Muscle pain, medically termed myalgia, is pain perceived in muscle tissue or the surrounding tissues and can arise from exercise, injury, infection, medication effects, metabolic abnormalities, inflammatory muscle disease or broader pain-processing disorders. It may be localised to one muscle or widespread throughout the body. The underlying mechanism may involve direct tissue injury, accumulation or alteration of signalling molecules associated with exercise, inflammation, nerve dysfunction or changes in how the nervous system processes pain. Muscle pain should be distinguished from pain originating in tendons, joints, nerves or fascia, although these structures are anatomically interconnected and symptoms can overlap. The timing of pain, relationship to exertion, weakness, swelling, fever, medication use and distribution provide important clues about its cause.
Exercise is one of the most common causes of temporary muscle pain. Unaccustomed or strenuous exercise can produce delayed-onset muscle soreness, commonly abbreviated DOMS. DOMS usually develops several hours after unfamiliar or intense exercise and is often most noticeable one to three days later before gradually improving. It is associated particularly with activities involving eccentric muscle contractions, in which muscles produce force while lengthening, such as lowering a weight or descending stairs. Microscopic changes and inflammatory signalling contribute to the soreness, but the old explanation that DOMS is simply caused by lactic-acid accumulation is incorrect. Lactate is cleared relatively quickly and is not the primary cause of soreness experienced days after exercise.
Acute muscle strains occur when muscle fibres and associated connective tissues are overloaded, producing varying degrees of tissue injury. They commonly occur during sprinting, jumping, sudden acceleration, lifting or other high-force movements. Symptoms can include sudden pain, tenderness, weakness, swelling or bruising. Severity ranges from minor microscopic damage to substantial tears. Recovery depends on the extent of injury, location and individual factors. Repeated loading without adequate recovery can also contribute to persistent symptoms. Differentiating a muscle strain from tendon injury, ligament injury or fracture can be difficult without examination, particularly around complex regions such as the groin, hamstring and shoulder.
Infections can cause widespread or localised myalgia. Viral infections such as influenza and various other systemic infections commonly produce muscle aches through inflammatory and immune responses rather than direct destruction of every painful muscle. Certain bacterial or parasitic infections can directly affect muscle tissue, although these are less common. Fever, profound fatigue, weakness and other systemic symptoms help distinguish infection-associated muscle pain from ordinary exercise soreness. Muscle pain can also accompany autoimmune inflammatory myopathies, in which immune-mediated muscle injury produces weakness that is often more prominent than pain. Conditions such as polymyositis, dermatomyositis and immune-mediated necrotising myopathy require medical evaluation and laboratory assessment.
Medicines and metabolic disturbances can produce myalgia. Statins, for example, can cause muscle symptoms in a minority of users, ranging from aches to, rarely, serious muscle injury. Other medicines and drug interactions can also affect muscle. Electrolyte abnormalities, thyroid disorders and deficiencies or excesses involving certain nutrients can contribute to muscle symptoms. Severe muscle injury can cause rhabdomyolysis, in which damaged muscle releases intracellular contents such as creatine kinase and myoglobin into the bloodstream. Rhabdomyolysis can occur after extreme exercise, trauma, certain drugs or other insults and can lead to acute kidney injury, making severe muscle pain accompanied by marked weakness, swelling or dark urine a reason for urgent medical assessment.
Nerve disorders can produce symptoms perceived as muscle pain, particularly when nerve roots or peripheral nerves are irritated. Radiculopathy can cause pain radiating into a limb together with tingling, numbness or weakness, while peripheral neuropathies can produce burning or electric sensations. Fibromyalgia involves widespread chronic pain and altered pain processing, often accompanied by fatigue, sleep disturbance and cognitive symptoms; it is not simply evidence of damaged muscles. Myofascial pain can involve muscles and surrounding connective tissues and may be influenced by mechanical loading and nervous-system sensitivity. These distinctions matter because treating presumed muscle inflammation when the primary mechanism is neurological or nociplastic may be ineffective.
The distribution and behaviour of muscle pain help identify its likely origin. Pain limited to a muscle after an obvious activity change is more suggestive of local overload or injury, whereas widespread pain without a clear exertional trigger may point towards infection, medication effects, systemic disease or altered pain processing. Objective muscle weakness is particularly important because it differs from pain-limited effort.
Progressive weakness, difficulty climbing stairs or rising from a chair, swallowing or breathing problems, persistent unexplained symptoms, fever, rash or dark urine may require medical investigation. Clinicians may use examination, blood tests including creatine kinase, medication review, thyroid and nutritional tests, imaging or electromyography depending on the suspected cause.
Management depends on the mechanism. Ordinary post-exercise soreness generally improves with time and gradual return to activity, whereas significant muscle strains require appropriately staged rehabilitation. Inflammatory myopathies require specialist treatment aimed at suppressing the underlying immune process. Medication-associated symptoms may improve after a clinician reviews the drug or dose, but medicines should not be stopped without appropriate advice. Rhabdomyolysis requires urgent assessment and treatment. Muscle pain is therefore a broad symptom with many possible origins, and the key distinction is between expected short-lived adaptation to physical activity, structural injury, systemic illness, inflammatory disease, medication effects, neurological pain and serious muscle breakdown.
