By contracting hard during the impact of a car crash the muscles of the neck tighten to prevent undesired excess movement that may damage the bones of the cervical spine. Afterwards they may not automatically return to their original resting state. The nerves may remain in a state of over excitement keeping the muscles in spasm. Manual manipulation of the muscles and nerve organs can help reduce the trauma, calm the nerves and effect the required release by removing the spasm. Muscles in long term (chronic) spasm can also include the formation of trigger points and fascial adhesion's which will not self resolve without treatment leaving you with a long term problem.