Sometimes when getting up to start walking, starting from a stop, or various sudden starts I have a few seconds of muscle tension on one side of my body or the other (particularly face and hands) and after that it returns to normal. I know from experience when it may happen (like I stated) so I can prevent it sometimes but not all. I'm 34 and have had this for maybe half my life, probably a little longer. It started happening sometime after a concussion (CAT scans showed no brain damage) and initially I had doctors try to figure it out but none could so I stopped because, well, it's disheartening to hear you're a medical unknown. The episodes vary in severity and once it starts I have to let it go the 5-10 seconds it lasts. There's no convulsing, loss of consciousness, etc; just tensed muscles on the extremities and I feel fine before and after.