Prolonged immobilization will weaken a muscle, but the brain dysfunction is by far the most damaging. You can recondition muscle fast (unless you have something else wrong like ME) but the brain rewiring is hard to fix if it atrophies too much.
I went to a very talented and educated physical therapist about my low back pain, and this is what she found -- missing muscle-brain connections. Every exercise she gave me was designed to reconnect the brain wiring, as well as spare my energy stores. (Almost all of them are lying-down exercises.)
I have heard that babies dream about performing physical movements before they do them. It's why they spend so much time sleeping. (How on earth would anyone figure that out, though?)
When learning Argentine tango (or any other dance), it really helps to go over a sequence mentally. Over and over. The students who don't retain anything new from a workshop (most of them) are the ones who don't do this. Here today, gone tomorrow.
The brain is fascinating. When I was 16, my French teacher gave me a novel to read over the summer. I started dreaming in French. When I took Russian in college, French kept popping out of my mouth initially. When I taught college French, some students had interference from their first second-language. The brain just goes right into second-language mode, once it knows that mode. N'est-ce pas,
@Gondwanaland?