Post Exertional Malaise (PEM)

Part 1 - What is PEM?


Post-Exertional Malaise “has given rise to a lot of puzzlement and in some cases suspicion among family members, physicians who hear about the report and especially among the disability insurance companies. They don't understand how you can be in the stands watching the game one day and incapacitated for the next two. They see you at the store and can't understand why you can't make any other social engagements for the next couple of days. So as a result, individuals with this illness often have their integrity challenged.” ​

Dr Peter Rowe



PEM is often described as a delayed (generally 24 to 48h) exacerbation of symptoms following physical or cognitive exertion and resulting in reduced functional abilities. Considered by many patients and specialists to be a core defining symptom of MECFS, it is difficult to diagnose and understand for outsiders. This first of a three-parts review will focus on studies describing what the main features of PEM are.

Triggering factors

The use of the word "exertion" may give the impression that PEM is caused by intense activity, but that is far from reality. PEM appears whenever a patient exceeds the limits imposed by the disease. Depending on the severity of their condition, this threshold may be very low: a one-hour stroll, a few hundred meters walk, looking for the mail, or simply brushing one’s teeth for the most severely affected. A series of small efforts slightly exceeding the capabilities of the patient can have the same effect.

Onset / duration

Most of the time, PEM appears a day or more after an effort, but sometimes it can happen immediately.

The recovery can take days or even weeks. In a 2010 VanNess study, patients and a control group recorded their symptoms for 7 days after a maximum exercise test. 24 hours after the test, 87% of the controls had completely recovered and 100% after 48 hours. None of the CFS patients had recovered after 24 hours and only 4% 48 hours later. The majority (60%) needed at least 5 days to recover from this simple effort test.



Symptoms Exacerbation

Post-Exertional Malaise cannot be reduced to mere fatigue. As reported by the Institute of Medicine (IOM), "Patients may describe it as a post-exertional “crash,” “exhaustion,” “flare-up,” “collapse,” “debility,” or “setback." PEM amplifies all symptoms and in addition to fatigue and decreased functional abilities, can lead to flu-like symptoms.

As shown in the graph below, summarizing the 2010 VanNess study’s results, the impact of exercise is extremely different for the two groups. The symptoms reported by the ME/CFS group are more numerous, of greater severity and last much longer than those of the control group. None of the patients felt better immediately after the bike test against 75% of the controls.



Effect on pain

Many studies have shown that exercise increases pain in patients with ME/CFS, compared to healthy or sedentary subjects. A study conducted by Nijs in 2010 measured this paradoxical effect of exercise on painful sensation for people with ME/CFS. Before exercise, the patients and the control group had approximately the same pain threshold (measured with pressure on the legs, hands and back with an algometer), but whereas in healthy subjects, exercise raises the pain threshold, in ME/CFS it is lowered.


Cognitive activity

Although cognitive problems are common in ME, few studies looked at the effect of exertion on cognitive function. While some studies found no differences before and after exercice, others, like this study by Cook and Light, highlighted an exacerbation of cognitive symptoms (difficulty concentrating, slow processing of information)*.

People with ME/CFS and a control group underwent a battery of 3 tests before and after exercise. In the first phase, both groups start at the same level. Controls do better over time, and 24 hours later, even better. For the ME/CFS group, the exact opposite happens. Their cognitive performances decline over the 3 exercises. And 24 hours later; they start at a worse level than the day before and continue to make more mistakes afterwards.

A cognitive effort can also lead to a decline in intellectual performance. But this field has been little studied.


*According to the IOM report, “these inconsistent results may be attributable to variations in subject selection, exercise testing procedures, and cognitive testing, which inhibit direct comparisons among studies.”

Inability to sustain activity over time

In a 2005 Christopher D Black study, participants enrolled in a program aimed at a 30% daily step number increase, were initially able to reach the goal. But between 4 and 10 days after the start of the program, patients were unable to maintain this rhythm and their capacity began to decline very significantly, unlike the control group.