Author Biography:

Daniel Cousins, MSc

Helmets are expensive and we are paying for products that does half of its job. Through this post, I am going to outline what a helmet it is, the kinds of injuries that occur, what kinds of impacts they sustain, and strategies used to protect against them. I will then conclude with the main issue of HEMA helmets with a possible solution. When I say HEMA helmet, I mean the mask plus an overlay/ back of head protection and like any good rule there must be an exception which is the Wukusi Cobra, which is both. The HEMA mask is typically made of spring steel or stainless steel with a variety of options for the overlay such as puncture resistant fabric, high-density polyethylene (HDPE) or ABS plastics. The majority of helmets then have a very thin liner to help with fit and to act as a buffer between skin and the mesh.  

Other sports helmets are made up of multiple layers and for simplicity, I will only talk about two layers: the shell and the lining. Starting with hockey helmets, the shell is typically made of plastic such as high-density polyethylene, polycarbonate, fiberglass or carbon fiber. The liners are then made of a soft material such as expanded polypropylene (EPP), vinyl nitrile (VN) or high end will use carbon lattices. The liner is typically where majority of helmets differ by using their own formulae, structures and alternating layering of multiple materials. Football helmets are typically made of ABS plastic or polycarbonate, with liners made of polyurethane, polyethylene, or nitrile (NBR) foam. Similar to hockey helmets, the liner is what sets them apart.

Now that I have outlined the construction of the helmets, you may have already noticed the key missing characteristic in HEMA helmets. The liner. There is almost no actual liner on the inside of a HEMA helmet. As I move on to talk about the impacts, injuries and protection, you will understand why this is such a massive oversight.

When it comes to injuries that can be sustained from impacts to the head, we can create two main classes: localized and diffuse. A localized injury is anything that can only occur at the site of impact which varies in severity from a scratch up to a puncture/ skull fracture. The diffuse injuries are anything that does not need to happen at the site of impact, these are things such as whiplash and concussions. These two different kinds of injuries are due to the differences in how our body will react to the force of a blow. One way is to deform by our soft tissue (e.g. skin, fat, muscles) compressing, if we cannot deform enough then we break. The other way we can absorb the impact is by moving. By moving, you can spread the impact across multiple joint that way none of them experience a critical amount of force that would cause an injury. With impacts to the head, the neck and spine are the main joint involved in spreading that force around. A good helmet should be able to absorb some of the force and be strong enough to not break if the force is too high to absorb.

Helmet testing, on the surface, is quite simple. Hit the helmet and measure what happens with the force. The difficult part is knowing how much force to hit it with, where to hit it and what you consider a pass or fail in its performance. The first part is done by evaluating the sport and recreating realistic impacts. In hockey and football, we know how fast people are skating, how much they weigh and how long the impact is, where the impact is and direction it is coming from and from that you can come up with a realistic value. Commonly a helmet will need to reduce the impact below a specific value for linear and rotational acceleration, and an aggregate measurement called severity index (SI). SI is a weighting of the different accelerations to determine the risk of a head injury sustained from an impact. The common value is a helmet must be below 1200SI. The respective organizations will then evaluate each helmet and determine if it is acceptable in their sport and athletes can only choose from approved helmets. HEMA helmets have none of this.

HEMA helmets operate under the FIE model of protection which only focuses on puncture. HEMA helmets will list the puncture rating of the bib at a minimum and higher end helmets will list if they are CEN2 rated, which is the FIE standard. To be CEN2 rated a helmet must have a bib that is puncture rated for 1600N and have specific mesh criteria in terms of the shape and strength of the metal. Nowhere in their testing is impact reduction. This is a massive problem considering the head impacts we are receiving.

I performed some preliminary testing of a longsword strike to a force transducer and calculated the impact characteristics then compared them to a low, medium and high velocity hockey hit and low medium high velocity puck shot. To do this, I asked members of my club to hit the force transducer with an oberhau at “tournament intensity”.

Sport	Impact Impulse (kg/m/s)
HEMA	51.9655
Low velocity hit	65.2556
Medium velocity hit	92.0368
High velocity	106.3704
Low velocity puck	4.981
Medium velocity puck	6.086
High velocity puck	7.191

This is good and bad. Bad because we do sustain a significant amount of impulse on our impacts and our helmets are not designed to take any of it. Good because hockey helmets have been fully designed to withstand that kind of impact with moderate ease. All we need to do is add a liner to our helmets. This is a band-aid on a problem. We do not currently know how well the helmets can handle the impacts, how much liner and what material to put in but at this point, anything is better than nothing. I have started putting a couple layers of EPP foam inside of a helmet to aid in the impact. From my own experience, it has made a slight difference and most notably has improved the fit of the helmet. There is a significant amount of redesign and testing that should be done for our helmets and hopefully as consumers, we can push manufacturers to do better. In the meantime, we need to work with what we have.