Marked, rapid physical and/or cognitive fatigability in response to exertion

Marked, rapid physical and/or cognitive fatigability in response to exertion, or rapid fatigability in response to exertion (which may be minimal such as activities of daily living or simple mental tasks) is a compulsory symptom of the International Consensus Criteria for myalgic encephalomyelitis. . The fatigability is not simply sudden but is frequently debilitating, and cause a relapse.

Prevalence
The marked, rapid and usually highly debilitating fatigability is one of the compulsory criteria of post-exertional neuroimmune exhaustion (PENE), the hallmark symptom of ME in the International Consensus Criteria. It is also mentioned in many other diagnostic criteria definitions of ME/CFS including the current CDC ME/CFS criteria.