Proprioception has nothing to do with head impact forces and the amount of these forces that cause injury. Proprioception is the term used to define the nerve function that tells the brain the position of a limb/muscle and the amount of force that muscle is exerting. I know this concept well because I had proprioception problems early in my recovery. The limits of our proprioception systems is taught to Little League Baseball infielders. They are taught to make a short bounce in their legs as the ball leaves the pitcher's had to excite the proprioception system so active motor/muscle control happens faster.

I addressed the geometry in post #33 when I showed how G forces are reduced when the impact is angled. A 45 degree angle reduces the speed by about a third and the G force by about half since the G force is related to the square of the speed.

The eurailsafe article had nothing that relates to the helmet research done. Nor did it show the qualitative data/graph mentioned. It is a weak attempt at looking at the issue. I can see what they are trying to do with the HIC formula but it appears they are missing some of the issues at hand. The assumptions they make far exceed the assumptions we have made.

But then, I only got an A in Physics 101A (Weight and Motion) and I did it with an old fashioned slide rule like my father used to build space launch vehicles. He was an expert in acceleration/vibration forces incurred during launch.