In a previous life, spent quite a bit of time as pax in helicopters.
I remember being in the back seat of a Squirrel once and being surprised at the movement of the pilot's head induced by vibration of the machine and the weight of the helmet, even in normal cruise flight.
It was a standard Gentex-type white civilian helmet, with no added extras.
Significant enough to think at the time "that's an OSH issue in the making over several years..."
(Presuming that the wobbling head wasn't due to any other factors
)