It's the interpretation that GJW's is implying helmets don't work, but he is not, wires crossed somewhere. He is only on about a, probably as I have no stats, small crossection of cases.
That of linear impacts of below 7mph and above 14mph against flat surfaces. And rotational impacts at all speeds. However you would be very wrong in believing that these types of impact are rare, as neuroscientists believe that the majority brain injuries and concussions are caused by the sudden rotations of the head.
Rotational brain injuries can occur as relatively low energy levels. Some of the most advanced rotational energy absorbing helmets have been produced for use in American baseball and football where multiple concussions were a big problem and traditional helmets did not prevent them.
One problem is that even linear impacts can produce rotations of the head. Anyone who has experienced the difficulty of knocking a nail into a piece of wood will have experienced this phenomenon. Therefore, next time I try to knock a nail into a piece of wood I intend to use a woodpecker instead of a hammer.
9/16" Suntour BMX MP-1000 pedals / 56 tooth TA 6-bolt chainring.(I'm not bothered if the teeth are worn or damaged)