Pain is not “weakness leaving the body”. If you have pain in your joints from exercise, this is not your body getting stronger. This is your body getter damaged, and ultimately weaker. Pain is one of the languages your body speaks. Sometimes it’s saying “stop, you don’t know what you’re doing”.
Our body is way smarter than we are. It does trillions of things we’re oblivious to. Don’t listen to meatheads who tell you to keep pushing yourself, they’ll just get you injured. There are a few high profile meatheads who recently had heart attacks in their mid-50’s — famous YouTubers who told young people to keep pushing themselves, to stop being weak.
Well, guess what? Your body is going to be weak no matter what you do. It’s inherently fragile. And the real mental weakness is believing you’re smarter than your body, and not educating yourself on good health.