Sometimes when I do some type of physical activity; such as sprint or leap to grab something; my hands involuntarily clench into a fist, while my jaw also clenches together. I can't move and I can't breath. Most of the time I have to lean up against something or else I will fall over, when nothing is around to lean on I quickly take a knee or lay on the ground. I usually have to wait it out, luckily I'd say it only last about 5-15 seconds. There is no pain when this happens.