You know, this knee thing is quite interesting. I'm sure lots of people have various problems with their knees and from different causes. When I trained in Taijichuan, our instructor was very adamant about doing knee rotations for fear of damaging the knee (as someone posted earlier), and of course not letting the knee extend past the toes, as we mentioned earlier, yet, I had some knee problems which did begin to concern me: the only way I could describe the sensation was that it felt that my tendons or ligaments or whatever were being stretched too much so that when I would walk around, my knee felt like it was too loose and caused a bit of pain. When I began training in Budo Taijutsu, I was shocked at first to see that our sensei encouraged us to rotated our knees (not strenuously, but lightly) in our warm up excercises. Again, care is taken not to extend the knee beyond the toe, but fortunately, I have found that not only has no further injury occurred, but the previous "loose" sensation totally disappeared within the first few weeks of training. I've been training in Budo Taijutsu for years now with no problem whatsoever with my knees and yes, I am "over 40". I can only assume that twisting the knees is not the same as rotating the knee - perhaps the difference being that the weight is not on the knee because it is bent and the rotations are small without straightening or locking the knee. In any case, if you can't afford a phsysiotherapist (who can, these days?) then perhaps you can ask your sensei to observe you as you do your technique. It's amazing what minute variances they can pick up by observing!