You're doing your favourite sport, and you're totally absorbed in it. Suddenly you hit your knee. You hardly notice it, because you have to and will go further. And you go further, finish what you started and get a good result.
If something's hard to predict, it's long-lasting pain. Mostly pain that has lasted longer than three months. One day you can exert yourself well and it doesn't seem to worsen the pain, the next day you can't do anything.
The year is 1979 (!) when many scientists come together to determine what pain is.
There is something special going on. For a long time it was thought that pain went hand in hand with damage to the body. More damage should simply lead to more pain.