My question really is about the semantics of 'crash' and 'PEM'. I guess I used to use both these words somewhat interchangeably and wanted to ask PRers what they think. In my experience with ME, PEM, or a worsening of symptoms, happens after mental or physical overexertion and only when I have too many periods overexertion over a short enough time without sufficient rest (or a continually long overexertion) then the PEM continues to build until I have a crash. Rarely too I've had crashes that seem to have come out of nowhere and didn't appear to be preceded by PEM. So these words are distinct but related phenomena to me. What do you think?