Chairs are a relatively new phenomenon
Have a listen to this podcast
. It talks about "sitting" as a part of the evolutionary process. It is a relatively new phenomenon and societies where kneeling or squatting are prevalent manage to avoid some of the major illnesses and diseases that we encounter in more "developed" societies.
How you rest has a huge impact on your health. Having your muscles turned on "just a little bit" will help your health enormously.