Exertion

Exertion is the physical or perceived use of energy. Exertion traditionally connotes a strenuous or costly effort related to physical, muscular, philosophical actions, and work, resulting in the generation of force, initiation of motion, or in the performance of work. It often relates to muscular activity and can be quantified, empirically and by a measurable metabolic response.

Exertion in ME/CFS
Only a minimal amount of exertion causes a marked increase in symptoms in people with ME/CFS, for instance chronic fatigue, chronic pain, cognitive dysfunction (e.g., brain fog), flu-like symptoms, muscle fatigability, unrefreshing sleep, and more, this is known as ME/CFS's hallmark symptom post-exertional malaise (PEM). Depending on a patient's disease severity exertion capabilities is different and results in varying symptoms and degree of symptom severity. Physical or mental exertion can trigger PEM.

Intolerance to exertion and exercise was built into the renaming of ME/CFS by the National Academy of Medicine in the 2015 Institute of Medicine report where the name Systemic Exertion Intolerance Disease was coined.