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Comment by etrautmann | Hacker News Reader

The helmet is a light mass, which can move relative to the head, and experience drastically higher g-forces for very short times. Sampling an accelerometer at kHz rates as a helmet is vibrating (without translating all of the vibrational energy into the head), will measure larger peak G's than the head experiences.

For this reason, when studying head impact for football players, they mount the accelerometers in the mouth guard. This more accurately reflects the real g-forces, as the teeth (ideally) don't move much relative to the brain during an impact, and have the appropriate reference frame.

The differences can be extremely large. The peaks reflected in this graph are absolutely not real and should not be taken literally.

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Yes, but notice that the shock goes sled -> spine -> head -> helmet in sledding and other player -> helmet -> head in football. So, strapping an accelerometer onto the helmet looks quite a bit better for sledding. (And especially for a first measurement.)

Yep, except in the cases where the head hits the sled or ice, as mentioned in the article. I suspect these are responsible for large (and artifactual from the perspective of the brain) transient spikes in measured G-forces

What would you say is the maximum frequency that the helmet is coupled to the head at? Filtering is a trivial operation and I would expect the g forces felt by sledders to be sustained for the better part of a second. You lose a lot of fidelity, but if you’re still over the line then you know there’s a problem.

According to the article, the tracks are designed for sustained g-forces of 5Gs. These are the accelerations that are sustained for greater than a second. According to the annotations on the NYT's plot, the g forces which are most alarming are from when the helmet makes contact with the ice. This is precisely when the helmet accelerations would be expected to differ wildly from what the head is actually doing.

A low pass filter could easily be used to remove the impacts with the ice, but minus the impacts with the ice there's nothing in that acceleration trace that should be alarming for a professional athlete. The high frequency content is the concern here.

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