What is Post-Exertional Malaise?

Part 1: The Basics & Reasons for Disbelief

Tired woman sitting on the edge of bed
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Post-exertional malaise (PEM) is such an important part of chronic fatigue syndrome (ME/CFS) that you really can't understand the disease without Understanding the symptom. It's guided a tremendous amount of ME/CFS research, is theorized to be the key to an objective diagnostic test, and is even behind the new suggested name for the condition — systemic exercise intolerance disease.

Still, however, some members of the medical community don't believe that PEM exists.

Instead, they blame the negative response to exercise on deconditioning; they blame exercise avoidance on a psychological condition called kinesiophobia. In a nutshell, they think a whole bunch of people are just out of shape and irrational. (Spoiler alert: research suggests otherwise!)

Meanwhile, a large and continuously growing body of evidence suggests a wide array of physiological abnormalities behind PEM. This symptom substantially limits the activity levels of people with ME/CFS and lowers the quality of life considerably. In severe cases, it defines their lives entirely.

Understanding Post-Exertional Malaise

PEM causes intense exhaustion as well as an upswing in other symptoms that last for at least 24 hours after physical exertion. That may not sound so unusual to those unfamiliar with it — after all, we all need time to recover after a tough workout.

When it comes to PEM, though, little about it is normal or familiar to people without ME/CFS.

It's not just about overused muscles or needing a little extra rest.

PEM can range from moderately stronger-than-normal symptoms to completely disabling. In a mild case, the person may have extra fatigue, achiness, and cognitive dysfunction. In a severe case, PEM can bring on intense flu -like symptoms on top of extreme fatigue, pain, and brain fog strong enough that it's hard to even form a sentence or follow the plot of a sitcom.

That's hardly what the rest of us go through after a hike or a trip to the gym. Also abnormal is the amount of exertion it can take to put people in this state.

As with the severity, the exertion needed to trigger PEM theories case-by-case. For some, it might kick in after a little bit of exercise on top of a day's regular activities. For others, is incredible as it may seem, it can just take a trip to the mailbox, a shower, or sitting upright for an hour.

The Belief That it's Not Real

If PEM is so disabling, how can some doctors believe that it doesn't even exist?

Part of the problem is the lingering skepticism that ME/CFS itself is real. Adding to that is how significantly activity levels change after the onset of the disease coupled with how long it takes for a diagnosis.

Current diagnostic criteria require the symptoms have been constant for at least six months. That's plenty of time for somebody to become deconditioned. The reality of this condition, though, is the diagnosis often takes much longer. If someone's been unable to tolerate much exertion for two or three years, it's hardly a surprise that they'd be out of shape.

Research supports PEM being more than mere deconditioning. (Bazelmans) A study published in Psychological Medicine demonstrated that there was no significant difference in physical fitness between those with ME/CFS and healthy, deconditioned people in the control group.

Another study (VanNess) involved exercise on two consecutive days. Researchers found that people with ME/CFS were unable to repeat their performance on the second day, in contrast to the control group.

They also found that oxygen consumption dropped in the ME/CFS patients, but not controls, on the second day. Researchers concluded that it was not deconditioning but more likely metabolic dysfunction causing the diminished exercise capacity. Later research also suggests differences in oxygen consumption and metabolism are linked to PEM. (Miller)

Some doctors also say that the fear of exertion displayed by many people with ME/CFS is actually an irrational fear of exercise called kinesiophobia.

The research in this area is somewhat mixed. Some studies have concluded that kinesiophobia rates are high in people with this condition and that it does play a role. At least one agrees that kinesiophobia is common but state that it does not appear to determine daily physical activity. Others found no correlation between fear of exercise and exercise performance. (Nijsx3, Silver)

Many patients and advocates point out that fearing the repercussions of PEM is perfectly rational and has a protective mechanism rather than a phobia.

Causes & Physiological Differences

Learn more about PEM:


1. Bazelmans E, et al. Psychological medicine. 2001 Jan;31(1):107-14. Is physical deconditioning the perpetuating factor chronic fatigue syndrome? Controlled study on maximal exercise performance and relations with fatigue, impairment and physical activity.

2. Miller RR, et al. Journal of translational medicine. 2015 May 20;13:159. Submaximal exercise testing with near-infrared spectroscopy in myalgic encephalitis/chronic fatigue syndrome patients compared to healthy controls: a case-controlled study.

3. Nijs J, et al. Physical therapy. 2004 Aug;84(8):696-705. Chronic fatigue syndrome: lack of association between pain -related fear of movement and exercise capacity and disability.

4. Nijs J, De Meirleir K, Duquet W. Archives of physical medicine and rehabilitation. 2004 Oct;85(10):1586-92. Kinesiophobia in chronic fatigue syndrome: assessment and associations with disability.

5. Nijs J, et al. Disability and rehabilitation. 2012;34(15):1299-305. Kinesiophobia, catastrophizing and and anticipated symptoms before stairclimbing and chronic fatigue syndrome: an experimental study.

6. Silver A, et al. Journal of psychosomatic research. 2002 Jun;52(6):485-93. The role of fear of physical movement and activity in chronic fatigue syndrome.

7. VanNess JM, Snell CR, Stevens SR. Journal of chronic fatigue syndrome. 2007 14(2): 77-85. Diminished cardiopulmonary capacity during post-exertional malaise.