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Post-Exertional Malaise (PEM)

The central feature of ME/CFS, and a common symptom of Long COVID, is post-exertional malaise (PEM). PEM is when symptoms get worse after physical or mental activity.

PEM is very disabling and causes changes in symptoms and a further reduction in ability to do every-day activities: what you may be able to do one day, you might not be able repeat at the same level the next.

The level of activity that triggers PEM is different for each person. For some, it may be a going for a jog, a short walk or writing a homework assignment. For others who are severely unwell, it could be brushing teeth, reading a sentence from a book, or rolling over in bed. This makes ME/CFS and Long COVID difficult to manage.

The increase in symptoms and reduction in ability may be immediate or delayed by up to 72 hours. PEM can take 24 hours, days, weeks or months to settle back to baseline. It all depends on how much time you spend pushing through once your battery went flat.

Some of the most common symptoms that increase with PEM include:

  • pain
  • sleep problems
  • brain fog (problems with concentrating, processing information and memory)
  • flu-like symptoms (sore throat, muscle pain, increased fatigue)
    muscle weakness
  • increase in sensitivity to noise, light or touch.

Everyone is different and you may need to work out what PEM feels like for you.

What does the research say about PEM?

Emerge Australia’s 2018 Health and Welfare Survey found that:

89% of the 610 people who did the survey told us that increasing their level of exercise/activity resulted in their symptoms getting worse.

Similar results have been found in patient surveys in the UK, Norway and elsewhere. In one large study, physical exertion was the most common trigger for PEM and the onset happened within minutes.  When PEM was caused by mental activity, there was a delay before people experienced PEM. Pacing and avoiding specific triggers were common approaches to prevent PEM.1

What are the potential causes of PEM?

Research suggests that PEM might be the result of many different systems in the body not working as they should1.

These include:

  • problems with how cells make energy
  • inappropriate immune response to physical or mental activity
  • problems with directing blood and oxygen to places such as the brain and muscles
  • how your genes “express” themselves.

These problems are not due to deconditioning. They are signs from your body that you need to rest.

How do we know what happens during PEM?

One way to measure PEM is by using a specialised test called the “two-day cardiopulmonary exercise test (CPET)”3.

The CPET test triggers PEM and there is a risk it may cause a flair or an overall worsening of ME/CFS. In Australia, it is difficult to access this test and there are not enough healthcare practitioners who know how to do the test accurately for people with ME/CFS.

How does CPET measure PEM?
During this test, you exercise to exhaustion and measurements are taken to see how your heart and lungs are working.
The amount of oxygen in your blood and the amount of carbon dioxide you breath out are measured.
You then repeat the test the next day and if you have PEM, you will not be able to perform the test at the same level as the day before.
This shows that there is a problem with how the body responds not only to exercise, but normal activities like walking, cooking or showering.

What should you do when experiencing PEM?

Stop, rest and learn to pace

For more information visit our Stop, Rest, Pace information page.

Building your Post-Exertional Malaise (PEM) Toolkit

LIVE webinar hosted on Wednesday 11 May, 2022 at 2pm (AEST)

Watch the webinar here

Join Emerge Australia’s Manager of Patient Support and Information Services Laura Allen and Nurse Educator Kate Herbert in an interactive session to support you in building your PEM toolkit! This powerful session pools the collective wisdom of our amazing ME/CFS community to support you to come away with new tools to manage PEM.

This session is designed for those living with ME/CFS or caring for a person with ME/CFS. It will address various levels of severity and provides all attendees with a chance to both share their personal experience and learn from the experience of others.


1 Bateman, L. et al. Myalgic Encephalomyelitis/Chronic Fatigue Syndrome: Essentials of Diagnosis and Management. Mayo Clinic Proceedings 96, 2861–2878 (2021).
2 Light, A. R. et al. Gene expression alterations at baseline and following moderate exercise in patients with Chronic Fatigue Syndrome and Fibromyalgia Syndrome. Journal of Internal Medicine 271, 64–81 (2012).
3 Keller, B. A., Pryor, J. L. & Giloteaux, L. Inability of myalgic encephalomyelitis/chronic fatigue syndrome patients to reproduce VO₂peak indicates functional impairment. Journal of translational medicine 12, 104 (2014).
4 Décary, S. et al. Humility and Acceptance: Working Within Our Limits With Long COVID and Myalgic Encephalomyelitis/Chronic Fatigue Syndrome. Journal of Orthopaedic & Sports Physical Therapy 51, 197–200 (2021).

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