You have a good day, so you clean the kitchen, run errands, and maybe take a walk. You feel fine that evening. Then, a day or two later, you’re flattened: exhausted, achy, foggy, and barely able to get off the couch. It can take days or weeks to climb back.
That delayed crash has a name: post-exertional malaise (PEM). Recognizing it is one of the most important steps in managing long COVID.
What PEM is
PEM is a worsening of symptoms after physical, mental, or emotional exertion that would not have caused problems before. Its defining features:
- It’s delayed. The crash often starts 12 to 72 hours after the activity, which makes the connection easy to miss.
- It’s out of proportion. The trigger can be small, like a shower, a phone call, or a grocery trip.
- It lasts. Recovery can take days, weeks, or longer.
- It involves more than tiredness. Brain fog, pain, sleep disruption, sore throat, gut symptoms, and a racing heart can all flare.
PEM is the hallmark symptom of myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS). It is also common in long COVID. The RECOVER study identified it as one of the symptoms that most strongly distinguished people with long COVID.
It’s biological, not a matter of willpower
A 2024 study in Nature Communications (Appelman et al.) took muscle biopsies from people with long COVID before and after an exercise test that triggered PEM. Compared with people who had recovered from COVID, the long COVID group showed:
- lower mitochondrial function in muscle,
- more muscle tissue damage after exertion, and
- deposits of abnormal proteins in muscle tissue.
The research is ongoing, but it points to measurable changes in the body. This is not deconditioning or lack of motivation.
Why “push through it” backfires
Traditional advice for fatigue is to gradually increase exercise. For people with PEM, that approach can cause repeated crashes and setbacks.
In 2021, the UK’s National Institute for Health and Care Excellence (NICE) updated its ME/CFS guideline. It no longer recommends graded exercise therapy, which is based on fixed increases in activity, and advises against programs that don’t account for PEM.
That doesn’t mean movement is off the table. It means activity has to be matched to what your body can tolerate without triggering a crash. This approach is often called pacing, and it works best when guided individually.
Signs you may be dealing with PEM
- You feel worse a day or two after activity, not during it
- “Good days” are often followed by bad ones
- Mental effort, like a work meeting or a difficult conversation, can trigger a crash
- Resting doesn’t quickly reverse the crash
- Your tolerance for activity has shrunk since COVID
How it is evaluated
- An activity-symptom timeline: tracking what you did and how you felt over the following three days often makes the pattern visible.
- Related findings: a racing heart on standing, sleep disruption, brain fog, and gut symptoms often travel with PEM. See our page on POTS after COVID.
- Ruling out other causes: anemia, thyroid problems, heart and lung conditions, and sleep disorders can all cause exercise intolerance.
- Coordination: with cardiology or pulmonology when exertional symptoms raise specific concerns.
When to get emergency care
Call 911 for:
- chest pain or pressure during or after activity,
- fainting,
- severe shortness of breath, or
- a racing heart with chest pain or confusion.
Serving Oklahoma City, Tulsa, and all of Oklahoma
Venturis Clinic sees patients from Oklahoma City and surrounding areas, including Tulsa, and from across Oklahoma. Learn more on our Long COVID Treatment in Oklahoma City page.
Frequently asked questions
What does a PEM crash feel like? Most people describe a flu-like collapse: profound exhaustion, aching, worsened brain fog, poor sleep, and sometimes a sore throat or racing heart. It usually starts a day or two after the trigger.
Should I stop exercising completely? Not necessarily. The goal is finding a level of activity that doesn’t trigger crashes, which is best worked out individually.
Is PEM the same as being out of shape? No. Deconditioning improves with gradual training. PEM often gets worse when activity is pushed.
Can mental activity cause PEM? Yes. Cognitive and emotional exertion can trigger crashes, not just physical activity.
Stuck in the crash cycle? Book a free 15-minute discovery call, or learn more about long COVID care in Oklahoma City.
This page is for educational purposes and is not a substitute for individual evaluation.
By Alvin Philipose, DC, ICCP
Sources
- Appelman B, et al. Muscle abnormalities worsen after post-exertional malaise in long COVID. Nature Communications. 2024;15:17.
- National Institute for Health and Care Excellence. Myalgic encephalomyelitis (or encephalopathy)/chronic fatigue syndrome: diagnosis and management (NG206). 2021.
- Thaweethai T, et al. Development of a definition of postacute sequelae of SARS-CoV-2 infection. JAMA. 2023;329(22):1934–1946.