Cross, True makiwara does cause callous on the knuckles but not always problems in later life. Depends on how you train on it. Alos they do harden the hands quite a bit allowing one to hit things much harder without damage to hand. But one thing people have to understand is it take years to develop this. Not weeks or months years if you want to do it correctly and not cause immediate damage to your hands. Note Makiwara training is not just for the knuckles, it is also used for kicking (instep Ball and toes), forearm, elbow, shuto, palms, and any other striking surface you can think of.