You may have been hearing some—okay, a lot—of noise about something called exertional rhabdomyolysis (or rhabdo, as it’s colloquially known in the fitness industry). In fact, a recent New York Times article detailed the story of a woman who, post-spin class, came down with the life-threatening health condition brought on by extreme exercise.

Although uncommon, rhabdo is real. So in the age of HIIT and other fast-paced classes, should you be worried? And how much is too much exercise?

What EXACTLY is rhabdo?

Rhabdo is a condition in which there’s a rapid breakdown of muscle tissue, resulting in the death of muscle fibers that wind up leaking into the blood stream, explains Michele Olson, Ph.D., F.A.C.S.M., and adjunct professor of sport science at Huntingdon College in Montgomery, AL.

Normally, your kidneys, which process and remove waste through urine, would take care of those leaking muscle fibers. But with rhabdo, the kidneys can’t handle the amount of damage, Olson says, and eventually (if not treated), they shut down.

That’s why you want to catch symptoms quickly. Rhabdo can lead to a very dangerous situation: People may experience kidney failure, electrolyte disturbances, cardiac arrhythmia, and even death.

Rhabdo is considered pretty rare, with about 26,000 cases per year occurring in the United States. (That may sound high, but a disease is considered ‘rare’ if it affects less than 200,000 people per year.)

How do you get rhabdo?

“Exertional rhabdomyolysis is the term used when rhabdomyolysis is associated to physical activity,” explains Gerardo Miranda-Comas, M.D., an assistant professor of rehabilitation medicine at the Icahn School of Medicine at Mount Sinai.

With this kind of rhabdo, the activities that cause the most muscle damage are the most likely to cause a problem, says Olson. “This includes ultra-endurance events such as marathons and heavy-intense weight lifting.”

And while healthy people who follow their training plans to a tee can unfortunately succumb to rhabdo, often it’s people who don’t properly build up to endurance exercise or heavy lifting who suffer, says Olson.

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Other kinds of rhabdo are caused by underlying medical conditions (think: diabetes, thyroid disease, chronic electrolyte disorders, or acidosis); medications (stimulants, antihistamines, and statin drugs), and illicit drugs can also put people at risk, notes Miranda-Comas.

Also: Anyone who’s damaged their muscles—whether it’s from a car accident or a dangerous infection—can experience rhabdo, notes Olson. So it’s not exclusive to your spin or HIIT class—you don’t have to cancel your membership!

What are the symptoms?

If you’re suffering from rhabdo, you might feel weak and have trouble with ‘normal’ movements, notes Olson. Pain in the shoulders, upper back, and thighs is also common, as is confusion, or vomiting, she says.

“Delayed onset muscle soreness (DOMS)—muscle soreness that occurs one to three days after an intense bout of exercise—is considered a mild form of rhabdomyolysis,” says Miranda-Comas. “So persistent soreness is an early sign and should be evaluated.”

Dark red or brown urine—which could mean there is blood and degraded protein (from destroyed muscle fibers)—can also be a sign of rhabdo, notes Olson.

Unfortunately, not all cases (or symptoms) look the same, so it can be difficult to diagnose. Miranda-Comas notes that symptoms can vary from no soreness to mild soreness or extreme muscle tightness and pain with weakness and extreme difficulty moving.

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Adds Olson: “A person may have all or very few of the symptoms—which is why rhabdo is clearly dangerous and can harm an unsuspecting exerciser.”

If you think you’ve got rhabdo, it’s crucial to seek medical attention immediately, both experts say. Miranda-Comas notes that hydration is the standard form of treatment, but severe forms may require dialysis or surgery.

How can you best prevent getting rhabdo from exercising?

Your best bet is to stay safe with exercise (regardless of rhabdo, but also because of it), and take things slow. “Build up gradually,” says Miranda-Comas.

Remember, endurance training should be a gradual progression, with no more than a 10 percent increase in volume weekly, he says. That means if you’re trying to build up mileage when running (let’s say you’re doing 10 miles a week), you should only do 11 miles the next week.

Rest days are crucial, too. The average exerciser shouldn’t be doing high-intensity training on consecutive days, notes Miranda-Comas.

The bottom line? “Do not ever jump into something intense if you haven’t been exercising, have been sick, have experienced a break from exercise due to vacation, or have a chronic medical illness.”