Like Mammoth was saying the limbs are easy to thicken because the body does lay down calcium...like the Thai boxers kicking trees, Karate guys hitting Makiwari boards, etc., but my understanding surrounding the nose is that once it's been broken the more likely it is to break again. You can strengthen the muscles of the face/jaw line but I think you're pretty much S.O.L. with the nose. Max bone density is reached between 20 and 35 years of age, and resistance training is effective at maintaining it, but it will not increase beyond it's max point. But you can't do it for your nose. Maybe try improving your bobbing and weaving skills so you don't deflect so many hands with your nose. I'm sorry that's not a lot of encouraging info.
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