My dad taught us that the human body is like an automobile. If you take care of it, it will function. If you don’t it wears out. Wise words indeed. Before I build my argument that he was only partially right let me make a key differentiation. A car is static. On its own without intervention it can’t repair. On the other hand a human is a living organism that can self-protect. In fact, it is designed to heal itself. Left to itself it can deal with most things. Remember we didn’t just get here without our predecessors coping with unimaginable challenges.
When the automobile is confronted with a rampaging tiger, it does not suddenly grow wings. A human body on the other hand responds by secreting super enzymes that change the way your body responds. You may not visibly see it, but your body literally did grow wings. It was those wings that gave you that extra speed as you ran across the prairie to save your hide. An automobile and a human body deal with stress differently.
I would not recommend that you constantly test your agility against a tiger. Shock though is an excellent tool to develop your body. The shock of ice-cold water in a otherwise warm shower. The shock of your sensei pounding the bones of your body as they harden. The shock of running 6 miles on the muscles around your knees. The shock of not eating anything at all. You get my point.
Each time, your body reacts and responds to the challenges thrown at it. It is just not visible to you dear human. With time and proper guidance, your body is able to overcome the challenge. This is what gives Buddhist monks the ability to sit in freezing cold wearing almost nothing at all.
Now hold on, don’t go around breaking your bones and claiming I told you to do so. Your goal is to shock not break your body.