I would not necessarily see it as not including muscle fatiguability, although it could be a bit wooly. ICC says this regarding PEM:
A. Postexertional neuroimmune exhaustion (PENE pen’‐e): Compulsory This cardinal feature is a pathological inability to produce sufficient energy on demand with prominent symptoms primarily in the neuroimmune regions.
Characteristics are as follows:
1. Marked, rapid physical and/or cognitive fatigability in response to exertion, which may be minimal such as activities of daily living or simple mental tasks, can be debilitating and cause a relapse.
2. Postexertional symptom exacerbation:e.g.acute flu‐like symptoms, pain and worsening of other symptoms.
3.Postexertional exhaustion may occur immediately after activity or be delayed by hours or days.
4. Recovery period is prolonged, usually taking 24 h or longer. A relapse can last days, weeks or longer.
5. Low threshold of physical and mental fatigability (lack of stamina) results in a substantial reduction in pre‐illness activity level.
So we have in point 1, physical and/OR cognitive fatiguability in response to exertion
but in point 5, low threshold of physical AND mental fatiguability.
Fatiguability is reducing ability to carry out work (activity) over time. Thus to me it implies weakness (muscle weakness is not required in point 1 but seems to be required in point 5.). Although it doesn't specifically say that so I suppose fatiguability from other sources could potentially be included (e.g. joint instability).
Muscle weakness could feasibly be included in points 2 and 3 but not well explained--though "flulike symptoms" is generally understood to include a debility that's perhaps a sort of immunological rather than neurological muscle weakness, and "exhaustion" could potentially include this, too, as serious exhaustion, heat exhaustion, and so on, would make a person weak
This is probably why some people prefer the very old definitions, or obscure definitions modified from those, as they explained the muscle weakness more clearly.
Fukuda, I think it was, specifically said "not asthenia" and "not somnolence" (not weakness and not sleepiness) which I think is straight-up wrong in the first instance, and wrong at least at some times or for some patients in the second instance. The field has been struggling to recover ever since, IMO.
However, the IOM report specifically mentions "weakness" in the
PEM section.