Continuously drive a machine too hard for too long and it will breakdown. Repeatedly exercise your body too hard for too long and it will get hurt.
Rest a machine for a few days after a period of intense use and nothing changes. Rest your body for a few days after a bout of tough exertion and it will comeback stronger.
Unlike mechanical machines the human body can adapt to the demands placed upon it.
All it needs is the right exposure and a little rest.