Kennedy was held kind of rigid by the back brace he wore pretty much constantly in 1963.
There was a TV documentary that put together a sequence of all the motion picture footage and still pictures shot that day that we know of. Some of it is pretty amazing, not just the Z-film. Orville Nix's film is quite impressive.
One of the possibly best films of the shooting has disappeared. It was taken by Beverly Oliver, a/k/a "The Babushka Lady" who was directly in front of JFK's Lincoln when the head shot occurred. She was across the street from the colonnade where Abraham Zapruder stood. This film was seized by government agents and never returned. When the House Select Committee on Assassinations reopened the case in the late Seventies, the FBI claimed that film had either been somehow lost or "inadvertently" destroyed.
Mary Moorman took a still picture at the exact moment of the head shot from almost the same spot Oliver filmed. Analysis of her picture allegedly shows a man wearing what looks like a cop's uniform firing a rifle from the fence atop the Grassy Knoll. He's become known as "Badgeman". Two theories exist about him. There was a famed Corsican assassin named Lucien Sarti who always wore police uniforms for public hits because it enabled him to blend in with all the cops in the aftermath and get away. There's also a theory that Badgeman was Dallas cop, (and known KKK member), JD Tippitt. He was eliminated by either Oswald or one of the many Oswald döppelgangers immediately afterwards to silence him...much the same way Ruby silenced Oswald.
Mary Moorman photograph:
Enhancement of Badgeman figure:
