I have gone through periods of some sort of "knee collapse" the last few years, when trying to pedal out of the saddle sometimes my left knee would just give out on me (collapse). That can hurt other parts. I do have some pain with the knee problem but it is strange how it will just give out on me. Been to Ortho Doc and the joint is ok still, Physical therapy seemed to help some but it just seems to come and go maybe once a year. I do have a Peripheral Neuropathy in the legs which is great to blame things on.