@Jonathan Edwards while I’m not disputing the fact that prolonged recumbency worsens OI, the idea that pwME should move a little more does not fit with my personal experience.
I certainly agree that pwME need to maintain a certain level of activity (whatever that looks like for a very severe/severe person) or sit upright for as long as they are able to, to prevent these additional physiological changes and OI worsening.
However I experienced OI for many years when I was undiagnosed and very mildly affected. I was working full time walking at a good pace around the hospital wards and during 23 hour on call shifts. By lunchtime and certainly by the end of the shift I would feel like I was on a boat, extremely brain fogged and was struggling to walk with postural and postprandial tachycardia.
With some rest these symptoms would die down. So my OI would exacerbate during PEM and clearly was not related to deconditioning.
I then had to stop working completely a few years ago as I was struggling to make it up a single flight of stairs at work with severe muscle pain and weakness. I was then bedbound for a year making it to the toilet and back, this felt like chronic flu. Now I’m moderately affected but still housebound and can’t stand up for long due to muscle pain/weakness which still remains.
My VO2 max at onset was 45% of expected! So exertion intolerance is a big factor for me not baseline fatigue.
If I try and push the boundaries of activity or not lay down/rest when I need to my OI flares along with my muscle pain and weakness.
So my feeling is that OI is tightly related to whatever pathology is going on in the brain in pwME which flares during PEM. Whether it’s immune driven changes in autonomic control, we cannot say yet. I also feel that the oxygen extraction problems that are appearing in Systrom’s work may be of significance too.
So I don’t think we can speculate how to best manage OI/PoTS etc until we have an understanding of the pathology. Until then pwME can only be guided by what feels right and if too much upright activity is making things worse overall then you’d be silly to ignore that and push through.