Being sore after workout is a sign you hit the gym hard enough to build new muscle. But there is definitely such a thing as being too sore—and not just because it makes sitting, standing, and laughing painful. Feeling excessively stiff and tender is a sign you’ve actually overtrained, and if you’ve gone too far, you could actually put your health at serious risk because of rhabdomyolysis.

Often called "rhabdo" for short, this potentially dangerous condition occurs when muscle fibers work too hard and ultimately die, thereby releasing their toxic contents into the bloodstream. While most milder rhabdo cases involve achy muscles, the condition affects your kidneys and can actually land you in the hospital.

Here's what fit guys need to know about rhabdo: Its causes, symptoms, prevalence, and what you should do if you suspect you might have a case.

What exactly is rhabdomyolysis?

Rhabdomyolysis can occur as a result of genetic and metabolic disorders, as well as trauma like a crush injury. But for the sake of this conversation, we’re talking exclusively about exertional rhabdomyolysis, which is caused by one thing: someone working out way to hard too freaking hard.

Some basic gym biology: When you lift weights, you’re causing micro tears in your muscles. When the body repairs those tears, the muscle becomes stronger and bigger. Technically, rhabdo is the same muscle damage, but taken to such an extreme that the fibers can’t repair, trainer and kinesiologist Joel Seedman, Ph.D., C.S.C.S., owner of Advanced Human Performance in Atlanta.

During a workout, pushing your body to this edge depletes your muscles of all their energy, leading to a chain of events culminating in the death of muscle cells, explains David Wang, M.D., a primary care sports medicine physician at the Hospital for Special Surgery in New York. “As the cell dies, it releases its contents into the bloodstream, causing pain, swelling, and potentially organ damage.”

Yes: Working out too hard can actually cause organ failure. As blood moves toward damaged muscle tissue to clear it out, the blood is also shuttled away from the rest of your body, including your kidneys, Wang explains. Simultaneously, the damaged tissue releases myoglobins—proteins toxic to the kidneys—that quickly collect in the kidneys and cause a blockage.

This is rhabdo's real life-threatening risk: Your kidneys are responsible for filtering waste and maintaining your fluid and electrolyte balance, so when they get clogged, you can’t get rid of any of the toxins in your system as you become severely dehydrated, Wang says.

How common is rhabdomyolysis?

It’s hard to say just how common rhabdo is. There is a scale of damage, and mild cases often don’t get reported, Seedman points out.

All the experts we talked to agree rhabdo isn’t all that common. However, people who work out really hard all the time are obviously at a higher risk for exertional rhabdomyolysis, simply because they're frequently pushing the limits of their bodies. And cases are on the rise—one study in a Norwegian hospital found that among the 22 people admitted with rhabdo between 2010 and 2015, three quarters of cases were between 2014 and 2015 alone.

How do you get rhabdomyolysis?

When rhabdo does occur, it's typically after a single, hard-as-hell workout when your energy supply can’t keep up with the demand, Wang says—not a prolonged case of overtraining.

There are two scenarios for how a smart guy like you might get yourself into this mess. The first: You haven’t worked out in a while (or ever), and suddenly you're getting after it like Dwayne "The Rock" Johnson. Even lifting hard after just two to four weeks off can do damage, simply because your strength is there to handle the load, but you’ve lost your recovery adaptations, Seedman says.

A veteran gym rat status doesn’t necessarily protect you, either. The second case: You're a super fit dude who hit it especially hard one day. “We also see this in trained athletes adding a new routine or being pushed past their bodies’ limits,” Wang says. He points to clusters of cases of teammates developing rhabdo after a workout heavy on pushups, squats, or hill training—especially when they're trying to keep up with, or outdo, an instructor or workout partner. That kind of approach quickly escalate to danger zone.

It's surprisingly easy for a well-intentioned ass-kicking workout to turn so bad. For example, a 2013 Clinical Journal of Sports Medicine study of collegiate football players found the players who suffered rhabdo during a back squat workout were simply more likely to have performed extra squats. The researchers also reported that the rhabdo-inflicted players went to muscle failure and didn't think they could complete the workout, compared to the guys who worked hard but not past the limit. Sounds similar to what you’re aiming for when you step up to the weight rack, right?

In reality, exertional rhabdomyolysis is a relatively uncommon condition, Wang says. And there’s no one factor that guarantees someone will develop it. But studies show that extreme exertion, dehydration, hotter temperatures, infection, trauma, drugs and medications, and endocrine abnormalities are all risk factors—the biggest being a sudden increase in your workout load.

The muscle groups most often affected are the shoulders, lower back, thighs, and calves, Wang adds. And while researchers haven’t pinpointed exact moves that increase your risk, they do know that one common culprit is eccentric moves—the phase of a contraction when the muscle lengthens instead of shortens, like the lowering of a squat or the lowering of a bicep curl. “Eccentric loading doesn’t use as much ATP (which provides energy to your muscles) as concentric move, so you don’t feel as fatigued and you can keep going and going, leading to excessive damage,” Seedman explains. Rhabdo is also more commonly seen after repetitive eccentric activities, like push-ups, curl-ups, squats, cycling, and hill training, Wang adds.

Do certain workouts increase your chances of developing rhabdomyolysis?

In the popular imagination, rhabdomyolysis is often associated with CrossFit—and there was a time where rhabdo one of the most common injuries in the CrossFit community. “There are a lot of great things about CrossFit, but unfortunately it’s predicated on higher volume and higher quantity," Seedman says. "When you train with the goal of quantity—as many reps as possible—you increase your chances of serious muscle damage."

Furthermore, when CrossFit first gained mass popularity, it meant a ton of newbies were inevitably thrown in to workouts alongside WOD veterans, which sometimes led to competition and overextending. Fortunately, most CrossFit affiliates have moved away from the one-size-fits-all model, and will now group training by experience, so there’s a lot less risk of rhabdo (as well as other injuries). Any CrossFit coach worth his or her salt knows when to push an athlete and when to ask an athlete to throttle back.

To be clear: CrossFit isn't inherently dangerous, and high-volume workouts aren't exclusive to CrossFit. However, those styles of high-volume workouts are more conducive to incurring rhabdomyolysis, Seedman says. In fact, any sport or training method has the opportunity to induce rhabdo if you’re going after it especially hard. A 2016 study published in the American Journal of Medicine found a few cases of fit folks getting rhabdo just from high-intensity spin classes, as they were new to the workout.

What are the symptoms of rhabdomyolysis?

It’s hard to tell you have full blown rhabdo mid-workout, but there are subtle warning signs you’re entering overtraining territory, Seedman says. "Watch out for an extreme drop in strength suddenly," he says. "It’ll be different than systematic fatigue, but it feels like the muscles have no strength left to fire," he explains. The other sign oft used by bodybuilders: "If your muscle pump decreases at some point in the workout, that’s an indication that glycogen levels are depleted, which is a pretty accurate gauge that you’re starting to tear your muscles down," he adds.

Be wary if you feel sore immediately after—or even during—a workout. “That’s huge—you shouldn’t get sore during a workout and if you do, it’s an indicator you’ve done too much,” Seedman says.

The reality, though, is you probably won’t know until after the workout. The biggest post-workout tell-tale is when you are excessively sore and it’s taking longer than usual to go away. “If you’re sore for more than three days after the exercise, that’s excessive,” Seedman says.

But how can you tell if you overtrained Monday or are newly sore from Wednesday’s workout? With healthy muscle soreness, another workout will alleviate the stiffness; with excessive training, the gym will make you feel worse, Seedman says.

Other markers: You feel weak—as in a significant drop in force—in the days after a workout; soreness is so extreme it impairs your ability to move; your muscles are extremely swollen from inflammation; cramps (a sign of potassium and sodium depletion); nausea; vomiting. The most helpful (and unarguable) sign is sometimes your urine becomes brown no matter how much water you drink. It’s a sign your kidneys aren’t filtering toxins out. While it’s an irrefutable sign something’s amiss, it only occurs about half of rhabdo cases, Wang says, so don't wait for your pee to turn brown before you suspect it.

You may have noticed these symptoms are also outcomes of your typical kickass workout. So: How can you tell the effects of a healthy, muscle-building training session from a damaging over-training session? Both our experts agree the line is tough, but your best marker is the duration of symptoms, especially soreness.

What should you do if you think you have rhabdomyolysis?

“If you develop atypical muscle pain and weakness, especially after increasing your workout load or introducing a new repetitive exercise, you should be evaluated,” Wang says. Yes, a lot of people who develop rhabdo don’t go to the doc, but the majority of the time these cases are mild and hydration and rest helps. But there have been a few cases of people dying from untreated rhabdo, so bear that in mind. You know how much soreness and what recovery timeframe is typical for your body, so if you feel like something’s wrong, you’ll heal a lot faster (and smarter) with medical expertise.

The definitive diagnosis of rhabdomyolysis can only be made with a blood test measuring the muscle enzyme creatine kinase, Wang says. That means you need to go to the doc. A sports medicine expert or even a primary care physician office can run the blood test, but if you can’t be seen within a day, it can definitely warrant an ER visit, Wang adds. And if you develop that dark urine, 100 percent go see someone.

What are the steps a doctor can take to treat rhabdomyolysis?

Unclogging your kidneys is priority here—all the other symptoms will cascade into healing from here. The mainstay of treatment for rhabdomyolysis is rest and fluid resuscitation, Wang says. Sounds simple enough, but the fluids the doc can give you—either to be taken orally at home or via an IV at the hospital depending on the severity of your condition—will help you heal a heckuva lot faster (typically within three to five days).

There are no specific guidelines on how quickly you can get back into the gym, but once your blood tests are clear and your doc gives you the okay, you can start easing yourself back in—gradually.