Confused
Elbows you only supposed to slice with, and the opponents skin will give before your elbows.
Knees...I'm not sure on this one....do you mean like bones density conditioning, correct elignment...skin again?
A lot of it will come through good old pad work and bag work like you surmised. Baring breaking/fracture something through poor technique, I don't see any damage that could be done that wouldn't take till after a fight is over to manifest itself. Its like thigh conditioning, you don't really need it when you fight, its after and your recovery that any conditioning will make a impact on