Physical activity is essential for the development, strength, and health of bodily organs, whereas disuse leads to decay and weakness.

Original passage

Action is a law of our being. Every organ of the body has its appointed work, upon the performance of which its development and strength depend. The normal action of all the organs gives strength and vigor, while the tendency of disuse is toward decay and death. Bind up an arm, even for a few weeks, then free it from its bands, and you will see that it is weaker than the one you have been using moderately during the same time. Inactivity produces the same effect upon the whole muscular system.