Re: unusual seiza problem - pain in BACK of knee
Doctor's answer: bone spurs rubbing on tendons and other soft tissues.
I went back to the Osaka Chuo Byo-in knee clinic yesterday with the help of a bilingual Kyoto University student from their Good Samaritan Club.
The doctor in the knee clinic x-rayed my knees and did a short physical exam, then pronounced that my pain is from bone spurs. I apparently have them around the top of the tibia (as well as "many behind the patella"). One was clearly visible in the x-rays he showed me. It appears like a small devil's horn sticking out of the lateral side of my right tibia bone. The pain is worst in my lateral right hamstring tendon, so that fits. I couldn't see any other spurs in the x-ray, but I'm not trained in radiology, so maybe they were there.
His recommendation is, of course, to stop training. Don't do things that make it hurt. In the case of bone spurs, "making hurt" also means "inflaming and worsening the condition." He didn't mention surgery, although I think you can also get the ends of the bones shaved to take away the spurs.
His diagnosis fits okay with the x-ray and physical exam he did. It doesn't quite explain to me why the problem comes from seiza--why do the spurs rub so badly in an essentially immobile position of extreme flexion but not when riding bikes or lifting weights? Also, if the problem is rubbing, it seems like it would get worse with more activity. E.g., the end of an aikido class should be more painful than the beginning, but that doesn't seem to be the case; if anything, it's the opposite of that.
I was a little disappointed that the doctor didn't seem to be interested in the course of the problem or in examining me in attempting to perform seiza. He asked me basically no questions. So when he said that if I didn't stop, I could develop inflammation that might lead to effusion, I said, "yes, I know, my knees were swelled up like this [hands around knees] a week ago," he gave me a wide-eyed look and said, "oh, really" (he spoke a little English). I had hoped that he might take a look at the joint while it was flexed in seiza to see if it looked normal, but no such luck.
Thanks again for all the replies. I have to decide now what to do. I suppose I will just keep training and see if the pain goes away.