Why not? To me (a layperson in the physics of helmets and brains), they would seem to be quite directly related, especially when the cause of the gees is not related to an impact.
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.
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.
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.
Actually the whole point of having a helmet is to not have your head track its movements - if the acceleration transferred to your head would be the same as that suffered by the helmet, you might as well not wear one at all. Same as with old cars with a stiff body, where in an accident the passengers were subjected to almost the same deceleration as the front bumper. Using the front portion of the car as a crumple zone to reduce the deceleration of the passenger cabin was one of the first major safety improvements...
Newer helmets are designed to be loosely coupled in twisting motions as well as direct impacts too, reducing the ability for the helmet to transfer angular momentum (at least that's the stated goal)
Yeah, for anyone interested there is a technology called MIPS in newer helmets that is intended to handle this better. There are some explanations and diagrams on the site https://mipsprotection.com/
It would depend on the material on the helmet, some are softer, some are more rigid.
Yes it does correlate directly, as in, a high acceleration will correspond to a high acceleration (maybe lower intensity, maybe low-passed in frequency) but it is a direct correlation
Yes it would be better to measure internal accelerations, but I wouldn't feel comfortable wearing a helmet that's being subject to 80G's
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.
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.
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.
Maybe, but if I had the option of recording something with noise, then filtering it out, and recording it without or with less noise to begin with, I'd always go for the latter. Never shun improving the initial data, regardless of what neat things you can do after that.
Helmets are padded, deform, or have suspension systems to decelerate any impulse that the helmet takes. This should cause a very different g-force spike between what the shell of the helmet experiences, and what the wearer experiences.
If the helmet didn't decelerate any impulse to the head, the range of impacts that it could theoretically protect the wearer from would actually be quite narrow.
> If the helmet didn't decelerate any impulse to the head, the range of impacts that it could theoretically protect the wearer from would actually be quite narrow.
This is wonderful understatement: “If the g-forces between the helmet and the brain weren’t different, the helmet wasn’t a helmet at all!”
A helmet without padding can protect against low energy penetrating impacts; think BBs or similar. They do this by spreading the impact point out over the entire head; preventing penetration or fracture.
But for pure G forces, you’re right. A helmet with no padding is effectively similar to not having a helmet at all.
A helmet can just protect the skull So a helmet doesn't have to be padded or help with g-forces to be a helmet. I think helmets were initially just to prevent skull fracture.
Knight's helmets were actually heavily padded, with most knights wearing heavy padded linen garments over their torso, arms, and head. Nicer helmets would have a leather strap system as well. Many helmets appeared to have integral liners, although the linen has rotted away and only left the rivets where they were attached.
With the advent of plate armor, offensive techniques began to focus on concussive force in order to disable combatants, including war hammers and maces. A blow to the head with only a bit of steel between your skull and a mace would be a fatal experience without any padding. Even with padding it was notorious for deafening and disorienting knights, due to noise and (presumably) brain injury.
It's a lucky thing that Sir Ulrich Von Liechtenstein was such a cunning champion in both the sword and joust or he might have ended up with all the brains of poor Wat.
Your head is in a suspension within the helmet, much like your brain is in a suspension within the skull.
Your skull can accelerate at extremely high G's without harming the brain as long as it stays well within the modulus of the suspension. i.e. not for too long. The helmet adds another suspension on top of this.
You are confusing the ability to sustain the g-force by the brain with the force experienced. A padding on the helmet helps in event of collision, not the g-force which is due to the acceleration
Sure the helmet helps with g-forces. Say you're going down the track and enter a tight turn. The helmet contacts the side of the sled at say 5 g's because the helmet is hard and so is the side of the sled. The brain impacts the helmet, which impacts the side of the sled, but the padding in the helmet spreads that impact over a greater period of time, since the padding compresses, so the brain only experiences a g-force of 2.
In a collision, the helmet gets hit and, subject to some degree of padding and such, transfers the impact to the head. If you’re sitting in a sled and the sled accelerated rapidly, your head and your helmet must accelerate with it, although you may not perfectly track its path. Unless the helmet is also secured to your torso, all the forces on the helmet come from your head. You would need tremendous acceleration of your head to meaningfully deform any but the softest padding.
