What Causes Quivering Sensation In Leg?

On occasion I get a quivering-tugging sensation in my upper right leg right about where my leg bends. I get this on occasion when I stand up and it grips me and it just stops me from walking and I have the urgency to just grab hold of that area. It is not painful but just this terrific sensation of quivering and I find it very embarrassing at times. Can you tell me what this is and is there anyting I can do to stop it?