What really causes muscle soreness after exercise?

By Sina Hartung, MMSC-BMI, Harvard Medical SchoolReviewed by Eureka Health Medical Group

Published: July 22, 2025Updated: July 22, 2025

Key Takeaways

Most next-day muscle soreness is delayed-onset muscle soreness (DOMS). Tiny tears in muscle fibers, local inflammation, and fluid shifts trigger pain sensors 12–36 hours after unfamiliar or intense exercise. Eccentric contractions—lowering a weight, running downhill—stretch fibers while they’re contracting, causing the most micro-damage. Soreness peaks at 48 hours, fades by day seven, and improves as muscles adapt.

Why do my muscles ache after a workout?

The most common cause is delayed-onset muscle soreness (DOMS), a normal repair response. Mechanical stress from eccentric contractions creates microscopic tears; your immune system rushes in to heal, producing inflammation and tenderness.

Eccentric movements strain fibers the mostLowering into a squat or slowly lowering a dumbbell can double the tension on muscle fibers compared with lifting the same weight, leading to more micro-tears.
Inflammation sensitizes pain receptorsWhite blood cells release prostaglandins and histamine; these chemicals irritate nerve endings, making ordinary movement feel sore.
Fluid shifts create pressureAfter hard exercise, intracellular fluid moves into the muscle belly, increasing its volume by up to 10 %. The resulting pressure stretches pain receptors.
Muscle repair is an adaptive signalSatellite cells fuse to damaged fibers, laying down new contractile proteins; this adaptation is why repeated workouts hurt less over time.
Soreness typically starts within a day and peaks by 72 hoursDOMS usually begins 12–24 hours after exercise and is most intense between the second and third day, explaining why the ‘second-day’ ache is often the worst. (Healthline)
Aches fade naturally in two to five daysMost post-workout muscle pain eases without treatment, subsiding in roughly 2–5 days as the healing process completes. (NHS)

When is post-workout pain a red flag rather than normal soreness?

Severe or strangely located pain can point to injury or a medical emergency. Knowing the difference keeps you out of trouble.

Sharp, immediate pain suggests a tearIf pain snapped in at the moment of exercise and you heard a pop, think tendon rupture rather than DOMS.
Swelling that grows by the hour signals compartment syndromeA calf that becomes rock-hard and numb within 6 hours needs same-day evaluation; untreated acute compartment syndrome can cause permanent damage.
Dark cola-colored urine hints at rhabdomyolysisWhen muscle cells break apart massively, myoglobin spills into urine; this can stress kidneys and needs ER-level labs.
Fever over 100.4 °F is not normal DOMSSystemic infection or flu masquerading as post-workout soreness is possible and requires medical assessment.
Soreness that lingers beyond a week raises concernDOMS usually peaks 24–72 hours after exercise and resolves within 2–5 days; pain that sticks around for 7–10 days or more may signal a strain or other injury that warrants evaluation. (BCBSM)
Pain centered in a joint, especially with redness or swelling, is not typical DOMSDiscomfort focused on a joint rather than muscle tissue can reflect ligament, cartilage, or bone damage and should be checked by a clinician instead of pushed through your next workout. (HFH)

How long should normal DOMS last before I worry?

Typical soreness appears 12–36 h after exercise, peaks at 48 h, and resolves within a week. Anything longer or worsening may be an injury.

Resolution by day seven is the ruleStudies in trail runners show 90 % report full recovery of strength by day seven.
Pain intensity should decline dailyRate your pain 0–10; a drop of at least 1 point per day is expected.
Symmetry mattersDOMS usually affects both sides; one-sided pain after a barbell row could be a strained lat muscle.
Peak tenderness strikes by day two or threeMedicineNet notes DOMS pain generally reaches its maximum within 12–72 h; if pain is still intensifying after 3 days, consider evaluation for injury. (MedNet)
Soreness that starts 12–24 h after exercise is typicalMedical News Today explains that normal DOMS begins the day after a workout, whereas pain felt immediately often signals an acute strain. (MNT)

What self-care eases normal post-exercise soreness fastest?

Simple habits can shorten recovery by up to 30 %.

Active recovery boosts blood flowCycling at 40 % of max heart rate for 20 minutes the day after leg day reduced soreness scores by 28 % in a 2022 meta-analysis.
Contrast showers may flush metabolitesAlternating 1 min cold (60 °F) and 1 min warm (100 °F) for 10 cycles lowered perceived soreness in rugby players.
Protein within 2 hours supports repairAim for 20–40 g of high-leucine protein like whey; muscle protein synthesis peaks when amino acids are abundant.
Gentle stretching restores lengthHolding a hamstring stretch for 30 s, 3 sets, reduces stiffness without impeding strength gains.
Quote from the team at Eureka Health“Most athletes recover quicker when they treat low-grade soreness as a sign to move, not to rest completely,” say the doctors at Eureka Health.
Foam rolling 30–60 s per muscle group cuts stiffnessUK HealthCare recommends sweeping a foam roller along large muscle groups for 30–60 seconds before and after training; regular use lessens tightness and speeds return to full range of motion. (UKHC)
Hydrating with an extra 16–32 oz water aids recoveryAdding about 500–1,000 mL of fluid after a tough session supports circulation and metabolite clearance, which the Kane Footwear guide lists as a key step to ease next-day soreness. (Kane)

Which lab tests and medications matter for serious muscle damage?

Lab work isn’t needed for routine DOMS but becomes critical if red flags appear.

Creatine kinase (CK) above 5,000 IU/L indicates rhabdomyolysisEmergency physicians use CK thresholds to decide on IV fluids to protect kidneys.
Serum creatinine tracks kidney stressA rise of 0.3 mg/dL within 48 h suggests acute kidney injury from muscle breakdown.
NSAIDs can ease pain but slow repairResearch shows taking 400 mg ibuprofen every 8 h cut soreness by 20 % but delayed satellite-cell activity; ask a clinician before use.
Topical menthol creams have fewer systemic risksA 4 % menthol gel reduced perceived soreness by 23 % in a randomized trial without affecting muscle protein synthesis.
Quote from Sina Hartung, MMSC-BMI“Lab confirmation guides whether we hydrate aggressively or monitor at home; guessing based on symptoms alone isn’t safe,” notes Sina Hartung.
Urine myoglobin detects muscle breakdown before creatinine risesBrigham and Women’s notes that a positive urinary myoglobin test should prompt rapid IV hydration because pigment can obstruct renal tubules even when blood creatinine is still normal. (BWH)
Statin-associated rhabdomyolysis affects roughly 1.5 in 100,000 patientsMayo Clinic advises stopping the drug and obtaining an urgent CK level if severe aches or cola-colored urine develop after starting a statin, since this rare complication can progress quickly. (Mayo)

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