The human spine is designed to carry weight in the vertical plane. It is not designed to take a sudden heavy impulse at 90 degrees. This can crack or displace vertebrates and permanently damage the spinal cord. The strongest possible strike that a person can deliver is a dropping stomp with their body weight directly in line behind it. A rearward side kick, sometimes called a back kick or mule kick is essentially the same. It is a flying one leg stomp with the kicker is leaned over with his torso lined up with the heel of the foot. You would be essentially have your back turned 3/4 toward the target, looking over your right shoulder, skip with the left foot and stomp with the right heel while your entire body is in motion. This kick hits like a battering ram when delivered by a trained practitioner and can easily generate in excess of a thousand foot pounds of force which is concentrated on a very small hard striking surface of the heel. Nobody's spine can withstand that, especially if they are relaxed and don't see it coming. The reason that the man who kicked Arnold failed to hurt him is that his feet made contact before he extended so it just turned out to be a hard shove.