Myalgia is the medical term for muscle pain, and it ranks among the most common physical complaints people bring to a doctor. The causes range from a hard workout to a viral infection to a prescription medication side effect, which is exactly why the symptom can feel confusing. A pulled muscle after weekend gardening and the all-over ache of the flu are both technically myalgia, yet they arise through completely different mechanisms and call for different responses.
Exercise-Related Muscle Pain and the Lactic Acid Myth
The most familiar form of myalgia is the soreness that sets in a day or two after unfamiliar physical activity, usually called delayed-onset muscle soreness (DOMS). This happens primarily after eccentric contractions, the type where a muscle lengthens under load, like lowering a heavy box or walking downhill. That kind of movement causes tiny tears in muscle fibers and surrounding connective tissue. The body responds with inflammation, fluid shifts, and immune-cell activity that collectively produce the stiffness and tenderness you feel roughly 24 to 72 hours later.1PubMed. Muscle soreness and delayed-onset muscle soreness The process closely mirrors classic acute inflammation, complete with swelling, loss of function, and infiltration of immune cells like macrophages.2PubMed. Acute inflammation: the underlying mechanism in delayed onset muscle soreness?
One of the most persistent misconceptions about muscle soreness is that lactic acid causes it. Research debunked this idea decades ago. In a classic experiment, runners on flat ground had high lactic acid levels but no significant delayed soreness, while runners going downhill had no lactic acid elevation yet experienced substantial soreness afterward.3PubMed. Is Lactic Acid Related to Delayed-Onset Muscle Soreness? Lactic acid clears from muscles within an hour or so of stopping exercise, which is far too early to explain pain that peaks a day or two later. That said, more recent work suggests lactate may not be entirely irrelevant. Within the muscle’s internal sensory structures, lactate could play a role in triggering the initial damage phase by interacting with pain-signaling molecules like bradykinin.4PubMed Central. Should We Void Lactate in the Pathophysiology of Delayed Onset Muscle Soreness? Not So Fast! Let’s See a Neurocentric View! So the old “lactic acid buildup” explanation is wrong as traditionally stated, but lactate’s story is more nuanced than a clean dismissal.
Infections That Make Your Muscles Ache
Almost everyone has experienced the deep, all-over muscle ache that accompanies a bad case of the flu. Viral infections are one of the most common causes of widespread myalgia. The pain is not always from the virus directly attacking muscle tissue. In many cases, the immune system’s own inflammatory response does most of the damage. Circulating cytokines, the signaling molecules that coordinate your immune response, can sensitize pain receptors throughout the body. In severe infections like COVID-19, a surge of cytokines including interleukin-6 can act on muscle tissue and trigger pain by stimulating production of prostaglandin E2, the same compound involved in fever and inflammation elsewhere in the body.5Frontiers in Pain Research. A Review: The Manifestations, Mechanisms, and Treatments of Musculoskeletal Pain in Patients With COVID-19
Direct viral invasion of muscle cells is another possible mechanism. Influenza, for example, can occasionally cause a true viral myositis, an inflammatory condition in the muscle itself. In rare cases this progresses to rhabdomyolysis, where muscle tissue breaks down rapidly and releases its contents into the bloodstream. The proposed explanations include direct infection of muscle cells, the cytokine storm from the immune response, and toxicity from circulating viral factors.6MedLink Neurology. Viral and retroviral myositis For the average person with a cold or flu, myalgia resolves as the infection clears. But if muscle pain is severe, the urine turns dark, or weakness develops, those are signals to seek medical attention quickly, since rhabdomyolysis can damage the kidneys.
When Medications Are the Culprit
Statin drugs, prescribed to lower cholesterol, are probably the best-known medication-related cause of myalgia. Muscle pain affects a meaningful minority of people on statins, and the mechanism appears to involve changes in how muscle cells produce energy. Research on simvastatin-treated patients found that coenzyme Q10 content in muscle was reduced and the maximum capacity of mitochondria to generate energy was decreased compared with controls.7PubMed. Simvastatin effects on skeletal muscle: relation to decreased mitochondrial function and glucose intolerance A separate study confirmed that patients taking statins who developed muscle pain showed lower mitochondrial respiration compared to both statin users without symptoms and non-users, though coenzyme Q10 levels in muscle were not significantly different between groups in that particular investigation.8The Journal of Clinical Endocrinology & Metabolism. Statin Treatment Decreases Mitochondrial Respiration But Muscle Coenzyme Q10 Levels Are Unaltered: The LIFESTAT Study The picture is complicated, and some researchers suspect that a portion of statin-related muscle complaints are amplified by the nocebo effect, where expecting side effects makes you more likely to experience them. Still, the mitochondrial changes are real and measurable.
Statins are not the only medications to watch. Fluoroquinolone antibiotics, a class that includes ciprofloxacin and levofloxacin, are best known for tendon problems but can also cause myalgia, joint pain, and peripheral neuropathy.9PubMed Central. The Risk of Fluoroquinolone-induced Tendinopathy and Tendon Rupture: What Does The Clinician Need To Know? If you develop unexplained muscle pain after starting a new medication, mentioning it to your prescriber is always worth doing. Drug-induced myalgia usually resolves after stopping the offending agent, though recovery time varies.
Nutritional Deficiencies and Hormonal Causes
Vitamin D deficiency is remarkably common and can produce a vague, widespread muscle pain that is easy to dismiss or misattribute. Low vitamin D is associated with proximal weakness, the kind that makes it hard to climb stairs or rise from a chair, along with an increased risk of falls.10PubMed Central. Vitamin D and muscle Because the signs are so nonspecific, vitamin D deficiency often goes unrecognized. A disruption anywhere in the vitamin D pathway can lead to bone pain, muscle weakness, and fractures.11PubMed Central. Recognizing the musculoskeletal manifestations of vitamin D deficiency If your muscle pain has been dragging on for weeks without an obvious cause, asking for a simple blood test to check your vitamin D level is a reasonable step.
Thyroid disorders, particularly hypothyroidism, can also cause myalgia. An underactive thyroid slows metabolism throughout the body, and muscles feel stiff, achy, and weak as a result. Magnesium deficiency is another underappreciated contributor. Magnesium plays a role in muscle contraction and relaxation, and low levels can produce cramps and soreness. A systematic review of magnesium supplementation studies found that supplementation reduced muscle soreness, improved recovery, and offered a protective effect against muscle damage across different types of physical activity.12PubMed Central. Effects of magnesium supplementation on muscle soreness in different type of physical activities: a systematic review
Chronic Myalgia and When Pain Rewires the Nervous System
Sometimes myalgia does not go away. When muscle pain persists for months without a clear ongoing injury or disease, the problem may have shifted from the muscles themselves to the nervous system. Fibromyalgia is the most widely recognized example. Research supports the idea that fibromyalgia involves a long-standing change in how the pain-processing nervous system functions, effectively amplifying pain signals that would not normally register as painful.13PubMed. Pathophysiological mechanisms in chronic musculoskeletal pain (fibromyalgia): the role of central and peripheral sensitization and pain disinhibition Fibromyalgia may represent the far end of a spectrum that starts with localized chronic pain and progresses to widespread disabling pain.
An important finding is that even in fibromyalgia, where the brain’s pain processing is clearly altered, peripheral input from the muscles still matters. When researchers injected lidocaine, a local anesthetic, into painful muscles of fibromyalgia patients, not only did local pain thresholds increase, but pain sensitivity at remote body sites decreased too.14PubMed Central. Enhanced central pain processing of fibromyalgia patients is maintained by muscle afferent input: a randomized, double-blind, placebo-controlled study This suggests that ongoing signals from the muscles help maintain the heightened pain state, which has practical implications: treating local sources of pain may help even when the problem has partly “gone central.”
Myofascial pain syndrome is a related but distinct condition characterized by painful taut bands of muscle containing hypersensitive spots called trigger points. These trigger points can refer pain to distant areas of the body, which makes them tricky to diagnose. The underlying mechanism likely involves a complex interaction of cellular signaling, neuromuscular inputs, local blood flow, and energy metabolism within the muscle.15PubMed Central. Mechanisms of Myofascial Pain
Stress, Posture, and Occupational Myalgia
Muscle pain does not always require a dramatic trigger. Repetitive low-level muscle use, the kind that comes from sitting at a desk for hours with poor posture, can produce trigger points and chronic myalgia. Sustained or repetitive low-intensity contractions can overload muscles when use exceeds capacity and normal recovery is disrupted.16PubMed Central. Etiology of myofascial trigger points Research on work-related myalgia found that affected workers showed an inability to relax the painful trapezius muscle during repetitive movements, a pattern that could both initiate and sustain the pain.17PubMed. Muscle performance, electromyography and fibre type composition in fibromyalgia and work-related myalgia
Psychological stress compounds the problem. A study of myofascial pain patients found that during periods of high stress, such as academic examinations, muscle tenderness increased significantly and pain scores rose, regardless of whether participants already had a preexisting pain condition.18PubMed. The impact of stress and anxiety on the pressure pain threshold of myofascial pain patients Stress does not just make you notice pain more; it appears to physically lower the threshold at which pressure on muscles becomes painful.
When to Investigate Further
Most myalgia is self-limiting and does not require extensive workup. But certain patterns warrant a closer look. If muscle pain comes on during exertion, is accompanied by dark urine (a sign of muscle breakdown), or involves actual weakness rather than just soreness, the differential diagnosis broadens to include metabolic myopathies and other conditions that need specific testing. McArdle disease, for instance, is a rare inherited condition where the body cannot break down glycogen in skeletal muscle, leading to exercise intolerance, cramps, and episodic weakness.19PubMed Central. McArdle Disease: Insights Into a Rare Metabolic Myopathy in a Young Boy With Recurrent Exercise-Induced Muscle Weakness
Creatine kinase (CK) is the blood test most commonly used to assess muscle damage. After intense exercise, CK levels can rise dramatically even in healthy people. Research on military recruits found that widely accepted laboratory thresholds for rhabdomyolysis were routinely exceeded, suggesting that CK levels need to be more than 50 times the upper limit of normal to reliably indicate exertional rhabdomyolysis rather than a normal post-exercise response.20PubMed. Serum creatine kinase after exercise: drawing the line between physiological response and exertional rhabdomyolysis Context matters enormously when interpreting CK results.
For chronic unexplained myalgia, electromyography (EMG) turns out to be a strong screening tool. A prospective study of patients with chronic diffuse muscle pain and no obvious clinical abnormalities found that EMG was excellent for detecting underlying muscle disease, while muscle biopsy added little diagnostic value in that population.21PubMed. Electromyography and muscle biopsy in chronic isolated Myalgia: A prospective study Biopsy becomes more useful when specific red flags are present: exertional pain with dark urine, a “second wind” phenomenon where exercise gets easier after an initial struggle, actual weakness, muscle wasting or enlargement, significantly elevated CK, or abnormal EMG findings.22PubMed. EFNS review on the role of muscle biopsy in the investigation of myalgia
It is also worth noting that conditions can masquerade as one another. A case series described elderly patients initially diagnosed and treated for polymyalgia rheumatica, a relatively common inflammatory condition, who actually had polymyositis, a different disease requiring different treatment. Elevated creatine kinase prompted muscle biopsies that corrected the diagnosis.23PubMed Central. Polymyositis, not polymyalgia rheumatica The takeaway is that when myalgia does not follow the expected pattern, or does not respond to initial treatment, revisiting the diagnosis is reasonable.
Managing and Treating Muscle Pain
For acute myalgia, especially exercise-related soreness, the evidence supports a few straightforward approaches. Heat and cold therapy both reduce DOMS pain when applied within an hour of exercise. A meta-analysis of 32 randomized trials found that cold water immersion reduced pain in the first 24 hours, while hot packs had even stronger effects and continued to reduce pain beyond 24 hours as well.24PubMed. Heat and cold therapy reduce pain in patients with delayed onset muscle soreness: A systematic review and meta-analysis of 32 randomized controlled trials If you have access to both, heat appears to edge out cold for DOMS specifically.
Foam rolling has become popular as a recovery tool, but the effects are modest. A meta-analysis found that foam rolling slightly reduced muscle pain perception and modestly preserved sprint and strength performance after exercise, though the overall effects were described as minor and partly negligible.25PubMed Central. A Meta-Analysis of the Effects of Foam Rolling on Performance and Recovery Foam rolling is not harmful and some people find it helpful, but expectations should be calibrated accordingly.
For more persistent pain, anti-inflammatory medications remain a first-line option. A study of acute low back pain found that combining a muscle relaxant with an anti-inflammatory drug produced greater pain reduction than the anti-inflammatory alone.26PubMed Central. Efficacy and Safety of Combination of NSAIDs and Muscle Relaxants in the Management of Acute Low Back Pain For chronic myalgia conditions like fibromyalgia, mindfulness-based approaches have shown promise. A study comparing mindfulness training with cognitive behavioral therapy found that mindfulness produced greater reductions in daily pain catastrophizing, morning disability, and fatigue.27PubMed Central. Mindfulness and Cognitive-behavioral Interventions for Chronic Pain: Differential Effects on Daily Pain Reactivity and Stress Reactivity
Supplements for Muscle Soreness
Omega-3 fatty acids have attracted attention as a supplement for reducing exercise-induced muscle pain. A trial of omega-3 supplementation for four weeks before a high-intensity cycling workout found that the supplemented group had lower CK levels, reduced calf pain, and faster strength recovery compared to controls.28PubMed Central. Effects of Omega-3 Supplementation on the Delayed Onset Muscle Soreness after Cycling High Intensity Interval Training in Overweight or Obese Males A separate trial using a marine oil blend derived from green-lipped mussels, which is rich in omega-3s, showed reduced markers of muscle damage and inflammation at multiple time points after a muscle-damaging exercise protocol, along with less soreness at 72 and 96 hours post-exercise.29PubMed Central. The effects PCSO-524®, a patented marine oil lipid and omega-3 PUFA blend derived from the New Zealand green lipped mussel (Perna canaliculus), on indirect markers of muscle damage and inflammation after muscle damaging exercise in untrained men: a randomized, placebo controlled trial These are promising findings, though both studies were relatively small, and the practical size of the benefit for a typical person remains an open question.
Muscle Pain in Children and Adolescents
Myalgia in kids deserves its own mention because parents often worry about it and the most common cause is entirely benign. “Growing pains” affect a substantial proportion of children, typically presenting as bilateral lower-extremity pain in the evening or at night. The pain is not triggered by activity and does not cause a limp. When the history fits this classic pattern and the physical exam is normal, blood tests and imaging are not needed.30PubMed Central. Growing Pains: When to Be Concerned Red flags that warrant further evaluation include pain that is one-sided, present during the day, associated with limping, or accompanied by fever, swelling, or weight loss. Those patterns suggest something other than growing pains and should prompt a visit to the pediatrician.