text: why does weight training build bone mass?   imo this is one of the ways our body is built to be adaptive.  for people who are exposed to a constant need to lift heavy objects, the body sends a message to the bone cells to add more bone to handle the extra demand that they will need.  another example of this adaptivity is modifying the metabolism to use fewer calories when experiencing a famine and keeping this change after the famine ends.  unfortunately for us modern folk who never experience famine unless we are dieting, this makes losing weight even harder!