Today they have no - slip, no - slide surfaces covered with rubber or synthetic forms of rubber or plastic. (articles.orlandosentinel.com)
Friction only arises at the locations where the highest hilltops on the two sliding surfaces touch one another. (sciencedaily.com)
I think the answer to this lies in what kind of changes or flexibility you are trying to develop: fasical, muscular, capsular, neural, other sliding surfaces? (rugbystrengthcoach.com)