I've suffered from painful knees since I was 18, it started playing roller hockey, I was getting into it and my knees started clicking, pain under kneecap etc. Physio gave me an exercise to do, standing on one leg and bending it a bit for reps, his theory was that my quad was over developed on the outer side and that this exercise would train my knee correctly. It helped, maybe, but quitting rollerhocky helped more. After decades of skateboarding, kickboxing I started really lifting heavy and playing American football. Shit got bad after 3 years of that, I was having to really curb what I did, avoiding running, no squats etc. To the point I was medicating and trying all sorts of shit, BPC-157 etc to fix. Saw another physio last year and he had me rest for months, reverse slide lunging with increasing weights. It helped and cured my patella tendonitis but when I cranked shit up and returned to football and lifting the familiar pain returned, not being able to sit with knees bent for more than 20sec or so. Having to get out of the car to stretch every 10 min etc. A random youtube clip on the science of hamstrings described the bicep femoris which attaches below the knee, that isn't activated at all by squats, deadlift, stiff legged deads, cable pull throughs etc. It's only really effected by resisted leg curls, an exercise I've never done. After one session of 3x12 things felt 60% better and it persisted. After two weeks of doing the same I can sit with my leg totally bent for an hour with no pain, something I havn't done for a decade. I've entirely healed myself, and it's incredible, literally the only thing holding me back athletically has gone, a new lease of life. I've sprinted without knee supports, squatted heavy and no pain whatsoever. I don't blame anyone for not figuring it out 20yrs ago but man, it seems like such a simple thing to miss. Balance of joints should be taught in school.