The intense-workout trend has a real if uncommon downside: rhabdomyolysis
A review explains why HIIT and CrossFit-style training can push exercise-induced muscle breakdown to the point of kidney injury, and what helps catch it earlier.
High-intensity interval training, CrossFit-style workouts, and other short, maximal-effort training formats have become some of the most widely practiced ways to build strength, speed, and endurance in both competitive and recreational sport. A narrative review of the current evidence lays out the mechanism behind a less-discussed downside of that intensity: a real, if generally uncommon, risk of muscle breakdown severe enough to injure the kidneys [s1].
What rhabdomyolysis actually is
Rhabdomyolysis is the breakdown of skeletal muscle tissue severe enough to release muscle cell contents — including a protein called myoglobin — into the bloodstream [s1]. Ordinary hard exercise causes some degree of muscle breakdown as a normal part of training adaptation; rhabdomyolysis describes a more extreme version of that same process, where the volume of breakdown products released overwhelms the body's normal clearance mechanisms [s1].
The review identifies the specific downstream danger: when myoglobin and related muscle breakdown products flood the bloodstream faster than the kidneys can filter them, the result can be acute kidney injury (AKI) [s1]. In severe cases, that kidney injury is serious enough to require hospitalization and renal replacement therapy — dialysis — and can limit an athlete's ability to return to sport [s1].
Why HIIT and CrossFit-style training specifically raise the risk
The review points to a specific structural feature of these training formats as the driver: they're explicitly designed to develop strength, speed, and endurance within a compressed, high-intensity window, which is precisely the combination — unfamiliar or excessive intensity and volume — most associated with triggering exertional rhabdomyolysis [s1]. This isn't a criticism unique to any single branded program; the mechanism the review describes applies to any training format that pushes muscle exertion sharply past what an individual's muscles are conditioned to handle, whether that's a new participant's first CrossFit class or an experienced athlete adding an unfamiliar new movement at high volume.
What's changing: better biomarkers
The review's specific contribution is less about the basic mechanism, which has been documented for decades, and more about emerging tools for catching it earlier. It surveys newer biomarkers of kidney injury beyond the traditional blood tests, and argues that tracking these markers after exercise could offer better insight into the dynamics of post-exercise change, help indicate the severity and location of exercise-induced kidney damage, and ultimately support earlier prevention of exertional kidney injuries before they progress to a severe case [s1].
What this means for someone starting an intense training program
The review's framing is explanatory rather than alarmist: rhabdomyolysis is a known, mechanistically well-understood risk of high-intensity training, not evidence that HIIT or CrossFit-style workouts are broadly unsafe for the large number of people who do them without incident. The practical implication sits with intensity and progression — the risk described in the review is tied specifically to exertion that exceeds what a person's muscles are currently conditioned to handle, which is a describable, if not always predictable, threshold rather than a fixed property of any particular workout format. This review does not offer dosing guidance or training prescriptions, and none should be inferred from it; it is a summary of a mechanism and an emerging diagnostic approach, not a set of instructions for how hard to train.
Sources
- Risk of Rhabdomyolysis and Kidney Injury after Intensive Exercise. Potential of Novel Biomarkers of Kidney Injury: A Narrative Review — Journal of Human Kinetics, 2026-02-01
Sources
- Risk of Rhabdomyolysis and Kidney Injury after Intensive Exercise. Potential of Novel Biomarkers of Kidney Injury: A Narrative Review — Journal of Human Kinetics , February 1, 2026
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