ALL of your ones CAN be strengthened by "micro-fracturing" (Wolf's Law). In my lifetime, I have been shot six times. Twice in the head! HOWEVER: Those were low angle, glancing blows to my head that gave me a concussion, and my ears still ring to this day. Even when strengthened so, there's still limits to the kinds of impacts that you can take that will cause organ damage within. I broke my collar bone at age 12... Getting launched across a creek and into a large maple tree by a Boy Scout catapult. They can break, but it takes a great deal to do it. The idea of the ULTRA, was to take a human agent to their farthest limits and make them more survivable to return from missions and become veteran in performing such. Most of us can be blown across a field and be back in full action within a few days or more for some of the worst injuries in the field.