[roguecoder]

The problem is the human neck. Any amount of force and deceleration is still being born by the head, and the only way it can dissipate force is the relatively-fragile human neck.

This is probably the origins of the grand bascinet, where a knight's outer helmet were strapped to his chest: so the force could be dissipated. But they severely restricted movement and athleticism: you wouldn't be able to turn your head in a different direction than you are running to look for the ball, for example.