"using any of the body's systems makes them stronger"
Only partly true. This is true in the short term for younger people. It's not true in the long term for older people. Your metabolism results in different kinds of intracellular junk and intercellular junk accumulating. After a certain point, it reaches the level of pathology. But before that happens, using those metabolic pathways does tend to make them stronger.
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"using any of the body's systems makes them stronger"
Only partly true. This is true in the short term for younger people. It's not true in the long term for older people. Your metabolism results in different kinds of intracellular junk and intercellular junk accumulating. After a certain point, it reaches the level of pathology. But before that happens, using those metabolic pathways does tend to make them stronger.