Experts say more intense exercise classes are putting people at risk for “rhabdo.” Here are some tips on how to avoid it.

A rare, serious side effect of exercise is not only affecting extreme athletes, but it’s also popping up in weekend warriors.

Rhabdomyolysis, or “rhabdo,” is a rare condition that can occur from exercising. Muscles become so overworked their cell contents are released into the blood stream, causing injury to the kidneys.

In recent years, exercise classes that push people to up their cardio — sometimes dramatically — have become more and more popular.

Dr. Robert Flannery, a sports medicine specialist at University Hospitals Cleveland Medical Center, said it’s these classes that can sometimes cause amateur athletes to push themselves a little too far.

“We’re seeing more of it with coach-driven exercise classes,” Flannery told Healthline. “The big one that seems to have the most is the spin classes … you see it a little bit in some of the Zumba intense workout classes.”

Flannery said it’s these cardio-based classes where the muscles can become overworked.

“You have this push where you do more than what your body is capable of doing,” he said. “I think we’re starting to see more of it in the less conditioned athletes.”

The overworked muscles start to release a protein called myoglobin in the blood, which is then filtered by the kidneys.

If the muscles are extremely overworked they can release a large amount of myoglobin, which breaks down into substances that can damage kidney cells.

“It will gum up the kidneys, for lack of a better word,” Flannery said.

Symptoms usually include aches, pains, flu-like symptoms, and dark brown or “tea-colored” urine that require IV hydration or possibly even kidney dialysis.

To prevent this from developing, Flannery stresses hydration.

“The myoglobin that gets broken down from the muscle will be washed out pretty quickly if you’re hydrated,” he explained.

Often people who are affected by “rhabdo” are in shape, but possibly unfamiliar with or not acclimated to a new exercise.

In 2012, researchers published an article about three amateur triathletes that developed rhabdo during their training.

“Whilst triathlon training is popular among amateur sports people, awareness must be raised to train appropriately under proper conditions,” the authors wrote.

In April, researchers from Westchester Medical Center in New York published a report in the American Journal of Medicine indicating that spin classes have led to a number of cases of rhabdo.

In one instance a woman showed up at the emergency room with signs of rhabdo after spending just 15 minutes on the spin bike.

The study authors said that people need to be made more aware of the possible side effects of extreme exercise, and that they should build up their endurance.

“Participants need to be informed of the risks of rhabdomyolysis,” the authors wrote. “Guidelines should include information about the signs and symptoms of rhabdomyolysis and the urgency of seeking hospital treatment when such manifestations occur.”

The condition has also been found in people with certain genetic conditions, drug or substance abuse problems, crush injuries, and in rare cases with certain drugs, including statins.

Unfortunately, Flannery said, there are no early warning signs that could alert you during your workout that you’re at risk for “rhabdo.”

“You just don’t have that immediate response, whereas you pull a muscle you know it right away,” he explained. “This is you overusing the muscle and the breakdown product doesn’t affect you until later.”

Instead, simply remember that if you’re trying out a new class or workout, simply take it easy.