In 2014, doctors at NewYork-Presbyterian/Weill Cornell Medical Center published a report on two patients who arrived at the emergency room with rhabdo shortly after their first spin class. One was a 24-year-old woman hobbled by pain, her legs swollen and feeling “as tight as drums.” She was rushed to surgery, where doctors sliced her thighs open to relieve a dangerous buildup of pressure.

Another study found that between 2010 and 2014, there were 29 emergency room visits for exercise-induced rhabdo at NewYork-Presbyterian alone. Weight lifting, CrossFit, running and P90X were the reasons for some visits. But the most common one was spinning. Dr. Todd S. Cutler, an internist at Weill Cornell Medicine and NewYork-Presbyterian and lead author of the study, said the patients all fit a similar profile.

“These are people who are not unfit,” Dr. Cutler said. “They are being pushed too hard, and they’re not trained to do this, and so they get really bad muscle trauma.”

There is some evidence that certain medications, including statins, stimulants and antipsychotic drugs, as well as genetic susceptibilities may contribute to the condition, said Patricia Deuster, a professor of military and emergency medicine at the Uniformed Services University of the Health Sciences.

But in general it occurs when people simply do not give their muscles time to adjust to an aggressive new exercise, experts say. A little damage to muscles is a good thing because that stimulates them to grow and adapt to stress. But when the stress is too great, fibers are destroyed. When that happens they break apart and release compounds that can be harmful to the liver, such as a protein called myoglobin, which causes brown or tea-colored urine, a classic symptom of rhabdo.

While almost any intense activity can cause rhabdo, it almost always strikes people who are doing something new. That is why people should always progress from light to moderate and then vigorous intensity when doing a new exercise, said Eric Rawson, chair of the department of health, nutrition and exercise science at Messiah College in Pennsylvania.

“You can be fit, and I can come up with a workout that you are unaccustomed to, and that could be what causes rhabdo,” he said.