The human body is a frugal contractor. It keeps the muscles and skeleton as strong as they need to be, no more and no less. “Use it or lose it” is a basic mantra of the human body. If you take up jogging or gain thirty pounds, your body will strengthen your bones and muscles as needed. Quit jogging or lose the thirty pounds, and your frame will be appropriately downsized. Muscle is regained in a matter of weeks once astronauts return to earth (and bed-resters get out of bed), but bone takes three to six months to recover.
