Hi, I don't think this is common, I am 20 years old, and my legs will at random go completely limp for about a half second and I will look like I stumbled but can then regain control of it before I hit the ground (almost like it feels when you black out, but only in my knee.) Usually it happens when I get a momentary excitement (I find something exceptionally funny, or like last night when I went to pop my dog for sticking her head in the trash to get a chicken bone) When something like that happens, it doesn't always happen, but like I said earlier, it does happen sometimes without any precursory events that I have been able to make a connection with it. This is something that has been happening over the last 3-4 years and I thought it was just a weird coincidence as it didn't happen very often. Now, it is starting to happen once or twice a week and I'm starting to think there may actually be something wrong. I don't have any problems like this with the rest of my body, and while I am overweight (currently 260 but loosing weight by healthy dieting/portion control) I don't believe that is the cause of the problem. Thanks in advance for any help you might be able to give me!