In August 2010, players at an Oregon high school football camp worked through an upper-arm drill, alternating dips and push-ups with no scheduled rest, in a wrestling room without air conditioning. Within one to three days, 22 of the 43 players had rhabdomyolysis, a breakdown of muscle tissue that can damage the kidneys. Twelve were hospitalized. Three developed compartment syndrome, a dangerous buildup of pressure inside a muscle.
Investigators from the Oregon Health Authority and the Centers for Disease Control and Prevention (CDC) asked each player to name the three hardest workers on the team. Among players who got at least one vote, 68% developed rhabdomyolysis. Among those who got none, 33% did.
Most sore muscles are nothing like that. The ache that makes the stairs miserable two days after a first spin class or a weekend of moving furniture is ordinary, and it fades. But ordinary soreness has a recognizable signature, and so does the dangerous kind. A third pattern runs the other way: muscle aches blamed on cholesterol-lowering statins that, in blinded trials, showed up nearly as often on dummy pills.
Why next-day soreness arrives late
Delayed-onset muscle soreness, usually shortened to DOMS, is the stiff, tender ache that follows exercise your muscles are not used to. A 2018 review led by Thilo Hotfiel of Friedrich-Alexander-University Erlangen-Nuremberg in Germany describes it as a mild injury to the muscle’s microscopic structure. The usual triggers are unfamiliar activity and eccentric work, meaning the muscle lengthens while under load: walking downhill, lowering a heavy box, the down phase of a squat.
The timing is the giveaway. The review puts peak soreness at 48 to 72 hours after the exercise. A September 2025 military clinical practice guideline on exertional rhabdomyolysis, from the Uniformed Services University and the Army Heat Center, agrees: DOMS usually appears within the first day and peaks around 72 hours. It can bring some temporary loss of strength, stiff and painful movement, and mild swelling. The exact chain of events is unknown, but the leading explanation is microscopic damage to muscle cells followed by local inflammation, which is why the pain trails the workout.
Lactic acid is not the cause
Many people still blame next-day soreness on lactic acid left in the muscle. A small 1983 experiment by Schwane and colleagues, published in The Physician and Sportsmedicine, tested that idea head on.
Volunteers ran for 45 minutes on a treadmill, either on the level or downhill at a 10% grade. Level running raised blood lactic acid but caused no significant soreness. Downhill running never raised lactic acid, yet it caused significant delayed soreness. The authors concluded that lactic acid is not related to the soreness.
It was one small study, but it fits the micro-damage explanation. A 2018 review in Cell Metabolism by George A. Brooks of the University of California, Berkeley, explains that lactate, once dismissed as a waste product, forms continuously even when oxygen is plentiful and serves as a major fuel. The lactic-acid explanation for next-day soreness therefore belongs among the claims this library calls tested and not supported.
Why the second time hurts less
Repeat the same unfamiliar workout a week later and it usually hurts far less. Researchers call this the repeated-bout effect. A 2003 review by Malachy P. McHugh of Lenox Hill Hospital in New York describes how a single bout of eccentric exercise protects a muscle against damage from later bouts. Nerve, mechanical and cellular changes may all contribute, and no single theory explains it yet.
A 1987 study led by Schwane, first author of the 1983 lactic-acid experiment, built the effect on purpose at the University of Texas at Tyler. One to two weeks of brief daily downhill running reduced, though did not eliminate, the soreness after a 45-minute downhill run. Uphill training did little.
That undercuts a popular belief. Soreness tracks novelty and lowering-type work, not simply effort: the 1983 level runners built up lactic acid and were not sore. None of this research treats soreness as a measure of how much good a workout did.
Ways to ease the ache, from stretching to ice baths, are weighed in the library’s recovery coverage rather than here.
When muscle breaks down: exertional rhabdomyolysis
Rhabdomyolysis means skeletal muscle cells break apart and spill their contents into the bloodstream. One of those contents, a protein called myoglobin, is toxic to the kidney’s filtering tubes and can lead to kidney failure, according to a 2026 surveillance report from the Defense Health Agency. Severity ranges from a symptom-free rise in muscle enzymes to emergencies such as kidney failure, irregular heart rhythms or compartment syndrome.
When exercise is the trigger, it is called exertional rhabdomyolysis. The 2025 military guideline diagnoses it when severe muscle symptoms come with a creatine kinase (CK) level of at least 5,000 units per liter after significant recent exercise. CK is an enzyme that leaks out of damaged muscle. The authors say it reflects their views rather than official Department of Defense policy, and a Uniformed Services University announcement noted that athletes, firefighters and first responders face similar risks.
The textbook picture is a triad of muscle pain, weakness and red-to-brown urine. The Defense Health Agency adds a caution: over half of patients do not have all three.
How often it happens
In 2025 the military recorded 521 cases among active-duty members, a rate of 39.7 per 100,000 person-years, and the share hospitalized hit a five-year high of 46.3%. Recruits had a rate more than six times that of other service members, and about three-quarters of cases came between April and September.
Civilian numbers are rougher. An analysis of national emergency department records led by Barry P. Boden, published in 2021, estimated 40,654 visits for exertional rhabdomyolysis from 2000 to 2019, with a 10-fold increase from the first decade to the second. That rise may partly reflect better recognition, and 65.3% of patients were hospitalized.
What the outbreaks have in common
- Oregon, 2010. The drill described above, in a hot room. Players rated hardest working were about twice as likely to get sick. Creatine supplement use was not associated with illness.
- Iowa. University of Iowa researchers reported in 2013 on a Division I football team assigned 100 back squats. Affected players’ first CK readings ran from 96,987 to 331,044 units per liter. Risk climbed with the time and number of sets a player needed to finish, and affected players were more likely to report going to muscle failure and doing extra squats.
- A Reserve Officer Training Corps program. In a 2018 report from Uniformed Services University researchers, 11 of 44 cadets were hospitalized after a mandatory timed workout. Better baseline fitness was not protective. Alcohol in the week before was linked to about four times the risk, and cadets knew their performance counted toward their ranking.
The guideline’s risk factors read like a description of those settings: intense, repetitive or prolonged exercise the body is not used to, excessive motivation, timed tests and coaches who push too hard. It adds heat and humidity, sickle cell trait, a recent viral illness, sleep deprivation, alcohol, stimulants including many supplements, and some prescription drugs, among them statins, antidepressants and antipsychotics. Sickle cell trait is carried by about 1 in 13 non-Hispanic Black Americans, the Defense Health Agency notes, and rates among Black service members run about double those of other groups.
So “no pain, no gain” has it backward in the cases that matter most: in two of these clusters, pushing harder went with more illness, and in the third the investigators pointed to pressure to perform for a ranking.
The guideline also tells clinicians to avoid nonsteroidal anti-inflammatory drugs (NSAIDs) in people with exertional rhabdomyolysis, noting that use before admission was tied to more kidney injury. What NSAID labels say more broadly is summarized in our piece on inflammation.
A high CK reading is not the whole answer
When University of Iowa researchers tested 32 football players during a preseason camp, reported in 2014, average CK climbed from about 285 units per liter on day one to about 1,562 on day seven. Those averages exceeded CK cutoffs often used in clinical practice to define rhabdomyolysis, yet every urine sample tested negative for blood. Hard training raises CK, which is why the guideline pairs its threshold with severe symptoms.
The National Institute for Occupational Safety and Health (NIOSH), part of the CDC, says only a blood test can confirm rhabdomyolysis, because dehydration and heat cramps can feel similar. It advises asking for repeated CK tests until two results in a row are falling, and it says a urine dipstick is not a good diagnostic tool. Symptoms can start hours or days after the injury.
Normal soreness or a warning sign: a side-by-side check
Our own sorting list, built from the military guideline and CDC guidance, puts urgent signs first.
Get emergency care now (call 911 or go to an emergency department):
- Dark urine, tea- or cola-colored, with muscle pain or weakness after exercise.
- A limb that is tense, swollen and very painful, especially if gently stretching the muscle makes it much worse, or if numbness or tingling sets in. The guideline lists these as warning signs of compartment syndrome.
- Severe muscle pain with little or no urine.
Get seen the same day:
- Pain far worse than the workout should have caused, or visible swelling of the muscle.
- Marked weakness, such as real trouble climbing stairs or lifting your arms, or being unable to finish a workout you normally can. NIOSH counts that last one as a symptom.
- Any of the above after a first-time, extreme, timed or hot-weather session, or in someone with sickle cell trait.
Call a clinician this week:
- Soreness that is not improving, or is getting worse, more than five to seven days after the activity. The military guideline lists persistence past that window as a feature of rhabdomyolysis, not of DOMS.
Usually ordinary:
- The ache starts within about a day of new or unaccustomed exercise, peaks around two to three days, and stays in the muscles you worked.
- The muscles feel stiff, tender and a little less strong, but daily life goes on.
- It fades steadily, and the same workout hurts less the next time.
Statin aches: where the blame often runs the wrong way
A 2019 scientific statement from the American Heart Association (AHA) said 1 in 4 Americans over 40 takes a statin. In US practice, it reported, roughly 10% of patients stop because of subjective complaints, most often muscle symptoms without a raised CK. In randomized trials, the difference in such symptoms between statin and placebo groups was less than 1%.
The most striking test came from Imperial College London. In the SAMSON trial, reported in full in the Journal of the American College of Cardiology in 2021, 60 people who had given up statins because of intolerable symptoms each received 12 monthly bottles: four held a statin, four held identical placebo tablets and four were empty. They rated their symptoms daily on a 0 to 100 scale.
Average scores were 8.0 in empty-bottle months, 15.4 on placebo and 16.3 on the statin, a statin-placebo gap that was not statistically significant. About 90% of the extra symptom burden from statin tablets also appeared with placebo tablets. The Imperial College team’s summary was blunt: “The majority of symptoms caused by statin tablets were nocebo.” Nocebo symptoms are produced by expecting harm, and they are real.
The details matter. SAMSON was small, tested one statin at one dose, enrolled only people whose symptoms began within two weeks of starting, and did not require blood tests. It counted symptoms of every kind, though muscle ache was the most common reason participants had quit. And the 90% figure pools everyone’s scores, a method an independent statistician recommended after the planned person-by-person calculation proved uninterpretable.
One finding goes straight at a common belief, that aches fading after stopping a statin prove the statin caused them. In SAMSON, scores fell by more than half within three days of stopping on 50% of occasions after statin months and 60% after placebo months. Because placebo produced the same pattern, the authors warned, neither the speed of onset nor the speed of relief shows the drug is responsible. Six months later, half the participants were back on a statin.
A second trial, StatinWISE, from the London School of Hygiene & Tropical Medicine and published in The BMJ in 2021, enrolled 200 people at 50 UK primary care sites who had stopped or were considering stopping a statin over muscle symptoms. Across blinded periods of statin and placebo, muscle symptom scores were essentially identical, and withdrawals for intolerable muscle symptoms were 9% on statin and 7% on placebo.
Statins do cause some muscle symptoms
The opposite belief, that statins never cause muscle problems, is also wrong. The Cholesterol Treatment Trialists’ Collaboration pooled individual data from 19 double-blind trials with 123,940 participants, published in The Lancet in 2022. Over a median of 4.3 years, 27.1% of people on a statin and 26.6% on placebo reported muscle pain or weakness.
In the first year, statins caused a 7% relative increase, about 11 extra reports for every 1,000 people followed for a year. So in that year only about 1 in 15 muscle complaints in the statin groups was due to the drug, and afterward there was no significant excess. More intensive regimens carried somewhat higher risk, and the extra pain was mostly mild. On the library’s scale, the harm is measured and real, but small.
Serious muscle injury is rare but documented. The AHA statement put the risk of serious statin-induced muscle injury, including rhabdomyolysis, below 0.1%. The current prescribing information for atorvastatin, the label approved by the Food and Drug Administration, warns of myopathy, meaning muscle pain, tenderness or weakness with a raised CK, and of rhabdomyolysis, with kidney injury and rare deaths. Risk is higher with age 65 or older, uncontrolled hypothyroidism, kidney impairment, certain other drugs and higher doses, and the label says more than 1.2 liters of grapefruit juice a day is not recommended with atorvastatin. It also describes a rare autoimmune muscle disease linked to statins that persists after the drug is stopped, a condition that belongs to our article on autoimmune disorders.
The FDA-approved label’s instruction to prescribers is plain: “Instruct patients to promptly report any unexplained muscle pain, tenderness or weakness, particularly if accompanied by malaise or fever.”
That is the routing the evidence supports. Whether an ache comes from the statin, the nocebo effect, a hard workout or something else, and whether any change to the prescription makes sense, is a question for the prescriber, who can check CK, review other medicines and weigh heart risk. The Lancet authors concluded that the small muscle risks are much lower than the known cardiovascular benefits.
Aches that come with a virus
Muscle and body aches are a standard flu symptom, according to the CDC. Less often, the agency says, flu can inflame muscle tissue directly, causing myositis or rhabdomyolysis. Its adult emergency warning signs include severe muscle pain, severe weakness or unsteadiness, and not urinating.
Children have their own version. A 2018 systematic review led by Gioele Capoferri, covering 451 patients in 72 published reports, described sporadic acute benign calf myositis, sudden pain and tenderness in the calves. Almost all patients were 18 or younger, 91% had a flu-like illness first, pain lasted a week and a half or less, and the kidneys were never involved. The review calls it self-limited. Even so, the CDC lists severe muscle pain in which a child refuses to walk among its emergency warning signs, and telling the benign version from something serious is a clinician’s job.
Three nearby patterns belong elsewhere. Aching and stiffness in both shoulders and hips in someone over 50, worst in the morning, is a pattern clinicians check for polymyalgia rheumatica, which our joint stiffness article explains. Symptoms that linger for months after an infection are the subject of the post-infectious syndromes article. And in ME/CFS, a crash after modest exertion, known as post-exertional malaise, is a different phenomenon from DOMS, one the ME/CFS article explains.
The takeaway
Judge muscle pain by its timing and its proportion. Ordinary soreness follows new or downhill-type work, peaks within about three days, fades, and is milder the next time. Pain out of proportion to the workout, swelling, marked weakness, dark urine, or soreness still getting worse five to seven days later breaks that pattern and needs medical care, urgently for dark urine or a tense, swollen limb.
If you take a statin, the label’s instruction is the useful one: report unexplained muscle pain, tenderness or weakness to the prescriber, especially with fever or feeling unwell. What caused it, and what happens to the prescription, is the prescriber’s decision to make with you.
Sources
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