I went on a week-long cave exploration trip in 2007 with a lot of crawling and squatting with heavy packs, after which my knees basically hurt all over and did not get back to 100% for two years. I chalked that up to a lack of physical fitness and a bad pair of knee pads. Doctors told me I didn't tear anything and that it was an overuse injury. That's the only injury I can think of. An x-ray was done and I naturally have a two-piece kneecap on the left side. Since then I have had numerous long pain-free periods where I can walk and hike with no pain. I just got back from going 4,500 feet deep in a cave in Croatia, ascending all of that on ropes with a 40 lb pack - no knee pain. But as soon as I add in cycling, the pain comes back. Walking and hiking can be painful then, but again only during periods of cycling. If I stop cycling, I can do just about anything I want with no pain. And through it all, the knee feels very stable and strong. Almost all of the pain feels like a swelling in the very center of the knee, behind the kneecap. However, there are occasional fleeting moments of pain when I step, like the force is passing right through the knee and hurting. And sometimes there are twinges of pain on the surface of the knee on either side of the kneecap. But both of those pains are few and far between and 95% of the pain is that internal swollen feeling that hurts when I squat.