4 ms·

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 k

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.

willis936 6y ago
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.
- mrfredward 6y ago
  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.
mam2 6y ago
That sounds like r/imsosmart ... a simple low pass filter would be sufficient to negate the effects you talk about
- ehnto 6y ago
  Have you used a helmet before? They don't track the heads movement very well. It's not that they lag behind or anything, they just move in completely different directions willy nilly. Even tight ones will have much sharper acceleration changes as they bounce around. I regularly wear a full face helmet with tight fitment and straps and it is moving way more than my head is. Is that notsosmart enough?
  - mam2 6y ago
    https://en.wikipedia.org/wiki/Low-pass_filter https://en.wikipedia.org/wiki/Low-pass_filter
    - DoingIsLearning 6y ago
      The helmet and head are too loosely mechanically coupled. Let's say you have acccelerometer data from a sensor on the helmet and you pass it through an LPF: If you have the helmet bobbying at really high forces you over-represent real G forces and you are right that you could negate that with filtering. _However_, if you have really high forces transmitted from the bobsleigh up to the torso/neck/head it is possible that the inertia of the loosely coupled helmet makes it not register the full force as propagated through the tightely coupled torso/neck/head and you underrepresent the real G forces experienced by the brain/skull. Accelerometer sensors should be coupled with the skull for proper risk/hazard assessment.
    - dvt 6y ago
      Please take this dismissive Wiki-linking clap-back garbage back to reddit. I come to HN to read insightful, diverse, thought-provoking discussion. It's definitely been slipping, but it's one of the few places on the internet that at least still tries.
    - ehnto 6y ago
      You could filter it and make it look like what you think it should look like, but it would still be a measurement of the forces applied to the helmet not the head.
yk 6y ago
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.)
- etrautmann 6y ago
  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

willis936 6y ago

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.

mrfredward 6y ago
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.

mam2 6y ago

That sounds like r/imsosmart ... a simple low pass filter would be sufficient to negate the effects you talk about

ehnto 6y ago
Have you used a helmet before? They don't track the heads movement very well. It's not that they lag behind or anything, they just move in completely different directions willy nilly. Even tight ones will have much sharper acceleration changes as they bounce around. I regularly wear a full face helmet with tight fitment and straps and it is moving way more than my head is. Is that notsosmart enough?
- mam2 6y ago
  https://en.wikipedia.org/wiki/Low-pass_filter https://en.wikipedia.org/wiki/Low-pass_filter
  - DoingIsLearning 6y ago
    The helmet and head are too loosely mechanically coupled. Let's say you have acccelerometer data from a sensor on the helmet and you pass it through an LPF: If you have the helmet bobbying at really high forces you over-represent real G forces and you are right that you could negate that with filtering. _However_, if you have really high forces transmitted from the bobsleigh up to the torso/neck/head it is possible that the inertia of the loosely coupled helmet makes it not register the full force as propagated through the tightely coupled torso/neck/head and you underrepresent the real G forces experienced by the brain/skull. Accelerometer sensors should be coupled with the skull for proper risk/hazard assessment.
  - dvt 6y ago
    Please take this dismissive Wiki-linking clap-back garbage back to reddit. I come to HN to read insightful, diverse, thought-provoking discussion. It's definitely been slipping, but it's one of the few places on the internet that at least still tries.
  - ehnto 6y ago
    You could filter it and make it look like what you think it should look like, but it would still be a measurement of the forces applied to the helmet not the head.

yk 6y ago

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.)

etrautmann 6y ago
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