I think we are on the same page here RichM. Your body is adaptable and cells renew in order to cope with the demands placed upon them. The areas of the body with the best blood supply renew quickest and the muscles which support a knee joint are definitely in this category. There is a small amount of tissue within a knee joint which is really not supplied with blood and hence not very good at replacing/strengthening. These are for the attentions of a surgeon, everything else is a case of challenge the tissue provoke the adaptation.