Much of who you are as an adult is a modified — stress-adapted — version of an original template. What would you be like if you had never had anything but extremely mild physical stresses in your life? What if you grew up in a padded, zero-G room? You would literally be fragile. Your bones would be like Styrofoam. You would look different: your skin would be a different colour and texture, your joints a different shape. You wouldn’t be able to do much of anything in normal gravity.
Most of who you are and what you are physically capable of is the result of adaptation to stress.
All of biology is organized around adaptive reactions to stimuli: immediate reflex (neurological) responses, nearly immediate hormonal and behavioural responses, tissue change responses. Every response is about improving conditions for the organism. Starting to fall down? The organism reacts to stop it. Regular pounding impact on your legs? Make the bones stronger. Etc.
Take away the stresses, and the body steadily stops investing energy in unnecessary adaptations. Adapting is “expensive.” You don’t do it if you don’t have to.
http://saveyourself.ca/articles/biological-literacy/use-it.php