I f'd my knees running track in HS - i suspect some kind of structural problem that alters the way my knees move. Sr year wsa awful, i was just about crawling to class in the morning. MRIs showed nothing and I learned to live with it and eventually it got better when I stopped running.

10ish years later I had started riding my bike a lot during the dotcom explosion induced downtime, I kept it up after I was employed again. Knee pain came back. Went to the ortho again, this time was told "oh you have a torn meniscus, lets go fix it.

So I went under and woke up to be told that my meniscus was fine, and that I had worn grooves through the cartilage and into the bone - doc told me to never run, jump, ski, ride, etc. I was to sit on the couch.

Somehow I ended up taking karate, and the knees hurt for a bit, but the stretching we did helped loosen up my hams and quads. It didnt make the knee pain go away, but it made it tolerable in that it no longer affected my day to day.