Muscle weakness after COVID stems from at least half a dozen overlapping problems, ranging from direct tissue damage and lingering inflammation to changes in how the brain signals muscles to contract. It is one of the most frequently reported long COVID symptoms, and research over the past few years has revealed that the weakness is not simply from being deconditioned during illness. Biopsies, imaging, and exercise testing all point to structural and metabolic changes inside muscle fibers themselves, along with nerve and blood-vessel abnormalities that compound the problem.
What Happens to Muscles During Acute COVID
Muscle loss can begin remarkably early. A retrospective study using CT scans found that patients hospitalized with COVID showed statistically significant drops in muscle area and density compared to their pre-infection scans, with the decline more pronounced in those who were intubated.1Radiología (English Edition). Acute muscle loss and early effects of COVID-19 on skeletal muscle in adult patients: A retrospective cohort study The severity of lung involvement on imaging correlated with the degree of muscle loss, suggesting that sicker patients lose more muscle faster.
People admitted to ICUs face an even steeper decline. ICU-acquired weakness affects a strikingly high proportion of critically ill COVID patients, manifesting as diffuse, symmetrical weakness that can appear within the first days of admission.2PubMed Central. Intensive care unit-acquired weakness: A review from molecular mechanisms to its impact in COVID-2019 The combination of sedation, paralytic drugs used during mechanical ventilation, high-dose corticosteroids, and complete immobility creates a perfect storm for rapid muscle wasting. But here is what catches many patients off guard: meaningful muscle weakness also appears in people who had mild infections and were never hospitalized. That pattern points to something beyond simple disuse.
Inside the Muscle Fiber
Muscle biopsies from people with persistent post-COVID weakness have been genuinely revealing. In one study of patients reporting fatigue months after infection, half had measurable muscle weakness on examination, and three-quarters showed abnormalities on electromyography. When researchers examined the tissue under a microscope, every single patient had histological changes.3PubMed Central. Myopathy as a cause of fatigue in long‐term post‐COVID‐19 symptoms: Evidence of skeletal muscle histopathology About a third had actual fiber atrophy, more than half showed signs of fibers trying to regenerate, and roughly six in ten had mitochondrial abnormalities such as loss of a key respiratory enzyme or structurally damaged cristae (the folds inside mitochondria where energy production occurs). Inflammation was equally common, with immune cells infiltrating the tissue. Perhaps most striking, three-quarters had damaged capillaries, the tiny blood vessels that deliver oxygen to muscle.
Those capillary findings connect to a broader vascular story. Persistent inflammation of blood vessel linings, overactive platelets, and abnormal fibrin-containing microclots have been identified as a unifying pathway across many long COVID symptoms.4PubMed Central. Long COVID: pathophysiological factors and abnormalities of coagulation When these clots are large enough to block capillaries within muscle, they can create pockets of local oxygen deprivation. Researchers have also observed thickening of the capillary basement membrane in long COVID patients, which further impairs the ability of oxygen to diffuse from blood into tissue.5Trends in Endocrinology & Metabolism. Skeletal muscle adaptations and post-exertional malaise in long COVID Even if your lungs are pulling in enough air and your heart is pumping normally, muscles starved of oxygen at the capillary level will feel weak and tire quickly.
Mitochondria and the Energy Shortfall
A healthy muscle fiber generates most of its energy through oxidative phosphorylation, a process that happens inside mitochondria and depends on a steady supply of oxygen. In long COVID patients, that process appears to be impaired at its source. Direct measurements of muscle oxidative capacity showed it was significantly lower in people with long COVID compared to healthy controls, and it dropped even further the day after exertion.6Nature Communications. Muscle abnormalities worsen after post-exertional malaise in long COVID
When mitochondria cannot produce enough energy aerobically, muscle fibers shift toward anaerobic metabolism, which is less efficient and produces lactate as a byproduct. Over time, this metabolic stress appears to push an actual remodeling of the muscle itself. Researchers have documented a shift in fiber composition: long COVID patients have fewer slow-twitch (type I) fibers, which are built for sustained aerobic activity, and more fast-twitch (type IIx) fibers, which rely on quick, glycolytic bursts.7Nature Communications. Skeletal muscle properties in long COVID and ME/CFS differ from those induced by bed rest This is not what you see in someone who has simply been lying in bed for weeks. Bed rest leads to different fiber-type changes, which strongly suggests that the muscle remodeling in long COVID is driven by disease-specific mechanisms rather than inactivity alone.
Why the Brain Plays a Role
Weakness does not always originate in the muscle itself. Using transcranial magnetic stimulation to probe brain-to-muscle signaling, researchers found that people with post-COVID fatigue had reduced intracortical facilitation, a measure of how readily the motor cortex amplifies signals to muscles. Inhibitory circuits were normal, meaning the brain was not actively suppressing movement. Instead, the excitatory drive was simply turned down.8Brain Communications. Neural dysregulation in post-COVID fatigue The practical consequence is that for the same volitional effort, corticospinal neurons fire less vigorously, so contractions feel harder than they should. This mismatch between effort and output is central to the sensation many long COVID patients describe: it is not just that their muscles are weak, but that everything requires disproportionate effort.
Peripheral nerves are also vulnerable. In one evaluation of long COVID patients with lingering symptoms, roughly six in ten had at least one test confirming neuropathy, and many received diagnoses of small fiber neuropathy, a condition affecting the thin nerve fibers that serve muscles and regulate autonomic function.9PubMed Central. Peripheral Neuropathy Evaluations of Patients With Prolonged Long COVID A separate case-control study confirmed small fiber neuropathy by skin biopsy in long COVID patients, the vast majority of whom also reported post-exertional malaise and showed signs of dysautonomia on cardiopulmonary exercise testing.10PubMed Central. Case-Control Study of Individuals With Small Fiber Neuropathy After COVID-19 Damaged small fibers can cause pain, tingling, and weakness, and their involvement in autonomic regulation helps explain why many people with long COVID also experience heart rate irregularities and blood pressure swings.
In rare cases, COVID can trigger full-blown autoimmune muscle disease. A case report documented new-onset dermatomyositis after infection, with massively elevated muscle enzymes and multiple autoantibodies.11Frontiers in Immunology. New-onset dermatomyositis following COVID-19: A case report Such cases are uncommon, but they illustrate how aggressively the immune system can be redirected against the body’s own tissues after a COVID infection.
The Diaphragm Problem
One muscle group deserves special attention because its weakness masquerades as a lung problem. The diaphragm, the main breathing muscle, is frequently affected. In a cohort of severe COVID survivors admitted to rehabilitation, over three-quarters had at least one ultrasound abnormality of the diaphragm, and the group as a whole showed reduced diaphragm contractility compared to non-COVID patients.12PubMed Central. Diaphragm dysfunction in severe COVID-19 as determined by neuromuscular ultrasound A weak diaphragm makes you feel short of breath even when your lungs themselves are undamaged, which is one reason why some post-COVID patients have normal chest X-rays and oxygen levels yet still struggle to take a satisfying breath.
Diagnosing diaphragm weakness accurately turns out to be harder than you might expect. Ultrasound is widely used to assess diaphragm function, but a study comparing it against the gold standard test (magnetic stimulation of the phrenic nerves combined with pressure catheters) found that ultrasound caught only about a quarter of confirmed cases of diaphragm weakness, despite being fairly good at ruling out weakness when it truly was not present.13Respiration. Poor Correlation between Diaphragm Ultrasound and Invasive Gold Standard Technique Derived Respiratory Muscle Strength Assessment in Patients after Hospitalisation for COVID-19 In that study, six out of ten post-COVID patients tested had objectively confirmed diaphragm dysfunction. Many of these cases would be missed by routine ultrasound alone, which matters because unexplained breathlessness in long COVID may be attributed to anxiety or deconditioning when the actual culprit is a muscular one.
Post-Exertional Malaise and Why “Just Exercise” Backfires
A hallmark feature that separates post-COVID muscle problems from ordinary deconditioning is post-exertional malaise, a worsening of symptoms (including weakness, pain, fatigue, and cognitive fog) that hits hours to days after physical or mental exertion. This is not simply soreness from a workout. Research shows that muscle tissue damage and immune cell infiltration into muscle occur rapidly after exercise in long COVID patients, and mitochondrial function, already impaired at baseline, deteriorates further.14PubMed. Skeletal muscle adaptations and post-exertional malaise in long COVID The oxidative phosphorylation capacity that was already lower than controls dropped even more the day after exertion.6Nature Communications. Muscle abnormalities worsen after post-exertional malaise in long COVID
This is why the standard rehabilitation advice of graded exercise can be counterproductive for people who experience post-exertional malaise. Pushing through makes the underlying muscle pathology measurably worse, at least in the short term. That does not mean all movement is harmful, but the approach needs to be carefully paced and led by the patient’s symptom response rather than by a predetermined escalation schedule. This has been a source of genuine tension in rehabilitation medicine, where progressive exercise is a cornerstone for nearly every other form of deconditioning.
Who Is Most at Risk
Several factors raise the odds of developing significant post-COVID muscle problems. Being female, being older, having obesity, being hospitalized (especially with mechanical ventilation), prolonged immobility, and having pre-existing conditions all emerge as clinical predictors.15PubMed Central. Musculoskeletal involvement: COVID-19 and post COVID 19 For ICU patients specifically, the list expands to include sepsis, multi-organ failure, neuromuscular blocking agents, corticosteroid use, and poor blood sugar control during the stay.16PubMed. Intensive care unit-acquired weakness and the COVID-19 pandemic: A clinical review Vaccination status also appears relevant, with unvaccinated individuals being more prone to long-term musculoskeletal symptoms.
The sex difference is worth noting in more detail. Research profiling immune, hormonal, and gene-expression markers in long COVID patients with ME/CFS-like presentations found that women showed stronger pro-inflammatory immune responses, a shift toward production of inflammatory blood cells, depletion of regulatory T cells, and markers of gut barrier dysfunction compared to men.17PubMed Central. Integrated immune, hormonal, and transcriptomic profiling reveals sex-specific dysregulation in long COVID patients with ME/CFS Hormonal disruptions ran in both directions: women had lower testosterone and men had lower estradiol. The study’s authors emphasized that these sex-specific patterns argue for tailored treatment approaches rather than one-size-fits-all protocols.
The Overlap With ME/CFS
Long COVID and myalgic encephalomyelitis/chronic fatigue syndrome share enough features that researchers have increasingly studied them side by side. When balance and physical capacity were tested, people with long COVID and people with ME/CFS performed almost identically: both groups had worse postural sway and slower performance on timed physical tasks than healthy controls, with no significant differences between the two patient groups. About 87% of participants in both groups exceeded the threshold for muscle weakness on a sit-to-stand test.18PubMed. People with Long Covid and ME/CFS Exhibit Similarly Impaired Balance and Physical Capacity: A Case-Case-Control Study
The muscle fiber changes overlap too, but they are not identical. Both conditions show fewer slow-twitch fibers and more fast-twitch fibers than healthy controls. However, ME/CFS patients had an even more pronounced loss of slow-twitch fibers and showed reduced capillary density, while capillary measures in long COVID patients were closer to normal.7Nature Communications. Skeletal muscle properties in long COVID and ME/CFS differ from those induced by bed rest Brain chemistry also diverges: a magnetic resonance spectroscopy study found that ME/CFS patients had elevated brain lactate (suggesting energetic stress), while long COVID patients had reduced brain choline, a marker potentially linked to coagulation abnormalities and cognitive symptoms.19PubMed Central. Brain and muscle chemistry in myalgic encephalitis/chronic fatigue syndrome (ME/CFS) and long COVID: a 7T magnetic resonance spectroscopy study The researchers cautioned that the two conditions should not be lumped together for study purposes until the underlying mechanisms are better understood, even though their clinical presentations look similar.
Getting Tested
There is no single test that captures all the ways COVID can weaken muscles. Standard blood work may show elevated creatine phosphokinase in some patients, a marker of muscle damage, and electrophysiologic studies can pick up both neuropathy and myopathy. In one study, patients with persistent musculoskeletal symptoms after COVID had significantly different results on these tests compared to healthy controls.20PubMed Central. Prevalence of peripheral neuropathy and myopathy in patients post-COVID-19 infection But electromyography and nerve conduction studies can miss small fiber neuropathy entirely, which requires a skin biopsy for confirmation.
For people whose primary complaint is breathlessness or exercise intolerance rather than limb weakness, the diaphragm evaluation challenge mentioned earlier is relevant. A normal chest X-ray and normal spirometry do not rule out diaphragm weakness. If the clinical suspicion is high, specialized testing using phrenic nerve stimulation may be warranted, though it is not widely available outside academic medical centers.
The general picture is that post-COVID muscle weakness involves at least three overlapping layers: systemic inflammation and poor nutrition driving muscle wasting, intrinsic mitochondrial dysfunction starving fibers of energy, and physical inactivity compounding both.21PubMed Central. The Impact of Long COVID-19 on Muscle Health Any thorough evaluation should consider all three, along with the neurological factors discussed earlier.
Nutritional Support and Emerging Approaches
While no medication specifically reverses post-COVID muscle pathology, some early data suggest nutritional interventions may help. A study giving COVID survivors a multinutrient supplement found that grip strength improved from about 26 to 29 kg over the supplementation period, time to muscle exhaustion increased from about 32 seconds to 48 seconds, and performance on a sit-to-stand test improved by roughly a quarter.22PubMed Central. Fatigue in Covid-19 survivors: The potential impact of a nutritional supplement on muscle strength and function These are modest gains, and the study was not a randomized controlled trial, but they suggest that nutritional optimization is at least part of the picture for people whose muscle function is slow to return.
Research into more targeted treatments is ongoing. The microvascular hypothesis has led some clinicians to trial anticoagulant or antiplatelet approaches, though evidence remains preliminary. Identifying and treating small fiber neuropathy with immunomodulatory therapies is another avenue being explored. Understanding the sex-specific immune patterns may eventually lead to hormonal interventions. For now, the most evidence-supported approach remains careful pacing of activity, adequate protein and micronutrient intake, and addressing comorbidities that compound weakness.
Children and Adolescents
Muscle weakness after COVID is not exclusive to adults. Reviews of pediatric long COVID have identified fatigue, headache, and muscle weakness among the clinical manifestations reported in children and adolescents.23PubMed Central. Long COVID in children and adolescents: prevalence, clinical manifestations, and management strategies The mechanisms are assumed to overlap with adult cases, though less biopsy and exercise-testing data exist in younger populations. For parents, the practical takeaway is that a child who remains unusually tired or physically weak weeks after a COVID infection is experiencing something recognized in the medical literature, not malingering or simply being lazy. Pediatric cases may warrant evaluation if symptoms persist beyond a few weeks and interfere with school or physical activity.