“It can resemble overtraining in athletes,” says Professor Nina Vøllestad.

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“It can resemble overtraining in athletes,” says Professor Nina Vøllestad.

Researchers can argue about a lot. But in one area, researchers worldwide have been remarkably unanimous:

Physical activity is healthy for both body and mind.

Even though intense training can hurt in the moment, the activity has a positive impact on everything from the heart and lungs to cognitive abilities and mood.

Similar positive effects are linked to activity on other levels, such as social activities or various forms of mental training.

But for ME/CFS sufferers, and some patients with other illnesses like long Covid and fibromyalgia, this is often not the case.

Instead, fairly mundane activities can send them to bed for days, with symptoms like extreme fatigue, flu-like feelings, pain, brain fog, and – paradoxically – sleep problems.

This is called PEM – post-exertional malaise.

Post-exertional malaise (PEM)

  • Exercise-induced worsening of symptoms is a characteristic feature of ME/CFS. It has also been reported among some long Covid patients and people with fibromyalgia.

  • The phenomenon is often referred to as PEM (post-exertional malaise). Some also call it PESE (post-exertional symptom exacerbation) or PENE (post-exertional neuroimmune exhaustion).

  • PEM involves a worsening of symptoms after physical, mental, or emotional exertion that would not have caused problems before the illness.

  • PEM can be triggered by cognitive, physical, emotional, or social activity.

  • PEM can cause a wide range of symptoms. The most common are extreme fatigue, feeling like you have the flu, body aches, sleep disturbances, and cognitive problems like brain fog.

  • The symptoms are often delayed and are typically worst 12 to 48 hours after the activity. They can last for days or even weeks. In the worst case, PEM can lead to long-term worsening of the illness.