OTOH, muscle wastage in and of itself can reach a point where it starts to affect function and it takes you more energy to do with weak muscles than what you could do with stronger ones.
Whilst in principle this is a valid idea......
I would only apply it to muscles which are essential, such as core muscles. These need to be strong enough to support the weight of your upper body or they will fatigue, very rapidly, and they will be liable to overstrain and damage. These muscles are only likely to get too weak if you don't sit or stand enough e.g. if you are mainly bed bound, although my old friend poor posture can have an impact on this. For example if, like me, you sit slouched and propped up by pillows.
I personally haven't worked out a useable solution to this problem with this particular muscle "group", but in theory it could be helped by several simple floor based bodyweight exercises e.g. planks, leg lifts etc., but I find these sorts of exercises much more damaging than I found weights, there is bound to be something that "helps", if it's deemed medically enough necessary, I'm just in a situation were I can't afford the risk/cost of trying stuff out.
But...the only muscle groups I would consider worth
any attempt at strengthening are those that comprise the "core", nothing else (no other muscle group) is likely to weaken you enough if too weak to be worth the cost....and in general I've found that eventually, when in an upturn, my core is in fact strong enough, in such a situation my general strength is at least 5 times greater, I've measured it lol. It's the effects of ME that's making it not so, and training when in a downturn - quite a bad idea IMO.
ETA - sorry, somewhat garbled but I hope it makes the point I was intending, if a bit waffly, I did a bit too much yesterday, and I have a cat telling me to calm down and go to bed, using the medium of yelling in my ear every 10 seconds.