I asked doctor and speaker at an MECFS event a similar question, in relation to severe MECFS, whether the patient is completely bed bound. They said that although muscle atrophies, that the cells themselves don't die. For someone who is completely bed-bound, they said that this is good news because they can prioritise pacing and if they recover later, can their muscles.

This was part of a much wider context which I've omitted here but as a fragment of information it might be reassuring to anyone who finds themselves (hopefully only) temporarily debilitated.