edit: I wrote this considerably brain-fogged. Apologies for my clumsy wording.
I am interested in the idea that what needs to be identified is some sort of 'motor fatigue signature' for ME. They are doing this for Parkinson's disease. My impression as an observer at meetings where PWME mix with 'normal controls' is that just like people with RA, PWME have learnt activity conservation strategies that you can see in action. The number of movements is not necessarily less but the movements are made economically.
This sounds very interesting. Also good to see that you still think that motor fatigue could be of relevance,
@Jonathan Edwards.
I think the conservation strategies for PWME are probably quite different because they do not relate to specific joint pain problems but to a need to avoid certain types of muscle usage. I may be wrong but I am not aware that anyone has really tackled this aspect. It was suggested to me be
@Esperanza and it keeps coming back to me as potentially important.
I think you meant me (I'm not Esperanza, but another Esperanza

)
Apologies for going off topic a bit, but I would like to add that the type of motor fatigue that I have now first occurred not before the 10th year of being ill with ME. I experienced this type of motor fatigue first in my hands and then in my legs. I never would have described these impairments as motor fatigue. I experienced temporary stiffness, weakness and spasms in my hands and legs. But two docs said this was severe motor fatigue when I showed them a video documenting my strange gait (my partner had filmed this).
The most debilitating symptoms in the first years of my illness were not motor fatigue or muscle weakness, but 1) vertigo induced by fast movements or longer exercise and 2) delayed PEM which always felt as having a relapse of Mono (but wasn't).
My temporary impairments in moving or coordinating movements that I developed during the first years of being ill occurred in different types of fatiguability, either only few minutes after doing fast movements, or after longer exercise. I could still go for walks or cycling for up to 45 minutes, sometimes longer, before severe vertigo and cognitive issues occurred. Muscle stiffness or muscle weakness did not regularly limit my activities those days, perhaps because vertigo stopped me before they would?
However, I experienced phases of stiff legs or feet and sometimes pain or clumsiness or a kind of numbness in different limbs at different times over several days or weeks, always remitting completely. I don't recall any trigger for these phases in the first 10 years of being ill, except once, when I couldn't move my left arm properly after a gastrointestinal infection and having a high temperature.
So, in retrospect I am not sure when I began to develop motor fatigue or whether this would have been measurable in the first years of my illness. I surely had coordination problems with some movements when doing these repeatedly or fast, but I did not react with muscle stiffness or sometimes complete inability to move as I do now.
My neurologist still seems to think I could have some weird neurological or neuromuscular condition other than ME, after I was suspected to have MS over two years and high dose cortisone pulse therapy helped me significantly, but MS finally was found to be less probable. But this is another story I hope I will able to tell someday.
All I wanted to add is that, although I think motor fatigue could be an important part of ME, it
probably perhaps is not always present in ME or not in every stage of the illness? Maybe other impairments will limit activity before motor fatigue could occur?
However, I think that some typical ME symptoms like muscle weakness or stiffness might get confused with motor fatigue sometimes (edit: I mean that these impairmens might actually occur due to motor fatigue).