Took the subway to visit our daughter and SIL yesterday. Lots of walking (subway stairs, detour to catch streetcar, walking to their condo). When we got on the streetcar (the steps are deep) my knees suddenly buckled. I almost fell onto the steps. It was so sudden, and a shock. Has this ever happened to anyone here? I can't take step aerobics where the knee can 'twist', but I'm not aware of any other problems. Guess I will have to speak with my doctor about this.