does anyone have any idea what it is that stops bear's muscles and bones from wasting away when they hibernate?

Who's to say they don't? (No idea personally)

I think atrophy caused by disuse is controlled by the nervous system (which would be why nervous disorders like Lou Gehrigs disease result in it) so there would be no reason for the muscles and bones to waste away during a normal hibernation. The muscles might be broken down to provide nutrients for the body though.

Here's a google cache of a short Nature article on it.

 

(note, you have to use google cache, otherwise its password protected)

 

 

http://216.239.57.100/search?q=cache:dX1eXIXfZ2MJ:www.nature.com/cgi-taf/DynaPage.taf%3Ffile%3D/nature/journal/v409/n6823/full/409997a0_r.html+hibernating+atrophy&hl=en&ie=UTF-8

 

"Skeletal muscle protein and strength can be conserved by using alternative sources of protein, by recycling urea nitrogen back into protein synthesis, or by rhythmically stimulating the muscles. Hibernating bears may therefore be able to retain their strength by synthesizing new amino acids and protein from urea nitrogen11, or by shivering and undergoing isometric muscle contraction through the winter, or by drawing on labile protein reserves such as visceral smooth muscle and extracellular matrix. "

http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?holding=npg&cmd=Retrieve&db=PubMed&list_uids=1117761&dopt=Abstract