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Even strong neck muscles of military volunteers cannot make a difference in such an environment. Rather it is the rigidity of the bones in the neck, in combination with the connecting ligaments, that determines whether the spine will hold together and the spinal cord will remain intact within the confines of the vertebral column.
This works for adults, but very young children have immature and incompletely ossified bones that are soft and will deform and/or separate under tension, leaving the spinal cord as the last link between the head and the torso. Have you ever pulled an electric cord from the socket by the cord instead of the plug and broken the wires? Same problem.
This scenario is based on actual physiological measures. According to Huelke et. al. (1), "In autopsy specimens the elastic infantile vertebral bodies and ligaments allow for column elongation of up to two inches, but the spinal cord ruptures if stretched more than 1/4 inch." Real accident experience has also shown that a young child's skull can be literally ripped from its spine by the force of a crash. Yes, the body is being held in place, but the head is not. Is it a statistically rare event? Yes. If it's my child, does it matter that it's rare?
When a child is facing rearward, the head is cradled and moves in unison with the body, so that there is little or no relative motion that might pull on the connecting neck.
Another aspect of the facing-direction issue that is often overlooked is the additional benefit a child gains in a side impact. Crash testing and field experience have both shown that the head of a child facing rearward is captured by the child restraint shell in side and frontal-oblique crashes, while that of a forward-facing child is thrown forward, around, and often outside the confines of the side wings. This can make the difference between a serious or fatal head injury and not.