Wow, sounds very painful. Having your leg buckle can lead to serious injury. It happened to me last year climbing up stairs carrying just light shopping. I have a problem with loosing power in my knee (still haven't had it diagnosed). But this night anyway, it was a cold winters night and the cold must have affected my knee, as I have some arthritis, and my knee lost power and I went flying back down the stairs. Luckily didn't break any bones but has very sore bruising and was very shook up. Be careful on those steps and definitely hold on to the railings.