Every once in a while I'm running up the stairs and and my left knee gives me the middle finger and a troll face and I go down. Then its completely fine a half hour later.
A few years ago I was loading large homegrown pumpkins into a truck and on the last one, I twisted a little as I was setting it in the bed... GAME OVER. My back wasn't 100% for almost 2 months. Crazy realization how you use muscles in your back for almost every single motion.