I think anyone at the severe end in the bedbound state would experience that. I can certainly relate to what @lansbergen is saying. I can remember my father asking me why I hadn't turned the light off (he was very keen on saving electricity!) and I struggled to explain that it was because I would have had to lift my arm to flick the switch. Similarly I couldn't get up from the floor my legs just weren't strong enough. That said I could always get out of bed and just about creep around very small distances. I wouldn't even have thought of doing push-ups. But I also feel as you do that the weakness is due to very low energy not strength. Because as the energy comes back the strength comes back pretty quickly. This has been my experience over a lot of years - I have gone in and out of this level of disability several times.