You only develop strong muscles and bones if you put some stress on them in the form of exercise. Without that stress demonstrating the inadequacy of the existi

You only develop strong muscles and bones if you put some stress on them in the form of exercise. Without that stress demonstrating the inadequacy of the existing systems our efficient bodies don't waste energy building muscle and bone. It stands to reason that the other systems of the body similarly only develop when they are required. They need some level of stress put on them to develop to their capacity.