Joanne, some literature call it the Moro reflex and it does exist in adults. A simple explanation is that the body has a built in fight/flight/freeze response, and most of the time a “weak” muscle gets stuck in the “freeze response” and once you unstick it, it returns to normal function, most of the time instantly. The sad part of it Joanne, is that you will never find out how? in the Fitness World, you have to search for it in the Physical Therapy world. Joanne i hope this answers your question? Take care.