For as long as I can remember, I periodically have a sudden tightness in my right arm that causes it to involuntarily fling out - usually straight out to my side like I am trying to hit someone standing beside me. It doesn't hurt at all, and it lasts only for as long as it takes for my arm to fling out, then it's gone. It isn't a tremor or a continuous jerking, just one big muscle jerk...and then nothing. It also doesn't happen all the time, maybe once every few days.  I remember doing this as a child as well, and no one ever seemed to concerned about it. I am in my late 40's now, and it manifests exactly the way it always has, so I know it's not likely to be dangerous or degenerative, but I still wish I knew what was happening. Sometimes, if it is a particularly intense jerk, the bottom half of the right side of my face will tense and pull down. If someone is looking right at me when it happens, they are alarmed...but if no one is looking right at me, the whole thing is so fast, no one in the room notices. Any ideas?