system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does overstressing your back muscles (bad posture) make them weaker?

Normally, if you overstress a muscle, it gets stronger. But if you have bad posture, causing you to overstress certain back muscle groups, you get back pain. Then the physio gives you exercises to "strengthen" those muscles. How does that work?
gpt
Over stressing a muscle means that muscle is engaged. If you're not keeping your back straight, the muscles responsible for that action will not be engaged, making them weaker.

Your muscles get stronger by using them.