About 6 months before I retired, 10-15, I started doing more under sink inspecting by being on my knees, like my son's father-in-law does all the time who has been in the trades forever. After a while my knees started feeling a little weird. I backed off, but a few weeks later I came up with the brilliant idea of getting some nice knee pads to use in low clearance attics so I could knee my way out over ceiling joists. Wrong. Started making my knees feel crappy again. I could still go on hikes and bike rides with little effect. My retirement came and that was the end of those moves. Well, I went on a vacation to UK in late September, which was after no more joist action, and ended up doing a lot of pavement walking, like 7+ miles on 7 different occasions. That started my right knee talking to me. I also did a bunch of trail hiking, but it was after the concrete stuff that it would bother me. Since I got back there has been no more concrete walking for me. It's bicycle or no go. It's been 7 weeks since I quit the crete and am feeling much better. Just standing on concrete would bug me. It's that right angle move where your tibia is pushing up laterally across your femur that seems to be the problem.