Muscle pain has dozens of possible causes, but most cases trace back to one of a handful of common triggers: physical overexertion, viral illness, mineral deficiencies, or chronic pain conditions. The explanation depends on whether your pain started after activity, came on during an illness, or appeared without an obvious reason. Here’s what’s actually happening inside your body for each scenario.
Soreness After Exercise
The most common reason for muscle pain is delayed onset muscle soreness, or DOMS, which typically peaks 24 to 72 hours after unfamiliar or intense physical activity. For decades, researchers assumed this was caused by tiny tears in muscle fibers followed by inflammation. That picture turns out to be incomplete. Studies in animal models have found mechanical soreness developing one to three days after exercise with no visible microscopic damage to the muscle and no signs of inflammation at all.
What’s actually driving the pain appears to be two signaling pathways triggered by the muscle fibers themselves. After strenuous contractions (especially eccentric ones, like lowering a heavy weight or walking downhill), muscle cells and the satellite cells surrounding them release nerve growth factor and glial cell line-derived neurotrophic factor. These substances sensitize the nerve endings in and around the muscle, making normal pressure and movement register as pain. In other words, muscle fiber damage is sufficient to cause soreness, but it isn’t required. The nerve-sensitizing chemicals your muscles produce are the real culprits.
One persistent myth is that lactic acid causes the ache you feel the day after a workout. Lactic acid is cleared from your muscles almost immediately once you stop exercising; your blood levels return to normal within minutes. It plays no role in next-day soreness.
Heat, Cold, and Recovery for Sore Muscles
A meta-analysis of 32 randomized controlled trials involving over 1,000 participants found that both cold and heat therapy reduce exercise-related muscle pain, with no significant difference between the two. The timing matters, though. Cold therapy (particularly cold water immersion) applied within an hour of exercise reduced pain during the first 24 hours but had no clear benefit beyond that window. Heat therapy, especially hot packs, reduced pain both within the first 24 hours and in the days that followed. If your soreness is already a day old, heat is the better bet.
Mild DOMS resolves on its own within three to five days. If you strained the muscle more seriously, recovery depends on severity. A moderate strain with partial tearing typically heals in two to three weeks, with a return to full activity around one month. A complete muscle tear requiring surgical repair can take three to four months of rehabilitation.
Muscle Pain During Illness
That full-body ache you feel with the flu or COVID-19 isn’t just “feeling sick.” Viruses trigger a flood of inflammatory signaling molecules, collectively called a cytokine storm in severe cases. One of these molecules, IL-6, acts directly on muscle tissue and stimulates the production of prostaglandin E2, a compound that lowers the pain threshold of nerve endings throughout your body. Other inflammatory signals including TNF-alpha, IL-2, IL-7, and IL-10 pile on.
COVID-19 adds a second mechanism. The virus enters cells through a receptor called ACE2, which is present in muscle tissue and the spinal cord. When the virus occupies those receptors, it disrupts a pathway that normally helps dampen pain signals. The net result is heightened pain sensitivity on top of the inflammation already happening. Direct viral injury to muscle cells and reduced oxygen delivery to tissues during respiratory illness can also contribute. This is why muscle pain during a viral infection can feel so much worse than post-exercise soreness, even though you haven’t done anything physically demanding.
Low Magnesium and Potassium
Mineral imbalances are an underappreciated cause of muscle pain and cramping. Normal blood magnesium levels fall between 1.46 and 2.68 milligrams per deciliter. Even mild drops below that range can cause muscle spasms, cramps, and numbness in the hands and feet. Low magnesium and low potassium frequently occur together, which compounds the problem since both minerals are essential for normal muscle contraction and relaxation.
Common causes of depletion include heavy sweating, prolonged diarrhea or vomiting, certain blood pressure medications, and diets low in leafy greens, nuts, and bananas. If your muscle pain comes with frequent cramping, especially at night, and you can’t link it to exercise or illness, an electrolyte imbalance is worth investigating with a simple blood test.
Fibromyalgia and Chronic Widespread Pain
When muscle pain persists for months without a clear injury or illness, fibromyalgia is one of the more common explanations. The current diagnostic criteria require widespread pain in at least four of five body regions lasting at least three months, combined with significant fatigue, unrefreshing sleep, or cognitive difficulties (often called “brain fog”). An alternative set of criteria requires pain in at least six of nine body regions plus moderate to severe sleep problems or fatigue.
Fibromyalgia differs from a localized injury in that the pain is diffuse, soft-tissue-based, and not associated with swollen or red joints. If your joints are visibly inflamed, that points more toward a rheumatologic condition like rheumatoid arthritis. Importantly, having another painful condition does not rule out fibromyalgia. The two can coexist, which is part of what makes it tricky to identify.
Warning Signs That Need Urgent Attention
Most muscle pain is self-limiting, but one serious condition to be aware of is rhabdomyolysis, where muscle tissue breaks down rapidly and releases its contents into the bloodstream. The classic triad is muscle pain, muscle weakness, and dark urine that looks tea- or cola-colored. In practice, fewer than 10 percent of cases produce visibly dark urine, and up to 50 percent of patients have vague or minimal symptoms. That makes it easy to miss.
Rhabdomyolysis can follow extreme exertion (a new CrossFit class, military training, marathon running in heat), crush injuries, prolonged immobilization, severe heat exposure, or drug and alcohol intoxication. Diagnosis requires a blood test showing creatine kinase levels at least five times the normal upper limit, typically above 5,000 units per liter. Left untreated, it can cause kidney failure. If you develop significant weakness alongside your muscle pain, or your urine darkens after intense exercise or trauma, that combination warrants a trip to the emergency room.