Post-Exertional Malaise Overview

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.

Tired woman sitting on the edge of bed
JGI / Tom Grill / Blend Images / Getty Images

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 is the worsening of symptoms following even minor physical or mental exertion, with symptoms typically worsening 12 to 48 hours after activity and lasting for days or even weeks. 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.

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.

Early research showed no significant difference in physical fitness between those with ME/CFS and healthy, deconditioned people. Later, we learned that the bodies of people with ME/CFS appear to have problems with oxygen consumption the day after exercise, which a 2015 study by Miller, et al linked to a metabolic problem.

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.

Many patients and advocates point out that fearing the repercussions of PEM is perfectly rational rather than a phobia. After all, when you know something will take a major negative toll on you, why wouldn't you avoid it?

A Word From Verywell

The good news is that the research trying to blame deconditioning and kinesiophobia is becoming dated and no longer appears to be taking up the time and attention of researchers. Meanwhile, experts keep learning more about the physiological abnormalities of PEM, and as we understand it better, we learn more about how you can treat and manage this debilitating system.

2 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Centers for Disease Control and Prevention. Post-exertional malaise (PEM). Reviewed November 19, 2019

  2. Miller RR, Reid WD, Mattman A, et al. Submaximal exercise testing with near-infrared spectroscopy in Myalgic Encephalomyelitis/Chronic Fatigue Syndrome patients compared to healthy controls: a case-control study. J Transl Med. 2015;13:159. doi:10.1186/s12967-015-0527-8

Additional Reading

By Adrienne Dellwo
Adrienne Dellwo is an experienced journalist who was diagnosed with fibromyalgia and has written extensively on the topic.