Post exertional malaise in Long COVID is a worsening of symptoms after physical, mental or emotional effort that can be far greater than the activity itself would suggest.
Also known as post-exertional malaise (PEM) or post-exertional symptom exacerbation (PESE), it is one of the most disabling symptoms experienced by some people with Long COVID.
It is not simply feeling tired after exercise.
You might manage a shower, attend an appointment or concentrate through a work meeting. At the time, you may feel reasonably well. But hours later, or the following day, your symptoms become significantly worse.
For people who experience PEM, recognising this pattern is important. It can help explain why ordinary activities sometimes have extraordinary consequences.
Quick Answer: How Can You Recognise PEM?
PEM involves a worsening of symptoms after exertion that is disproportionate to the activity involved.
The main characteristics are:
- Symptoms become significantly worse after physical, cognitive, emotional or social activity.
- Worsening is often delayed, commonly appearing 12–48 hours later, although it can begin sooner.
- Recovery takes longer than expected, sometimes lasting days or weeks.
- Several symptoms may worsen together, including fatigue, pain, brain fog and flu-like feelings.
- The amount of activity that triggers PEM varies between people and can change over time.
PEM is a defining feature of ME/CFS and is also experienced by a substantial subgroup of people with Long COVID.
Key Insights
- PEM is different from ordinary fatigue or exercise intolerance.
- A person may appear relatively well during an activity but experience significant worsening afterwards.
- Research has identified measurable physiological abnormalities following exertion in some patient groups.
- Two-day cardiopulmonary exercise testing has contributed to our understanding of abnormal recovery, but it is not a routine test for diagnosing PEM.
- There is currently no single blood test or biomarker that can reliably diagnose PEM.
- Recognising the pattern through symptoms and clinical assessment remains important.
What Is Post-Exertional Malaise?
Post-exertional malaise describes an abnormal worsening of symptoms following exertion.
The word malaise can be misleading. It sounds like a vague feeling of being unwell, but PEM can involve profound and sometimes disabling changes across several bodily systems.
For someone with PEM, an activity that previously required little effort may now cause a substantial worsening of symptoms.
This can happen after physical exertion, such as walking, but also after concentration, emotional demands or social interaction.
The reaction is often delayed and recovery can be prolonged.
PEM is a central diagnostic feature of myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS). It is also recognised in Long COVID, although not everyone with Long COVID experiences it.
The NICE guideline for ME/CFS describes PEM in terms of disproportionate symptom worsening, frequently delayed onset and prolonged recovery.
These features help distinguish PEM from ordinary tiredness.
What Does PEM Actually Feel Like?
Clinical definitions are useful, but they do not always capture the experience.
PEM can affect different people in different ways.
Some describe overwhelming exhaustion, as though their body has suddenly lost the ability to function normally.
Others experience flu-like symptoms, muscle pain, headaches, increased dizziness or a dramatic worsening of brain fog.
For some, the most noticeable change is cognitive.
They may struggle to find words, follow conversations, process information or tolerate screens.
Others experience worsening physical weakness, orthostatic symptoms or sensory sensitivity.
A crash can also make existing Long COVID symptoms more intense.
What matters is not one particular symptom but the relationship between exertion and the subsequent deterioration.
The shower problem
Consider something as ordinary as taking a shower.
Showering can involve standing, raising your arms, dealing with warm water, regulating body temperature and completing several tasks in sequence.
For someone with POTS or orthostatic intolerance, standing and heat may also provoke immediate symptoms.
But if the activity is followed by a delayed and disproportionate worsening of symptoms, PEM may be involved as well.
The distinction matters because immediate dizziness during a shower and a PEM crash the following day are not necessarily the same physiological event.
Both deserve appropriate assessment.
The invisible cognitive trigger
Now consider a long video meeting.
You are sitting down. There is no obvious physical exercise.
But you are listening, concentrating, remembering information, processing facial expressions and responding to questions.
For someone with Long COVID, that cognitive demand can be substantial.
Afterwards, they may experience worsening brain fog, headaches, fatigue or other symptoms.
Sometimes the deterioration is immediate. Sometimes it becomes much more noticeable later.
Cognitive exertion is an important part of understanding PEM, particularly for people who are trying to return to work or education.
How Is PEM Different From Ordinary Fatigue?
Fatigue is common in many illnesses.
PEM is more specific.
| Ordinary fatigue | Post-exertional malaise |
|---|---|
| Often occurs during or shortly after activity | Frequently involves delayed symptom worsening |
| Usually improves with adequate rest | Recovery may remain prolonged despite rest |
| Often reflects the intensity or duration of exertion | Can be disproportionate to seemingly minor exertion |
| Primarily involves tiredness or reduced energy | May worsen multiple symptoms, including pain, cognition and flu-like illness |
| Gradual conditioning may help when deconditioning is the cause | Activity increases that exceed current limits may trigger symptom exacerbation |
These differences are useful, but they are not a substitute for medical assessment.
Fatigue, PEM, autonomic dysfunction, sleep problems and other conditions can overlap.
A person may experience more than one of these problems at the same time.
How Is PEM Different From Exercise Intolerance?
Exercise intolerance means having difficulty tolerating physical exertion.
It can occur for many reasons, including cardiovascular, respiratory, muscular and autonomic conditions.
For example, someone with POTS may experience a rapid heart rate, dizziness or weakness when standing or exercising.
Those symptoms may develop during the activity itself.
PEM is characterised by a disproportionate worsening of symptoms after exertion, often with delayed onset and prolonged recovery.
The two can coexist.
This distinction is especially important in Long COVID because people may have several overlapping causes of reduced activity tolerance.
Not every episode of breathlessness, dizziness or fatigue after exertion should automatically be labelled PEM.
Why Is PEM Often Difficult to Recognise?
One reason is the delay between activity and symptoms.
If you become breathless while climbing stairs, the connection is obvious.
But if you attend a family gathering on Saturday and feel significantly worse on Sunday or Monday, it may be harder to identify the relationship.
Another difficulty is that activity tolerance can fluctuate.
Something manageable last week may not be manageable today.
People may also underestimate cognitive, emotional or social exertion because these activities do not resemble traditional exercise.
This is why PEM can initially appear unpredictable.
However, not every worsening of symptoms is caused by PEM. Infections, medication effects, sleep disorders and other medical problems can also produce changes in symptoms.
Recognising PEM involves looking at the overall pattern rather than assuming that every difficult day is a crash.
How Common Is PEM in Long COVID?
PEM is frequently reported in Long COVID research, but estimates vary considerably.
Some studies of selected Long COVID populations have reported rates approaching 70–80%, while others have found lower proportions.
These differences can reflect who was recruited, how Long COVID was defined and which questionnaire or assessment method researchers used.
People attending specialist clinics or participating in symptom-focused studies may not represent everyone living with Long COVID.
Therefore, a single percentage should not be presented as the prevalence of PEM across all people with Long COVID.
The important conclusion is that PEM affects a substantial proportion of patients and deserves specific recognition in clinical care.
Can Scientific Tests Detect PEM?
Researchers have investigated several ways of measuring physiological changes associated with PEM.
One of the most studied approaches is cardiopulmonary exercise testing, known as CPET.
CPET measures how the heart, lungs and body respond during exercise, including oxygen consumption and workload.
In some studies involving ME/CFS and Long COVID, researchers have used two tests approximately 24 hours apart.
The aim is to examine whether physiological performance changes when exercise is repeated before full recovery.
What did the 2026 two-day CPET study find?
A study published in April 2026 by Davenport and colleagues compared results from 238 participants:
- 84 people with ME/CFS
- 79 people with Long COVID
- 72 non-disabled controls
The researchers analysed physiological responses during two maximal exercise tests performed approximately 24 hours apart.
They found differences in oxygen consumption and workload at the ventilatory anaerobic threshold between patient groups and controls.
This threshold is the exercise intensity at which the body’s response to increasing effort begins to change in a measurable way.
The study also found evidence of poorer performance on repeated testing, with some findings more pronounced among female participants.
The researchers interpreted these results as supporting physiological similarities between ME/CFS and Long COVID, particularly in exercise metabolism and recovery.
However, there are important limitations.
This was a retrospective analysis, not a study establishing a diagnostic test for every person with Long COVID.
The findings were not identical across all measurements or participant groups, and previous Long COVID two-day CPET studies have produced mixed results.
The results support the existence of measurable physiological abnormalities in the populations studied. They do not establish that every person with PEM will show the same pattern.
Two-day maximal CPET can also provoke substantial symptom worsening. It should not be attempted simply to find out whether you have PEM, and it is not a routine requirement for diagnosis.
Read the 2026 study by Davenport and colleagues.
Why Can Standard Medical Tests Be Normal?
Many people with Long COVID report significant symptoms despite routine investigations that do not reveal an obvious explanation.
This does not mean their symptoms are imaginary.
Routine blood tests, scans and clinical examinations are designed to investigate particular conditions or abnormalities.
They do not necessarily measure how symptoms change following exertion.
PEM may therefore be missed if an assessment focuses only on how someone feels at rest or during a brief appointment.
However, normal tests should not be used to assume that all symptoms are caused by PEM.
Appropriate medical evaluation remains important, particularly when symptoms are new, changing or potentially related to another treatable condition.
For a broader discussion, read Long Covid and Normal Tests: Why Results Can Look Fine When You Feel Unwell.
What Might Be Happening in the Body During PEM?
Researchers are investigating several possible mechanisms.
These include changes in energy metabolism, immune responses, muscle physiology, blood circulation and autonomic nervous system function.
Some studies have reported abnormalities after exertion in selected groups of people with Long COVID or ME/CFS.
These findings are important because they suggest that PEM cannot be adequately explained as ordinary tiredness or simply a lack of motivation.
However, the science is still developing.
Researchers have not established one biological mechanism that explains every case of PEM.
It is also not yet clear whether all people with PEM share the same underlying abnormalities.
Different biological processes may contribute in different individuals.
Understanding these mechanisms could eventually help researchers develop more reliable biomarkers and targeted treatments.
For now, the evidence supports taking PEM seriously while recognising that important questions remain unanswered.
How Can You Recognise PEM in Yourself?
The most useful starting point is to look for a repeated relationship between exertion and symptom worsening.
Questions worth considering include:
- Do your symptoms become significantly worse after activity?
- Is the worsening disproportionate to what you did?
- Does it sometimes appear hours later or the following day?
- Do several symptoms worsen together?
- Does recovery take longer than you would expect?
- Can cognitive or emotional effort produce a similar pattern?
You do not need to experience every symptom or have an identical delay after every activity.
The pattern over time matters.
A clinician familiar with Long COVID or ME/CFS can help assess whether PEM is present and whether other conditions may be contributing.
Can a symptom diary help?
A simple record of activities and subsequent symptoms may help reveal patterns.
For example, you could note whether a demanding appointment is followed by increased symptoms later that day or the next.
But detailed tracking is not suitable for everyone.
For people with severe fatigue, brain fog or sensory sensitivity, keeping extensive records can itself become burdensome.
A few brief notes may be enough.
The purpose is to recognise patterns, not to create another demanding daily task.
What is the DePaul Symptom Questionnaire?
The DePaul Symptom Questionnaire (DSQ) is a research instrument used to assess symptoms associated with ME/CFS, including post-exertional malaise.
PEM-related questions examine features such as symptom frequency and severity.
Different versions and scoring approaches have been used in research.
The questionnaire can help structure discussions about symptoms, but it does not replace a clinical evaluation or establish the underlying cause of symptoms.
Can PEM Improve Over Time?
PEM does not follow the same course for everyone.
Some people report improvement in activity tolerance and fewer symptom exacerbations over time.
Others experience persistent or fluctuating symptoms, sometimes with substantial disability.
There is no reliable way to predict an individual’s recovery from PEM based on a single symptom pattern or test.
Pacing and energy management are commonly recommended to help people stay within their current limits and reduce the risk of symptom exacerbation.
However, pacing has not been established as a cure or as a guarantee of long-term recovery.
For practical suggestions about rest, cognitive demands and everyday adaptations, read Living With Post-Exertional Malaise: Practical Tips That Actually Help.
Frequently Asked Questions
Is PEM a recognised medical symptom?
Yes. PEM is a defining diagnostic feature of ME/CFS and is recognised in Long COVID clinical guidance.
Does PEM always begin 12–48 hours after activity?
No. Delayed worsening is characteristic, but symptoms can begin sooner. The timing varies between individuals and episodes.
Can PEM happen after mental effort?
Yes. Concentration, studying, working and other cognitively demanding activities can contribute to PEM.
Is PEM caused by deconditioning?
PEM should not be equated with deconditioning. Research has identified physiological abnormalities in some patients with PEM-related illnesses, although the mechanisms are still being investigated. Deconditioning may coexist with PEM and other medical problems.
Can a blood test diagnose PEM?
No single routine blood test can reliably confirm PEM. Recognition generally depends on symptom history, clinical assessment and consideration of other possible explanations.
Do I need a two-day CPET to prove I have PEM?
No. Two-day CPET is primarily a specialised research or assessment approach and may provoke significant symptom worsening. It is not required for routine clinical recognition of PEM.
Can PEM happen in conditions other than Long COVID?
PEM is especially associated with ME/CFS and is also recognised in Long COVID. Exertion-related symptom worsening is reported in other conditions, but not all activity-related fatigue or deterioration meets the characteristics of PEM.
Can PEM be treated?
There is currently no established medication that specifically cures PEM. Management focuses on individual symptoms, recognising exertional limits and reducing avoidable exacerbations, with appropriate clinical support.
The Bottom Line
Post-exertional malaise is not simply feeling tired after activity.
It involves a disproportionate worsening of symptoms that may be delayed and can take a long time to resolve.
For people with Long COVID, recognising PEM can be difficult because symptoms fluctuate, triggers are not always obvious and standard medical investigations may not capture the changes that occur after exertion.
Research, including two-day exercise testing, is providing evidence of physiological abnormalities in some patient groups.
But there is still much to learn about the mechanisms, diagnosis and treatment of PEM.
Recognising the pattern matters because it can help people receive more appropriate care and avoid advice that may worsen their symptoms.
References and Further Reading
- NICE (2021). Myalgic encephalomyelitis (or encephalopathy)/chronic fatigue syndrome: diagnosis and management. Guideline NG206. Read the recommendations.
- CDC. Long COVID Clinical Guidance. Read the guidance.
- Davenport TE, Stevens SR, Stevens J, Van Ness JM (2026). ME/CFS and Long COVID Demonstrate Similar Bioenergetic Impairment and Recovery Failure on Two-Day Cardiopulmonary Exercise Testing. Clinical & Translational Metabolism. Read the study.
- CDC (2024). Strategies to Prevent Worsening of Symptoms. Read the guidance.
Related Articles
- Post-Exertional Malaise in Long COVID: Why You Crash After Activity
- Living With Post-Exertional Malaise: Practical Tips That Actually Help
- Microclots and Exercise Intolerance in Long COVID
Disclaimer
This article provides general educational information and does not replace personalised medical advice. PEM symptoms can overlap with other conditions. If you experience significant or changing symptoms after activity, consult a healthcare professional familiar with Long COVID or ME/CFS. New chest pain, severe breathlessness, fainting or other potentially serious symptoms require appropriate medical assessment.
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