I think your muscles gradually turn to tapioca pudding....

I really don't know the answer, medicaly, and it is an interesting question. I can add that I didn't ride my bike for 6 months (with the last 2 months doing NO exercise at all). Since starting again, my legs are definitely weaker - they hurt more quickly, are more achy the day after a hard ride than they used to. Somehow the muscles get "soft", but I don't know what that means from an actual muscle make up standpoint.

My lungs and lung capacity, on the other hand, are MUCH better than they were when I was riding last year - but that's a different story!!