There’s a feature of some Long COVID cases (~50%) which is also the defining feature of an illness called ME/CFS which has been caused by various forms of viral infections throughout history. (It is thought that a lot of Long COVID cases are ME/CFS). Anyways this feature is, Post-Exertional Malaise, a worsening of the illness after exertion beyond a certain threshold, which can entail hundreds of symptoms and be permanent.
This paper is a review of some of the biomedical studies looking at what could possibly cause this, and finds there is repeated data of Microvascular (blood vessels) and immunometabolic (metabolic markers relating to immune function) differences with healthy controls.
The leading hypotheses are that this is caused by mitochondrial dysfunction which is mediated by a dysregulated immune system.
Some of my colleagues were co-authors on this paper. I’ll forward the feedback that it is jargony.