I shake when i yawn. my upper body mostly. it happens every time. what causes it?

Normal response: It is common to have some muscle reaction to a good yawn as there is the bodies reaction to the yawn. Our muscles tighten and we shake in response to the trigger of the yawning episode. If it is a very light yawn it might not cause the response to occur but it is common with larger yawns. Hope this helps. It is nothing to worry about.

Need more info: Could be a benign neurological manifestation. If you have no other medical problems I don't see a serious problem but if this condition is recurrent, more than 10 times a month, it would be a good idea to talk it over with your family physician.