This is a question that occurred to me as I took a long motorcycle ride today. What’s happening in my bent legs that causes the kind pain whose only alleviation comes from straightening them out?
Just anecdotal, but I used to have chondromalacia, which was a roughening of the bearing surfaces of the cartilage in my knee. You can Google “Movie Theater Knee” for what might be some help.
I don’t know the answer but I used to have the same thing happen when I rode and never figured it out. I had other knee issues but nothing so serious as to seek treatment. I always figured it had to do with decreased circulation.
IANAD, but it seems to me that you might have patellofemoral pain syndrome:
http://www.m.webmd.com/pain-management/tc/patellofemoral-pain-syndrome-topic-overview
It might be worth your while to see an orthopedist and get your knees checked out. Good luck!
I had this for a time… my knee would get an agonizing pain if I kept it bent while sitting. Only straightening it out, even for a moment, would make it better, and then instantly and for a good duration.
This was after I’d fallen and smacked the shit out of my kneecap on concrete not once, but twice in a week. After about a year, it stopped happening. (I had no pain any other time once the initial pain was gone.)