Image, this and following pages, Alex Borland
Amendment XXVIII, (proposed), Section 4:
"When either House has too few Members able to attend to reach quorum, [...]"
Except regarding Presidents and Vice Presidents, the Constitution doesn't address incapacitation. Quorum, and continuity of government, can suddenly be lost if Senators or Representatives die or are incapacitated, for example after an attack on the Capitol. Medical incapacity is addressed, and inability to attend while in prison if not yet expelled.
This section allows for temporary replacement when quorum is lost for any reason if less than three months remain in the session, or before that if the President declares a state of emergency.
It doesn't require members to attend, just to be able to attend. Nor does it require members to take part. Members don't always vote as it is, whether that drops the chamber below a ‘functional quorum’ for that vote or not. Quorum is determined by their presence.
When members' attendance can't be meaningful due to medical conditions they usually resign. Implementing legislation should require it.