On page 315 of his 1993 book Case Closed, author Gerald Posner writes: “But if the President was struck in the head by a bullet fired from the rear, then why does he jerk so violently backward on the Zapruder film, which recorded the assassination? To most lay people, the rapid backward movement at the moment of the head shot means the President was struck from the front.”
When Itek Optical Systems did a computer enhancement of the Zapruder film for a CBS documentary, it discovered that when the bullet (the final of the three fired by Lee Harvey Oswald) hit JFK, he first jerked forward 2.3 inches and then began his movement backward.
So why did the president jerk backwards when hit in the back of the head by a bullet fired from behind him? The bullet destroyed the President’s cortex. That caused a neuromuscular spasm. That sent neurologic impulses from the brain down the spine to every muscle in the body. “The body then stiffens,” said Dr. John Latimer, “With the strongest muscles predominating. These are the muscles of the back and neck.”