensity, one-hour session, her legs throbbed with excruciating pain, and her urine gradually took on a dark brown hue.

Two days later, she checked into the hospital and was diagnosed with rhabdomyolysis, meaning her overworked muscles had begun to die, and were leaking their contents into her blood stream and straining her kidneys. She stayed in the hospital for two weeks.

D’Ambrosio’s case was featured in the April issue of The American Journal of Medicine, along with two other cases of rhabdomyolysis caused by an over-exhaustive cycling class and treated by the same doctors.

The article reported at least 46 other documented cases of rhabdomyolysis occurring after a cycling class. Of the documented cases, 42 were newcomers to a group setting indoor cycling group class.

Alan Coffino, chairman of medicine at Northern Westchester Hospital and co-author of the report told The New York Times, “I would never discourage exercise, ever. Spin class is a great exercise. But it’s not an activity where you start off at full speed. And it’s important for the public to realize this and for trainers to realize this.”

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According to the report, there is some evidence that certain medications or genetic predispositions can contribute to rhabdomyolysis, but generally speaking it is a symptom of simply doing too much, too soon without taking the time to adjust to a new activity.

Courtney Cameron is Editorial Assistant of Athletic Business.