Impact Mitigation in Modern Football Helmets: Advances and Limitations of Position-Specific Designs

Our research in the Hurt Lab focuses on how to keep people safe, and especially how helmets help us do that. We've seen in the past that head impacts cause structural, functional, and chemical changes in the brain. And so the most obvious thing for us to look at now is how do helmets contribute to safety?

To begin, obtain four different helmet models from two manufacturers for testing. Test three separate helmets for each helmet model to obtain a total of 12 helmets. Prepare a 50th percentile Hybrid III head and neck assembly testing rig secured to a steel base plate for impact testing.

After ensuring the data acquisition system and the computer running the lab view code are properly connected to power, arrange the tools for conducting impact tests on the rig to measure forces and accelerations on the bare head form and on each helmet. Plug all sensors into the data acquisition system. Double-click the custom lab view program titled Hammer Time 2017 to initiate data collection.

Enter the basic data into the appropriate fields, including the file path for the final data file, the number of hits per file set to five, the number of files per location set to four, and the headgear profile. Initiate the lab view code by pressing run to begin the data acquisition process. Use the modal impulse hammer to strike the front of the head form as a preliminary system check.

Strike the top of the head form to further confirm data collection and then strike the side of the head form to verify that the hammer and all nine accelerometers are recording data. Measure resultant accelerations at the center of mass during each impact using the nine accelerometer array arranged in a 3-2-2-2 configuration. Collect a 200 millisecond time series for each normal and oblique impact with a 70 millisecond pre-trigger, providing 130 milliseconds of acceleration and impact force measurements.

Acquire all signals at a sampling rate of 5, 120 Hz.Define 14 impact types representing two angles of incidents at seven impact locations for all test cases, including the bare head form and each helmet model. Define normal impacts as perpendicular to the surface and oblique impacts as delivered at an angle of approximately 45 degrees to the normal direction. Denote the impact types as shown.

To simplify position-specific analysis, group the defined impact types into aggregated regions such as frontal and rear aspect impacts. Administer impacts to the bare head form using a modally tuned impulse hammer to record the applied force during impact. Use the hammer impact to trigger a 200-millisecond acquisition window for each hit.

Visually inspect the recorded signals to confirm there are no recording errors. Deliver impacts across impulse levels with three repeats. Record five hits per file and collect four files per impact location to obtain 20 impacts at each location.

After completing the bare head form reference set, fit three examples of each helmet model onto the head form according to the manufacturer's specifications. Deliver all 14 defined impact types to each helmet, resulting in a total of 840 data points collected per helmet model. For the final and largest hammer strike at each location, record the impact using a high speed camera operating at a frame rate of 959 Hz.The H2T helmet had the highest mass at 2.243 kilograms, making it the heaviest helmet tested to date.

All helmets significantly reduced translational accelerations compared to the bare head form. The H2T helmet achieved the highest translational mitigation in 8 of 14 impact types, including four of six on the frontal aspect, while the H1 achieved none. The H2 helmet consistently outperformed the H1 in both translational and rotational mitigation.

On the rear aspect, the H2 and H2Q showed better translational performance in impact type seven. For rotational mitigation in impact type nine, the H1, H2, and H2T all surpassed 50%with the H2 highest at 0.529, while the H2Q scored substantially lower at 0.473. There's three things that we do that are important, that are different from the way that helmets are normally tested.

The first is that we actually measure the force going in and the acceleration at the center of mass. So having the input force and the output acceleration means that we can build a transfer function for every single head impact. The other thing that we do is we test it without the helmet, then we put a helmet on, test it again.

That gets rid of any asymmetries or any kind of non-ideology kinda, non-ideal characteristics of the neck. And then the third thing that we do is we measure translational acceleration and angular acceleration at the same time. And that's important because for many years people have only tested translational acceleration and it's widely understood that the angular acceleration or the rotational acceleration is the most damaging.

So it's important for us to get that data out here as well. What we found was that current helmets, current football helmets don't do a very good job with reducing rotational accelerations or angular accelerations, especially at the back of the helmet. So if you hit the backside of the helmet, that's where the helmet does the least to help you.

And so what we're looking for in the future is how do we design better helmets so that they absorb the most energy possible at every location around the head. And how do we extend that and do things like construction helmets or helmets in other walks of life.