Hockey helmet physics: me keep my damage from being brained.

[ausername1]

I recently got a concussion from playing hockey- I think it's time for a new helmet.
I was looking at a new-ish helmet that claims to have superior protection than the others but a video promo for it was sort of unsettling for me:http://www.youtube.com/watch?v=yAV99h1Aeg0e. At around 3:00 it shows the helmet being crushed along with the competitors' helmets being crushed. It claims that because it takes more force to crush this helmet, it is safer. Could someone explain why this is? To me it seems like the 'give' the other helmets have would help to cushion the blow, whereas this helmet would just transfer the force directly to my head since not as much energy is used up to distort it. I thought that was the guiding principle for building those cars that crumble upon impact- slower change in momentum means it's less violent?

 

[krd]

It claims that because it takes more force to crush this helmet, it is safer. Could someone explain why this is?

Yes it's safer...For the helmet....For your brain it's more dangerous. It means the helmet is going to pass more of the shock wave onto your head, than if it broke up itself.

The upside though, is after you're brain damaged, you'll still have a helmet in mint condition.

To me it seems like the 'give' the other helmets have would help to cushion the blow, whereas this helmet would just transfer the force directly to my head since not as much energy is used up to distort it.

I thought that was the guiding principle for building those cars that crumble upon impact- slower change in momentum means it's less violent?

Yep. You're dead right.

The whole idea of shock absorption is to disperse the force - send it in different directions - you don't want it directly hitting you head. It's like this. If you take a steel bar, and wack it against a stone wall, you'll feel force in your wrist - and it will hurt. Do the same with a child's plastic baseball back.

 

[Dale]

Hi ausername1, welcome to physics forums, and great thread title!

I recently got a concussion from playing hockey- I think it's time for a new helmet.
I was looking at a new-ish helmet that claims to have superior protection than the others but a video promo for it was sort of unsettling for me:http://www.youtube.com/watch?v=yAV99h1Aeg0e. At around 3:00 it shows the helmet being crushed along with the competitors' helmets being crushed. It claims that because it takes more force to crush this helmet, it is safer.

The key measure for safety vs. concussions is the "g-rating" that they mention at about 2:30. It is interesting that in all of their other statistics they compare their products to their competition, whereas for the "g-rating" they compare their product to the certification requirement. I often find that what a company's advertisement does not say is as informative as what it does say.

If you are primarily concerned about concussion then make that your primary decision point. Ignore most of the rest of the hype from any company.

Could someone explain why this is? To me it seems like the 'give' the other helmets have would help to cushion the blow, whereas this helmet would just transfer the force directly to my head since not as much energy is used up to distort it. I thought that was the guiding principle for building those cars that crumble upon impact- slower change in momentum means it's less violent?

I agree with you in principle wrt concussive injury. However, concussion is not the only head injury of concern in hockey. For projectile injury I could see why the stiffer helmet would be helpful, it would distribute the impact from the projectile over a greater area.

Ideally, I think that you would want a stiff exterior shell and a fairly squishy foam interior. You see similar things in cars, where the crumple zone you mentioned is designed to crush and absorb energy, but then other sections are designed to be much stiffer.

 

[ausername1]

Yeah I thought a hard exterior w/ softer inside would be best too. Thanks for the info- I just had to make sure I wasn't crazy or completely missing some concept for thinking the helmets with give might be better... for some hits anyway.

 

[Andy Resnick]

I recently had a similar discussion with a "sports medicine" doctor- one of the local hospitals has a specialized department that works with the professional sports teams here, and they see a lot of high school/college sports injuries as well.

He was very clear about two things- helmets, regardless of the sport, do negligible good and probable harm.

The 'harm' part is easy to understand- when you are suited up, you are more willing to be more physical and hit harder because you feel protected.

On the 'good' side: as far as he knew, there has not been any real testing to determine the efficacy of wearing a helmet. Lots of anecdotal evidence and the logic of impact dynamics seem to support the benefit of wearing a helmet, but the trauma mechanisms (swelling, twisting motions, etc) are not part of the initial impact and may have a closer connection with momentum transfer rather than energy transfer.

That said- there is a giant magnifying glass of attention being focused on concussions in sports right now (at least here), so my paragraph above is subject to change. Also, we didn't discuss the role of helmets in motorcycle/auto accidents, nor did we discuss pads designed to limit neck movement and keep the head in a fixed orientation.

 

[DragonPetter]

He was very clear about two things- helmets, regardless of the sport, do negligible good and probable harm.

I think a general statement like that gets him into trouble.

I got a concussion while snowboarding without a helmet once. After that, I always wear one, and many times I've hit my head and have been grateful that I had a helmet. Also, I once crashed a 4-wheeler and flew into a ditch head first. The top of my head smashed into a boulder like a missle; the helmet cracked and my head was fine, although the rest of my body was cut and bruised. I'm pretty sure I would have had a severe head injury if not for the helmet. I'm not trying to use this as anecdotal evidence, but rather just some examples that make me question what he says.

 

[mrspeedybob]

Yeah I thought a hard exterior w/ softer inside would be best too. Thanks for the info- I just had to make sure I wasn't crazy or completely missing some concept for thinking the helmets with give might be better... for some hits anyway.

How hard or soft the inside should be is going to be different for different impacts.

In a less severe impact the softer material will be better because it slows your head down more slowly. In a severe impact a softer material may allow your head to keep moving almost until it hits the inside of the shell, at which point G force will spike rapidly. In this scenario the firmer material will absorb more energy as it collapses. So for any given impact there will be an ideal firmness for the helmet lining material.

Given that the material cannot change properties on the fly in response to the impact, I think a relatively firm material optimized for a worst-case-scenerio would be the best bet. In a less severe impact a higher percentage of the total energy will be transferred to your head, but it's a higher percentage of a smaller number.

A helmet which may be hard enough to give you a concussion in a moderate impact may save your life in a severe impact, where a softer helmet would not.

 

[AnTiFreeze3]

I think a general statement like that gets him into trouble.

I got a concussion while snowboarding without a helmet once. After that, I always wear one, and many times I've hit my head and have been grateful that I had a helmet. Also, I once crashed a 4-wheeler and flew into a ditch head first. The top of my head smashed into a boulder like a missle; the helmet cracked and my head was fine, although the rest of my body was cut and bruised. I'm pretty sure I would have had a severe head injury if not for the helmet. I'm not trying to use this as anecdotal evidence, but rather just some examples that make me question what he says.

While snowboarding may be a sport, it's not a contact sport. It wasn't the basis of Andy's discussion. In your case, snowboarding would fall into the category, in terms of effectiveness of helmets, with the likes of motorcycle riding, bike riding, etc. Basically, anything where wearing a helmet doesn't make you want to hurt people more because you feel protected. Helmets in contact sports increase aggressiveness. Helmets in sports like snowboarding or bike racing are merely for protection in cases of falls or accidental collisions, which you are trying to avoid at all costs, as opposed to contact sports, where you're trying your hardest to hit other people as hard as you can.

 

[haruspex]

There are in principle three layers:
- a stiff outer layer to spread the impact of a localised contact over a large area of the second layer
- the second layer's job is to absorb energy while minimising the maximum transmitted force. The ideal is a small elastic range and a large plastic range: the transmitted force increases linearly over the elastic range and plateaus somewhat in the plastic range; to minimise the maximum force you want the plateau to be reached quickly. There is a trade-off here. Make the plateau too high and you've transmitted a dangerously high force; make it too low and you run out of range before absorbing enough energy. The ideal point depends on the worst case impact you plan to handle. Making the layer thicker helps.
- A relatively soft inner layer. The purpose of this is to adapt to the shape of the skull, spreading the load on you.
Note that a consequence is a helmet should be discarded once it has shielded a blow sufficient to push the middle layer into its plastic range.