Hang from a chin bar, practice mid fingers as well as full fist grip. If this is too easy, hang from one hand for a set time. Tendon strength involves stressing them with micro tears, rebuilding them after. Takes a while. Ever noticed many old school industrial mechanics or carpenters hands? years of twisting bolts in tight spaces and wrenching things. Another is wood chopping with an axe, or sledge work on hard objects. Ever used post hole diggers? Pain, feeling the burn in your hands but you have to keep going. Even the skinny guys that do this have catchers mitts for hands. After all that work your hands won't be as pretty, but the first throat you grab is in for some serious drama. Many men these days have some pretty hands.