When does exercise stop making you stronger—and start causing damage?

We’re constantly driven in our pursuit of fitness by a familiar mantra: harder, faster, stronger. We chase progress by pushing our bodies to the limit—but what happens when the body pushes back?

Muscles are remarkably adaptable—but they do have a limit.  At the extreme lies rhabdomyolysis (“rhabdo”), a condition in which skeletal muscle breaks down, releasing proteins and electrolytes into the bloodstream that can put organs such as the heart and kidneys at risk.

Exertional rhabdomyolysis (ER) is the subtype caused by physical exertion. “Evidence suggests ER cases have been increasing since 2020,” says Nick Kruijt, an academic researcher at Radboud University Medical Centre (Radboudumc). “It appears more common in military recruits, where extreme exertion and overheating are likely to play a role. But incidence is also increasing amongst recreational athletes, particularly in sports that involve eccentric training— where muscles produce force whilst being stretched.”

Researchers think several factors may be contributing. Greater awareness among athletes, trainers and physicians has likely increased diagnosis. At the same time, some experts point to post-pandemic returns to exercise and the growing popularity of endurance challenges and social media-driven fitness trends as possible contributors.

“We’re seeing cases of rhabdo reported in all sorts of physical activity,” says Tamara Hew-Butler, a sports performance researcher at Wayne State University. “It’s appearing in spinning classes, in football players, even swimming. Some extraordinary cases include a patient who spent a prolonged period gardening, and another who rather overzealously played the drums for hours on end”

Rhabdo has also been reported after high-intensity fitness regimes including CrossFit sessions, Murph-style challenges and HYROX. The issue is not necessarily that these formats are dangerous—for many participants they can be undertaken safely, provided training is built up gradually, and recovery between workouts is adequate. Risk arises because these programs often combine several recognized risk factors: highly motivated athletes, high training volumes, whole-body engagement, and overheating.

Still, rhabdomyolysis represents one extreme of a much larger biological question. Every workout pushes muscle toward adaptation, but every workout also carries the potential for damage. Scientists are now trying to define the biological tipping point where one becomes the other.

A spectrum of muscle damage

Not all muscle breakdown is bad news— “you need some breakdown in order to get better and stronger,” says Hew-Butler. Skeletal muscle is active tissue which constantly remodels during both exertion and rest. Exercise simply accelerates that progress.

Rather than being either normal or pathological, muscle damage exists on a spectrum. At one end lies the more familiar delayed-onset muscle soreness (DOMS)—the aches that often appear after unfamiliar exercise and usually resolve without treatment. At the other end lies rhabdomyolysis, where muscle breakdown becomes severe enough to threaten other organs.

So where is the line between the two?

The biomarker creatine kinase (CK), an enzyme released when muscle breaks down, is the key diagnostic marker for rhabdo. It helps clinicians gauge the degree of muscle damage, but it doesn’t draw a clear line between healthy muscle adaptation and pathology. CK levels can rise dramatically after exercise—without causing harm. In fact, some studies have found that CK levels up to 175 times the normal values can occur without rhabdo symptoms or complications.

Kruijt and colleagues recently led discussions to draw a clearer line. The group reached a consensus that the minimum CK level required to support an ER diagnosis should exceed 50 times the upper limit of normal. That threshold helps demonstrate the scale of muscle breakdown involved, but also exposes a major problem for researchers.

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“There’s enormous variation between responses to exercise in different individuals,” says Chris Gaffney, a senior lecturer in integrative physiology at Lancaster University. “This makes understanding what tips one individual from normal muscle break down into pathological very hard to determine.”

Rhabdo can prove difficult to spot too. The classical symptom triad—muscle weakness, pain and dark urine, often resembling black tea—doesn’t appear in every patient. And since some muscle pain and weakness after a workout is considered entirely normal, many athletes put their symptoms down to DOMS. Because symptoms of exertion are so often normalized, few individuals seek medical advice—meaning much of this spectrum can remain underrecognized.

Who’s really at risk?

Since no single factor determines whether someone develops rhabdo, researchers think it usually arises when several physiological stresses converge at once. Training load is one of the clearest examples. Stacked exercise days without enough rest, sudden leaps in intensity, and high-volume eccentric exercises involving the whole body can all push an individual beyond their limits.

That risk may be amplified by heat. “Both conditions share a high core temperature which might lead to muscle break down,” says Coen Bongers, a thermophysiologist from Radboudumc. “HYROX for instance, features a high intensity workout with both running and strength components.” In the wrong conditions—particularly heat, poor recovery, or inadequate pacing—the combination could create a perfect storm. This doesn’t apply only to outdoor events. Heat-exposed workouts happen indoors too, in poorly ventilated gyms, packed studios, spin classes, or endurance sessions with limited airflow.

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Whole-body exercise days or routines may also raise the risk, as they use larger volumes of muscle tissue. But individual muscle groups matter, too. “Biceps may be smaller muscles than quads, but can break down more easily,” says Hew-Butler.

Differences in risk between genders have recently blurred. “Original thought was that males were more susceptible than females, because of a larger muscle mass,” says Gaffney. “But newer data suggests this might not be true.” Women can also exercise at high intensities, pushing close to their own physiological limits—and have been historically under-represented in research, with older studies often failing to include enough female participants to assess their risk properly.

Individual susceptibility matters too, which might boil down to a genetic predisposition. “Patients who develop rhabdo more than once warrant testing to see if there is a genetic defect in their muscle tissue which makes it more likely to break down,” says Bongers.

Since there’s no established algorithm to predict whether any one subject will develop rhabdo, clinicians rely on known risk factors. Well-established contributors include unaccustomed intense exercise, heat stress, recent viral illness, and certain medications such as statins. Other possible contributors, including caffeine, alcohol, and some antidepressants—remain under scrutiny.

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Hydration appears to be an important variable too. “We’re clear that dehydration is a risk factor for rhabdo, but we’re now also concerned that overhydration might make it more likely,” says Hew-Butler. “Water-swollen muscle fibers are more unstable and break down more easily during exercise.”

Even so, scientists caution against overstating the danger. “Longitudinal intervention studies involving maximal eccentric exercise demonstrate little evidence of ER,” says Paul Greenhaff, a professor of muscle metabolism at Nottingham University. In other words, even hard exercise rarely causes rhabdo on its own. The danger seems more likely when several different stressors collide.

That makes prediction and prevention less about avoiding hard exercise altogether, but more about preparation, progression, and recognition of key warning signs.

“Exercise can be dangerous on rare occasions, but it’s also really good for you”, says Bongers. Gaffney agrees that adaptation is essential. “The important thing to remember is that your body’s physiology needs time to adapt.”