Discovery or Nat Geo did some thing on physical conditioning of the body for fighting.... they did various scans of boxers, MMA figthers, and other disciplines and found that they all had fibrous tissue around their rib cage that was a response to impact, and the bone density of their skulls had increased. Surface nerves on all areas of the body had been desensitized and the bone beneath had hardened and was covered in deposits that made them more durable.
Their stomach muscles in particular had thickened in a way that was different than resistance training (I don't remember the exact details), but it was all adaptations from taking hits. The body is incredible, it is so awesome to me that you can actually make yourself incredibly tough by taking beatings all your life.