Last reviewed and updated: October 2026.
Heavy legs, trembling arms, burning muscles and the mysterious disappearance of your former strength. What’s happening — and what is actually worth investigating?
Muscle weakness in Long Covid can involve changes in muscle energy metabolism, post-exertional malaise (PEM), autonomic dysfunction, circulation or other treatable conditions.
Research has found measurable muscle abnormalities in some Long Covid patients, including changes in mitochondrial energy production and muscle responses to exercise. But not everyone has the same problem.
The important question is whether your muscles are genuinely losing strength, becoming exhausted unusually quickly, or struggling because standing and exertion affect other systems.
Routine blood tests may be normal. Depending on your symptoms, investigations such as creatine kinase (CK), neurological examination, orthostatic testing or specialist muscle assessment may help identify what is happening.
Key Insights
- Heavy legs are not necessarily weak muscles. POTS, circulation and fatigue can make your legs feel like concrete.
- Mitochondria matter. Studies have identified impaired muscle energy metabolism in some patients, although it is not a universal explanation.
- PEM changes everything. If activity makes you worse the next day, pushing harder may be counterproductive.
- Normal CK doesn’t exclude every muscle problem. It measures certain types of muscle injury, not how much energy your muscles can produce.
- There are investigations worth exploring, particularly when weakness is progressive or accompanied by neurological symptoms.
- Supplements deserve a sensible discussion. Some have plausible mechanisms or early research, but none is an established cure for Long Covid muscle weakness.
When Your Legs Decide They’ve Had Enough
Remember when walking upstairs was just walking upstairs?
Now your legs occasionally behave as though you’ve completed the London Marathon.
Except you haven’t.
You’ve walked to the kitchen.
Or perhaps the problem is your arms. Washing your hair becomes an endurance event. Holding a hairdryer feels like lifting weights.
Sometimes your muscles burn. Sometimes they tremble. Sometimes they work reasonably well in the morning and seem to resign by lunchtime.
And the most confusing part?
Your blood tests may look perfectly normal.
The first thing worth understanding is that muscle weakness, muscle fatigue and muscle heaviness are not necessarily the same problem.
That distinction can change what is worth investigating.
1. What Kind of Muscle Weakness Do You Have?
Three experiences are particularly useful to distinguish.
Your muscles have lost strength.
You cannot rise from a chair without using your arms, climb stairs as before, or lift something you previously managed.
This may indicate genuine muscle weakness and deserves examination, especially if it is worsening.
Your muscles work, but run out of steam.
You can lift your arms, but keeping them raised is difficult. You can walk, but your legs tire disproportionately quickly.
This is closer to muscle fatigability or reduced endurance.
Your legs feel like concrete, especially when standing.
You feel shaky, heavy or exhausted while upright, but noticeably better lying down.
This pattern raises a different possibility: orthostatic intolerance, including POTS.
And then there is a fourth experience.
You manage an activity today, but tomorrow your muscles and possibly your whole body are dramatically worse.
That may be post-exertional malaise (PEM).
These experiences can overlap, but they should not automatically be treated as one condition.
2. What Is Actually Happening Inside the Muscles?
One of the most interesting Long Covid studies came from researchers in Amsterdam.
Published in Nature Communications in 2024, it compared 25 people with Long Covid and PEM against 21 people who had recovered from COVID.
The researchers examined muscle tissue before and after exercise.
They found differences in muscle structure, energy metabolism and the way muscles responded to exertion.
Some abnormalities worsened following exercise.
Read the study: Muscle Abnormalities Worsen After Post-Exertional Malaise in Long COVID
The mitochondria connection
Mitochondria are the tiny energy-producing structures inside our cells.
Think of them as the power supply. Your muscles need that power to contract, recover and keep working.
The Amsterdam study identified reduced mitochondrial oxidative capacity in its Long Covid group.
In plain English, the muscles appeared less efficient at producing energy through certain pathways.
That is particularly interesting for people who describe muscles that become exhausted after surprisingly little activity.
But there is an important limitation: this was a relatively small study involving patients with PEM.
It does not prove that every person with Long Covid has mitochondrial dysfunction.
What about oxygen and blood vessels?
The researchers also found evidence of reduced oxygen extraction by working muscles.
Other Long Covid research has investigated endothelial dysfunction and microcirculation.
These findings raise an interesting possibility: some patients may have problems involving both energy production and how oxygen is delivered or used.
However, muscle heaviness does not prove that microclots are blocking your capillaries.
The research is more complicated than that.
What matters for patients: Scientists are identifying measurable muscle abnormalities. But we still need better ways to determine which mechanisms affect an individual person.
Explore LCJ: Long Covid and Microvascular Dysfunction
3. Why Your Muscles May Be Worse After Activity
This is where PEM becomes important.
Imagine you feel reasonably well and decide to make the most of it.
A little cleaning. A short walk. Perhaps a supermarket trip.
Nothing extraordinary.
Then the following day, your legs are heavier, your muscles ache, your brain fog worsens and your energy disappears.
That’s not necessarily ordinary post-exercise soreness.
PEM can involve several systems and may appear hours or even a day or two after activity.
The Amsterdam researchers found muscle changes following exercise in their PEM group.
This helps explain why the familiar advice to just build your fitness back up may be inappropriate for some patients.
The practical lesson: Before trying to strengthen your muscles, establish whether exertion causes delayed deterioration.
If it does, avoiding repeated crashes becomes a priority.
That doesn’t mean everyone with Long Covid should avoid movement. It means activity needs to match the person’s physiology and tolerance.
Explore LCJ: Why Exercise Can Make Long Covid Worse
4. What Is Actually Worth Investigating?
This is probably the most useful part of the conversation to have with your doctor.
Not every patient needs specialist muscle investigations.
But if weakness is significant, new or worsening, there are questions worth asking.
Start with the basics — but understand their limitations
| Investigation | What it may tell you |
|---|---|
| Creatine kinase (CK) | Can indicate muscle injury, but normal CK doesn’t rule out all muscle dysfunction |
| B12, thyroid, ferritin and vitamin D | May identify deficiencies or hormonal problems contributing to symptoms |
| Electrolytes and calcium | Can detect certain metabolic causes of weakness |
| Neurological examination | Helps distinguish muscle weakness from nerve-related problems |
| Lying/standing heart rate and blood pressure | May identify orthostatic intolerance contributing to heavy legs |
What if those tests are normal?
That is where the discussion becomes more interesting.
Depending on the symptom pattern, there may be reasons to explore:
EMG and nerve conduction studies
These assess certain aspects of nerve and muscle electrical function.
They may be useful if you have genuine loss of strength, unusual reflexes, muscle wasting or symptoms suggesting a neuromuscular disorder.
Muscle MRI
This can identify certain structural muscle abnormalities, including inflammation or patterns of muscle disease.
It is not a routine test for Long Covid fatigue, but may be useful when a specific muscle disorder is suspected.
Small fibre neuropathy assessment
If weakness comes with burning feet, tingling, altered temperature sensation or autonomic symptoms, small fibre neuropathy may be worth discussing.
Skin biopsy and specialised sensory or autonomic tests can sometimes help, depending on the presentation.
Small fibre neuropathy primarily affects sensory and autonomic fibres; it does not usually explain major objective muscle weakness by itself.
Autonomic testing
If your legs become heavy when standing, particularly alongside palpitations, dizziness or heat intolerance, evaluating POTS or another form of orthostatic intolerance may be more useful than a muscle MRI.
Explore LCJ: Dysautonomia in Long Covid
Specialist metabolic or neuromuscular assessment
If there is progressive objective weakness, unusual exercise intolerance, muscle wasting or a strong clinical suspicion of muscle disease, a specialist may consider further investigations.
Research techniques such as muscle biopsy, phosphorus magnetic resonance spectroscopy or specialised metabolic testing can provide information in selected circumstances, but are not established routine Long Covid tests.
What about CPET?
Cardiopulmonary exercise testing can reveal certain abnormalities in exercise physiology.
However, it can also provoke PEM.
For someone who already experiences severe crashes, the possible benefit of the test needs careful consideration.
The LCJ question to ask isn’t “Can I have every test?”
It’s:
“Which test might explain my particular type of weakness — and would the result change what we do?”
5. Supplements: What’s Worth Exploring?
There is a reason supplements come up so often in Long Covid discussions.
If muscle energy production is affected, it seems logical to look at nutrients involved in mitochondrial function.
But supporting a biological pathway is not the same as proving that a supplement improves Long Covid.
Here is a more useful way to look at the options.
Coenzyme Q10 — the mitochondrial favourite
CoQ10 plays a role in mitochondrial energy production.
It sounds promising.
But a randomised placebo-controlled trial using high-dose CoQ10 in people with post-Covid condition did not establish a meaningful overall clinical benefit.
So while CoQ10 has a plausible biological rationale, it is not a proven solution for muscle weakness.
Creatine — interesting, but early evidence
Creatine helps muscles regenerate ATP during short bursts of activity.
It is widely studied in sports and muscle physiology.
Small preliminary Long Covid studies have explored creatine supplementation, but evidence is still insufficient to know whether it reliably improves muscle strength, fatigue or PEM.
It is a research avenue worth following rather than an established treatment.
Kidney health, medications and individual circumstances matter when considering supplementation.
Magnesium — useful if you need it
Magnesium is important for normal muscle and nerve function.
Low magnesium can contribute to muscle symptoms.
But having Long Covid does not automatically mean you are magnesium deficient.
Supplementation is most clearly justified when deficiency or another specific indication exists.
B12, B1 and vitamin D — check before guessing
These nutrients are involved in neurological, muscular or metabolic function.
Deficiencies can contribute to symptoms and may be treatable.
But taking large doses when you don’t need them is not necessarily helpful.
Excessive vitamin B6, for example, can itself cause neuropathy.
Electrolytes — particularly relevant when POTS is involved
If heavy legs and weakness worsen when standing, fluid and sodium management may help some people with orthostatic intolerance.
That is different from treating mitochondrial dysfunction.
Increasing salt or fluids is not suitable for everyone, particularly those with certain heart, kidney or blood-pressure conditions.
Explore LCJ: Long Covid and Electrolyte Imbalance
Protein and amino acids — don’t overlook the basics
Adequate protein is important for maintaining muscle tissue.
If illness has reduced your appetite, food intake or muscle mass, a nutritional assessment may be more useful than an expensive supplement stack.
Branched-chain amino acids and other metabolic supplements are being explored, but evidence for treating Long Covid muscle weakness remains limited.
So what would LCJ prioritise?
Before buying five supplements aimed at your mitochondria, explore whether you have:
- A correctable nutritional deficiency.
- Inadequate protein or calorie intake.
- Orthostatic intolerance affecting your muscles.
- PEM that is repeatedly worsening your function.
- Another muscular or neurological condition requiring treatment.
Then discuss any experimental supplement with a clinician or pharmacist, especially if you take prescription medicines.
There is no established mitochondrial supplement protocol for Long Covid.
But there are sensible avenues to investigate, and research is continuing.
6. What Can Help in Everyday Life?
Sometimes the most useful intervention is not a new supplement or another scan.
It is understanding what makes your particular symptoms worse.
If standing makes your legs heavy: Consider whether orthostatic intolerance is contributing. Sitting for tasks, avoiding overheating and discussing appropriate treatment may help.
If your arms tire during washing or cooking: Reduce the sustained effort. Sitting, supporting your elbows or splitting tasks into shorter stages can make a difference.
If activity causes PEM: Try to work within a tolerable activity level rather than repeatedly pushing until you crash.
If you are genuinely losing strength: Don’t assume it is simply Long Covid fatigue. Ask for a clinical examination.
If heat makes everything worse: Cooler showers and temperature control may reduce the added strain.
Explore LCJ: Why Heat Makes Long Covid Symptoms Worse
And remember that strength and stamina are not interchangeable.
You may still be able to produce normal strength for a few seconds while having very limited endurance.
That distinction matters.
Questions Worth Taking to Your Doctor
Rather than arriving with a long list of experimental tests, try these:
- Is this true muscle weakness, reduced endurance or PEM? Can we assess the difference?
- If my CK is normal, what muscle or neurological conditions have actually been excluded?
- Could orthostatic intolerance explain why my legs feel worse when standing?
- Would EMG, muscle imaging or neurological assessment be useful given my symptoms?
- Are there deficiencies or medication effects that could be contributing?
- If I’m considering creatine, CoQ10 or other supplements, are there reasons they might be unsuitable for me?
New progressive weakness, muscle wasting or repeated falls deserve prompt medical assessment. Sudden one-sided weakness or new difficulty breathing or swallowing requires emergency care.
Frequently Asked Questions
Why do my muscles feel weak after COVID?
Muscle weakness in Long Covid is linked to disrupted mitochondrial energy production, reduced oxygen delivery to muscles, nervous system dysregulation, and ongoing low-level inflammation. It is a biological problem, not a fitness problem.
Why does muscle weakness get worse after activity?
This is post-exertional malaise (PEM) a hallmark feature of Long Covid. The body struggles to recover from physical or mental effort, and symptoms worsen in the hours or days that follow. It is a physiological response, not a psychological one.
Is muscle weakness in Long Covid permanent?
For most people, symptoms do improve over time, though recovery is often slow, non-linear, and highly individual. Research into treatments targeting mitochondrial function and immune dysregulation is ongoing. It is too early to say what long-term outcomes will look like for everyone.
Why do my muscles burn after small tasks?
This is likely early lactic acid build-up caused by impaired mitochondrial function. Your muscles switch to an inefficient energy pathway far sooner than they should, producing the burning sensation associated with intense exercise but during everyday activity.
Should I exercise with muscle weakness in Long Covid?
Only very gently, and only with awareness of PEM. Structured, progressive exercise routines can cause significant harm in people with post-exertional malaise. Symptom-paced, gentle movement is generally safer, ideally guided by a physiotherapist familiar with Long Covid or ME/CFS.
Why do my muscles feel normal one day and terrible the next?
One of the most confusing parts of Long COVID is that muscle weakness often fluctuates. On a good day you may manage far more than expected, only to find your legs feel heavy and unreliable the following day. This isn’t inconsistency or lack of effort. It reflects the fluctuating nature of Long COVID itself, where energy production, autonomic function and post-exertional malaise can all vary from day to day.
Should I exercise with muscle weakness in Long Covid
One of the most confusing parts of Long COVID is that muscle weakness often fluctuates. On a good day you may manage far more than expected, only to find your legs feel heavy and unreliable the following day. This isn’t inconsistency or lack of effort. It reflects the fluctuating nature of Long COVID itself, where energy production, autonomic function and post-exertional malaise can all vary from day to day.
Why do my legs feel heavy or unstable?
This can relate to autonomic nervous system dysfunction, reduced circulation, mitochondrial impairment, or fatigue affecting muscle control signals. If it worsens on standing or walking, ask your GP about dysautonomia or POTS.
Can supplements help muscle weakness in Long Covid?
Some may provide modest support CoQ10, magnesium, B vitamins, omega-3s, and electrolytes are among those with a plausible rationale. None are cures. Always discuss with a healthcare professional before starting, especially if you take other medications.
Why is my recovery so slow compared to other people?
Long Covid recovery is highly individual and influenced by how severe your initial infection was, how long you have been ill, whether you have other conditions, and likely genetic factors affecting mitochondrial function. Comparing your timeline to others is rarely helpful and often demoralising. Your pace is your pace.
Research & References
1. Appelman et al. — Muscle Abnormalities Worsen After Post-Exertional Malaise in Long COVID. Nature Communications, 2024.
Muscle biopsies and exercise testing identified abnormalities in muscle metabolism, structure and post-exertional responses in a small group of patients with Long Covid and PEM.
2. Hansen et al. — High-Dose Coenzyme Q10 Therapy Versus Placebo in Patients With Post COVID-19 Condition.The Lancet Regional Health – Europe, 2023.
A randomised crossover trial examining whether high-dose CoQ10 improves post-Covid symptoms.
3. Davis et al. — Long COVID: Major Findings, Mechanisms and Recommendations. Nature Reviews Microbiology, 2023.
A broad review of Long Covid research, including vascular, neurological, immune and metabolic mechanisms.
4. Al-Aly et al. — Long COVID Science, Research and Policy. Nature Medicine, 2024.
A comprehensive review of Long Covid, its multisystem effects and ongoing research priorities.
Related Long Covid Journey Articles
- Why Exercise Can Make Long Covid Worse
- Understanding Dysautonomia in Long Covid
- Long Covid and Microvascular Dysfunction
- Long Covid and Electrolyte Imbalance
- Why Long Covid Tests Can Be Normal
A Final Thought From Long Covid Journey
There is something deeply strange about having to negotiate with your own muscles.
Will my legs manage the stairs?
Can I wash my hair without exhausting my arms?
If I walk today, what will tomorrow cost me?
These are questions most people never need to ask.
Long Covid research is beginning to show why muscles may behave differently after infection. But it is also showing that there isn’t one explanation for everyone.
The useful next step isn’t to assume your mitochondria are damaged or that you simply need to exercise more.
It’s to understand which part of the system may be struggling muscle strength, energy production, circulation, nervous-system regulation or recovery after exertion.
Because the better the question, the better the chance of finding something useful to investigate.
Medical disclaimer: This article is for information and education, not individual medical advice. Investigations, supplements and rehabilitation should be discussed with an appropriate healthcare professional.
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