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.

This post-exertional malaise is not limited to just musculoskeletal pain or fatigue, and does not occur in illnesses like depression, multiple sclerosis, or systemic lupus erthematosus or rheumatoid arthritis.

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 (SEID) was coined.

Notable studies
2008, Can exercise limits prevent post-exertional malaise in chronic fatigue syndrome? An uncontrolled clinical trial. (Full text)

2014, The delayed fatigue effect in myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS) (Abstract)

2015, "Beyond Myalgic Encephalomyelitis/Chronic Fatigue Syndrome" - IOM report (Full text)

2018, Deconstructing post-exertional malaise in myalgic encephalomyelitis/ chronic fatigue syndrome: A patient-centered, cross-sectional survey (Abstract)