system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: why do our muscles require constant training to be strengthened instead of strengthening them once and having strong muscles forever?
gpt
Our bodies strive to be as efficient as possible in order to increase our likelihood of continued survival.  Muscles, while being useful for motion, are quite expensive energy-wise, so the body only keeps enough muscle to meet the usual requirements for moving around.


Now if you regularly use your muscles to a degree that they get worn out a lot, such as by lifting heavier weights or running longer and farther, then the body builds those muscles bigger and stronger to meet the new requirements and prevent damage.  


However, if you stop the additional exercise that built those muscles up, then the body starts breaking them back down because maintaining them costs too much energy and it doesn't seem to need the extra strength anymore.