i really wonder about it, i asked my mentor about it one time, is it possible to condition your hands, and mentioned the iron palms training and so on. he said something that i think is basically true. that with the way the hand is configured that it's not really possible to make it so there isn't a big risk of breaking your hand. of course, if you have a static object, you can pretty much land the way you want but the human head is both hard, not consistently shaped and is probably going to be moving too, all of those make it seem impossible not to have hand problems. my hands haven't bothered me in years but I don't spar anymore and when i hit the bag, i always wear protection. the only thing i can say happens when i take long breaks from training is that my wrist buckles when i start back, but that's why i have the wraps and usually that stops after only a few training sessions. i must say though, that my right hand doesn't seem as facile as a pianist as it was when i was in my twenties, especially between my middle and ring fingers.