According to Wolff's law, a bone becomes more dense when contact is applied to it. So for example, if you run, your feet are going to be more dense than those of someone who doesn't.
Can this be applied to other things? I have heard that your skin gets wrinkles because of a loss of bone density.
In kung fu, there is a technique called Iron Body by which you hit yourself and it creates tougher bones and you can receive harder strikes without harm. This is when you see guys breaking boards over their chests, backs, etc. Could this be used as part of an overall fitness routine? At first, softly hitting your arms with canvas sacks filled with mung beans and then progressing over time. Discuss possible problems as well advantages of doing this kind of technique.