Whiplash injury: A hyperextension and flexion injury to the neck, often a result of being struck from behind, as by a fast-moving vehicle in a car accident. The mechanics of whiplash injury are thought to be as follows: The victim may be first pushed or accelerated forward, pushing the body forward, but the head remains behind momentarily, rocking up and back, and some muscles and ligaments in and around the spine may be stretched or torn. These muscles, in a reflex action, contract to bring the head forward again, to prevent excessive injury. There may be overcompensation when the head is traveling in a forward direction as the vehicle decelerates. This may rock the head violently forward, stretching and tearing more muscles and ligaments.