The only thing i know of for tendons of your hands is to put your hand in a bucket of sand or rice and close and open it and the like, i don't know what that will do if anything.
apart from that the advice above about messing up your body to be a better weapon, your body isn't designed to be a weapon its designed to get you through life and once you loose function of a part of it it cant be replaced as easily as a stick can. A small function of life would require you to beat something to death with your bare hands and not pick up a rock or a stick for example. (and a stick and someone who can use one quite well will probably out compete you in fighting any day if you don't have one)
Second best advice, you cant condition for every hit you can take.
i would probably say some of these training routines are meant as a toughness test and to get you to toughen up mentally rather than physically, so you get used to feeling pain and having to go through it. Or just a lack of modern understanding of the bodies functionality if not the former. never the less, if you pursue anything do it cautiously and get medical advice if you get any issues.