Muscle pain has dozens of possible causes, ranging from a hard workout to an underlying medical condition. The most common trigger is simple mechanical stress: you overworked a muscle, strained it, or held it in an awkward position too long. But muscle pain can also signal dehydration, medication side effects, viral infections, electrolyte imbalances, or chronic conditions like fibromyalgia. Understanding which category your pain falls into helps you figure out what to do about it.
Exercise and Overuse
The soreness you feel a day or two after an intense workout is called delayed onset muscle soreness, or DOMS. It happens when mechanical load exceeds what your muscle fibers can handle, causing microscopic damage at the cellular level. This triggers a cascade of protein breakdown, inflammation, and repair. Symptoms first appear 6 to 12 hours after exercise and peak between 48 and 72 hours.
One persistent myth is that lactic acid causes this soreness. It doesn’t. In a well-known study, runners who ran downhill (which produces significant soreness) never showed elevated lactic acid levels. Runners on flat ground did produce lactic acid but reported no meaningful soreness afterward. The real culprit is the structural damage to muscle fibers and the inflammatory response that follows, not any buildup of acid.
DOMS is most common after eccentric exercise, where your muscles lengthen under load (think: lowering a heavy weight, running downhill, or the first day back after a long break). It resolves on its own within a few days and, while uncomfortable, is not a sign of injury.
Muscle Strains and Tears
A strain is a step beyond normal soreness. It means the muscle fibers have been stretched or torn, and the severity determines how long you’ll be dealing with it.
- Grade I (mild): The muscle is stretched and pulled enough to cause minor damage, but no actual tear. This is the most common type and typically heals within a few weeks.
- Grade II (moderate): Some or most of the muscle fibers are torn. You’ll notice reduced strength and limited range of motion. Recovery can take several weeks to months.
- Grade III (severe): The muscle is torn all the way through. A complete rupture often requires surgery, and healing takes four to six months afterward.
Strains tend to happen suddenly during explosive movements, heavy lifting, or abrupt changes in direction. The pain is typically sharp and immediate, unlike the gradual build of DOMS.
Viral Infections
That full-body aching you feel when you have the flu isn’t your imagination, and it’s not the virus directly attacking your muscles. Your immune system produces signaling molecules called cytokines to fight the infection, and one of them, interleukin-6 (IL-6), triggers the production of compounds that cause pain. IL-6 has been directly correlated with the muscle aches that accompany influenza. When IL-6 is administered to humans in research settings, it reliably produces fever, headache, and muscle pain.
This is why muscle aches are so common across very different illnesses: the pain comes from your immune response, not from the specific pathogen. COVID-19, the flu, mononucleosis, and even some common colds can all produce widespread muscle pain through this same inflammatory pathway.
Dehydration and Electrolyte Imbalances
When you’re dehydrated, water shifts out of your muscle cells and into the bloodstream to maintain normal blood concentration. This cell-level dehydration impairs muscle function in several ways: it disrupts temperature regulation, interferes with the proteins that contract your muscles, and alters how electrolytes move across cell membranes. The result can be anything from vague achiness to sharp cramping.
Electrolyte imbalances amplify the problem. Low magnesium is a particularly common contributor to muscle pain and spasms. Normal blood magnesium levels sit between 1.46 and 2.68 mg/dL, but symptoms like muscle cramps and spasms typically appear once levels drop below 1.2 mg/dL. Magnesium deficiency often flies under the radar because mild cases produce no symptoms at all. Low potassium and low calcium cause similar issues, and these deficiencies frequently overlap since magnesium helps regulate the other two.
People at higher risk include heavy sweaters, those on diuretics, anyone with chronic diarrhea or vomiting, and older adults who may not eat enough mineral-rich foods.
Medication Side Effects
Statins, the cholesterol-lowering drugs taken by tens of millions of people, are one of the most recognized medication-related causes of muscle pain. In one study tracking patients over 12 months, 27.8% developed some form of muscle problem while on statins. The risk wasn’t uniform: it was higher in men (31.4%) than women (22.6%) and increased with age, reaching 34% in patients aged 60 to 79.
Statin-related muscle problems range from mild aching with no measurable muscle damage to, in rare cases, severe breakdown of muscle tissue. Most people who experience symptoms fall into the milder category of general pain or weakness. The type and dose of statin matters, too. Some formulations caused muscle problems in up to half of patients at higher doses, while others stayed around 10%.
Statins aren’t the only medications that cause muscle pain. Blood pressure drugs, certain antibiotics, antifungal medications, and some psychiatric medications can all produce muscle symptoms. If your pain started within weeks of beginning a new medication, the timing is worth noting.
Fibromyalgia
Fibromyalgia produces chronic, widespread pain that persists for months or years. It’s diagnosed when diffuse pain, fatigue, and sleep disturbances have been present for at least three months and the pain affects at least four of five body regions: left shoulder or arm, right shoulder or arm, neck or back, left hip or leg, and right hip or leg.
The pain in fibromyalgia isn’t caused by visible tissue damage. Instead, it appears to involve how the nervous system processes pain signals, essentially amplifying them. Patients often describe muscle stiffness and tenderness alongside the pain. Fatigue is nearly universal. Sleep is frequently disrupted, with difficulty falling asleep, frequent waking, or waking up feeling unrefreshed. Cognitive symptoms like poor concentration and forgetfulness (“fibro fog”) are also common. Comorbid anxiety and depression occur frequently, which can make the pain harder to manage.
Polymyalgia Rheumatica
If you’re over 50 and developed pain and stiffness in both shoulders seemingly out of nowhere, polymyalgia rheumatica is worth knowing about. It almost never appears before age 50 and most commonly strikes people over 65, with the average age of onset just above 70. The hallmark presentation is bilateral shoulder pain and stiffness that comes on quickly, often over days to weeks, with tenderness in both upper arms. Many people also develop hip pain and stiffness along with aching in the back of the neck.
The pain is typically worst in the morning and improves somewhat with movement throughout the day. It responds well to treatment, and most people notice dramatic improvement within days of starting therapy. It’s an inflammatory condition, so blood tests usually show elevated markers of inflammation, which helps distinguish it from other causes of muscle pain in older adults.
When Muscle Pain Points to Something Else
Most muscle pain resolves with time, hydration, and rest. But certain patterns suggest something more than a simple strain. Pain that affects both sides of the body symmetrically, lasts more than a few weeks, or comes with fatigue and sleep problems may point toward a systemic cause. Muscle pain accompanied by dark-colored urine can indicate severe muscle breakdown, which requires urgent attention. Pain that worsens steadily rather than improving, or that appeared after starting a new medication, also warrants investigation.
Your age matters in narrowing the possibilities. Younger adults are more likely dealing with overuse, strains, or viral illness. People over 50 need to consider polymyalgia rheumatica and medication side effects. And anyone with persistent, unexplained muscle pain spreading across multiple body regions should be evaluated for conditions like fibromyalgia or autoimmune disease.