It can be difficult to explain symptoms for the general public or even your physician to understand.
How would you describe PEM?
Post-exertional malaise (PEM) Post-exertional malaise (PEM) is the worsening of symptoms following even minor physical or mental exertion, with symptoms typically worsening 12 to 48 hours after activity and lasting for days or even weeks. PEM can be mitigated by activity management (pacing).
Myalgic Encephalomyelitis/Chronic Fatigue Syndrome - CDC
Why would the fatigue occur hours after the exertion?
How could exercise induce cytokine elevation?
What is the mechanism that makes this symptom most unique to MECFS and Long Covid?
Still a lot of research to cover.
Moneghetti, Kegan J., et al. "Value of circulating cytokine profiling during submaximal exercise testing in myalgic encephalomyelitis/chronic fatigue syndrome." Scientific reports 8.1 (2018): 2779.
Post Exertional Malaise and CD40L