I have a friend who suffered a beating at the hands of several bullies as a child. As a result, he cannot tolerate being held down. This beating damaged him psychologically, but I think it also caused his body to develop a new reflex pathway.
Since he is terrified of having his arms held down, if someone so much as barely touches his arm on the bicep, his muscle and skin twitches. His brain doesn't even have time to process the touch. It's unlike anything I've ever seen! The trauma gave him a new reflex pathway.