In January 2025, I attended my first bootcamp class.

I had spent the day hunched over my laptop, anxious and craving an intense workout that would dispel my worries. I booked the class at a nearby gym, and the five-star reviews promised the all-consuming exercise I wanted: “Militant style instructor, but very motivating,” read one. Another: “Hardest workout of my life; extremely rewarding.”

The gym was no-frills – just a room with a mirror. After a standard warm-up, we did four sets of lateral shuffle push-ups across the floor, interspersed with standing, weight-bearing exercises.

When my turn came, I dropped to plank position and started doing steady, shallow reps, focusing on my form.

But caution soon fell away. Upbeat music was booming and someone was always advancing beside me. When the instructor encouraged us to lower all the way down, I obeyed, even though my form suffered. I had rarely done more than a handful of pushups at a time, and in the final set, I was exhausted, collapsing on every rep and barely prying my torso off the floor.

The rest of class was a blur. I became nauseated, told the instructor I needed to pause, and stepped outside to suck cold winter air into my lungs.

Back in the studio, I sat on the sidelines and waited for the dizziness to recede before slinking to my spot for the core section and cooldown.

Later that night, I felt what I thought was typical post-workout muscle soreness. I was satisfied; the ache was proof of a successful workout.

But the next day, lifting my arms to wash my face was exhausting. Searing pain kept me awake that night. Two days after the class, my arms were so stiff I couldn’t raise them more than a few inches, even to brush my teeth.

When I Googled my symptoms – pain, weakness and a new one, dark urine – something frightening came up: exertional rhabdomyolysis, a condition wherein extreme exercise causes muscle cell contents to flood the bloodstream, potentially overwhelming the kidneys. One article warned that debilitating pain after a new, intense activity was a sign to visit the emergency room.

I went to the ER but suspected I was overreacting. After all, the internet always offers the worst possibility.

Doctors use bloodwork to test for exertional rhabdomyolysis; typically, they diagnose it if a patient has too much of a muscle enzyme called creatine kinase (CK) in their blood – at least 1,000 units per liter, or five times the normal range. However, there is little consensus on this number, says Dr Barry Boden, an orthopaedic surgeon at The Orthopaedic Center in Maryland, who specializes in sports medicine. Some recent guidelines suggest that only higher amounts – as much as 10,000 units – warrant diagnosis and inpatient treatment.

My CK count was so high the machine in the emergency room couldn’t measure it; a nurse had to do a second blood draw and send it to a more precise lab. They put me on an IV drip and eventually reported the exact number: 57,000.

Thus began my seven-day hospital stay. My mom and sister traded shifts, acting as my arms for the week – scrubbing my teeth, feeding me, washing my face. I had never felt so helpless and irresponsible. What had I done to myself?

What is exertional rhabdomyolysis?

Normally, during exercise, muscles tear a little and then rebuild. There may be a little extra CK in a person’s blood as a result, which healthy kidneys can filter out.

But excessive exercise can harm muscle cells so much that their contents – including CK and a protein called myoglobin – overload the system. “When the cell membrane starts to break down, the chemicals within the muscle cells start to get released, which can cause damage to other organs around the body,” said Boden. “ If there’s enough of those chemicals from the muscle that reach the kidney, it can cause damage to the kidney.”

The symptoms are muscle pain (even while at rest), weakness and dark urine, though few people experience all three. Treatment involves early and aggressive administration of IV fluids to help the kidneys filter the toxins. It’s possible to manage a mild case with at-home oral hydration, but it’s always important to consult a doctor because mild symptoms don’t always mean low CK elevation, said Dr Petr Schlegel, a CrossFit trainer and professor at the department of physical education and sports at the University of Hradec Králové in the Czech Republic.

Exertional rhabdomyolysis is dangerous and fatal in very rare cases; researchers estimate that 10% of patients develop acute kidney injury (AKI), and some suffer other serious complications.

How common is exertional rhabdomyolysis?

Before I had rhabdomyolysis, I had never heard of it; I thought the worst that could come from a workout was a broken bone or a regurgitated lunch. I now know it can happen to anyone, although people with certain conditions, such as sickle-cell trait and hypokalemia, are predisposed. Despite the common misconception that only unfit people can get it, even elite athletes are susceptible.

 “Anybody can get it – anybody that’s pushed to an extreme, taking a big jump in their exercise level, or doing something they’re not used to doing,” said Boden. “Everybody has muscles, and if the muscles are damaged enough, you can develop rhabdomyolysis.”

Boden authored a study estimating there were over 40,000 exertional rhabdomyolysis cases in US hospitals from 2000 to 2019. But he said it’s impossible to get a precise count, as no organization collects the data.

This number is likely an underestimate, as the condition is probably under-reported, said Schlegel. Since the symptoms closely resemble those of delayed onset muscle soreness – the normal ache people expect after a workout – individuals may not seek care.

Data indicate exertional rhabdomyolysis is on the rise. In Norway, Australia and the US, researchers have observed an increase in hospital records between the 2000s and 2010s. This year, hospitals in a Canadian province reported a surge in cases.

Researchers suspect the popularity of high-intensity workouts is behind the rise. They are efficient and produce measurable progress, but are risky if misused, said Schlegel: “Evidence suggests that high-intensity exercise, especially when combining strength and endurance elements, carries the greatest potential to induce [exertional rhabdomyolysis].”

Can you prevent exertional rhabdomyolysis?

Prevention guidance has been pretty consistent since the earliest studies from the 1960s: vary exercises to avoid overloading one muscle group, incorporate rest, and gradually build intensity when starting something new or after time off.

Starting low is especially important when targeting large muscle groups – such as biceps, triceps and quads. “It’s that hyperintense going from zero to 100, really intense workouts of large muscle groups, that puts people at risk,” said Dr Bryant Walrod, a sports medicine physician and the head team physician for the Ohio State Buckeyes. Weight matters, but so do reps; an outrageous number of low-weight exercises or calisthenics – hundreds of pushups or squats, for example – is the trigger in many cases. Walrod also advises doing a different kind of workout from one day to the next.

Eccentric exercises like push-ups – where muscles lengthen – are particularly likely to cause injury. In a 2024 article about how to prevent rhabdomyolysis in student athletes, the National Federation of State High School Associations called push-ups the “No 1 cause” of rhabdomyolysis. In April, Texas families filed a lawsuit against a charter school after 20 children were hospitalized after hundreds of pushups.

Walrod said collegiate sports professionals became more vigilant about prevention after University of Iowa football players were hospitalized with exertional rhabdomyolysis in 2011. “That case spurred better control of workouts, better monitoring, and better input from the trainers and strength coaches.”

“Where we see most of the cases is that athletes are being pushed or threatened or punished [contrary to industry standards],” said Dr Rebecca Stearns, COO at the Korey Stringer Institute for preventing sudden death in sports. Coaches are not exercise physiologists, and even when well-intentioned may not have adequate training to prevent overexertion, said Boden.

Similarly, there’s no guarantee fitness instructors understand the risk. “People may be going in unconditioned and doing too much too soon,” said Walrod of these classes. CrossFit incorporated prevention into its trainer curriculum after reports of severe cases among participants, but in general, class participants should self-monitor.

How do you identify overexertion?

I was discharged from the hospital with a firm instruction: no exercise except walking for a month. I had to learn what an appropriate challenge felt like – how to self-monitor, discern between safe discomfort and overexertion, and advocate for myself. But when is hard too hard?

Unfortunately, sensing that something is wrong is a subjective measure; there is no universal metric.

“I never have a very satisfying answer to that, but I do think it’s a line we need to be vigilant about always, and it changes from day to day,” said Dr Natalia Petrzela, author of Fit Nation: The Gains and Pains of America’s Exercise Obsession, longtime fitness instructor, and professor of history at The New School.

“You know your body the best, and if you feel like something isn’t right, it’s time to speak up,” said Walrod. Sports medicine physicians advise people to stop exercise immediately if unusual pain occurs; in the event that there is excessive muscle breakdown, it’s critical to stop the movement right away.

Speaking up in a class setting can be difficult. Many feel self-conscious about pausing or modifying activity, especially if an instructor is singling them out. Petrzela said that she motivates participants in her class but also expresses a key caveat: “Only you know what you can do today.” She said this language “helps [them] find that very important and difficult-to-discern line”, adding that this nuance might get lost in classes with less experienced instructors who give inflexible instructions.

During that fateful class, I ignored the alarm bells. Maybe I was desperately chasing endorphins or wanted to prove to myself that I wasn’t weak. Ironically, my arms atrophied to below baseline as a result.

Over a year later, I’m still building my strength back – but not at bootcamp. I opt for low-impact methods such as barre and pilates where I can modify if needed, and there is no need to keep pace with others.

Sometimes I tell instructors about my medical history, so they understand what’s happening if I pause. Verbalizing it also reminds me to be careful. Finally, I avoid anything new or especially difficult when I’m having a hard day.

Fitness culture taught me that pain is gain – but now I know that’s not always true.