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IN DEGREE PROJECT MEDICAL ENGINEERING,SECOND CYCLE, 30 CREDITS
, STOCKHOLM SWEDEN 2019
Evaluation of Test Methods for Football Helmets Using Finite Element Simulations
AÐALHEIÐUR GUNNARSDÓTTIR
KTH ROYAL INSTITUTE OF TECHNOLOGYSCHOOL OF ENGINEERING SCIENCES IN CHEMISTRY, BIOTECHNOLOGY AND HEALTH
Evaluation of Test Methodsfor Football Helmets UsingFinite Element Simulations
Aðalheiður Gunnarsdóttir
Degree Project in Medical EngineeringStockholm, Sweden 2019Supervisor: Madelen FahlstedtReviewer: Svein KleivenExaminer: Mats NilssonSchool of Engineering Sciences in Chemistry, Biotechnology andHealthSwedish title: Utvärdering av Metoder för Test av Hjälmar förAmerikansk Fotboll Genom Finita Element-Simuleringar
Abstract
Introduction: Concussions in American Football are of a major concern due tohighly reported injury rates. The importance of properly designed helmets haveshown effect in reducing the risk of injuries, such as skull fractures. However, they arenot as effective in reducing the risk of concussion. Helmets designed are required topass standards and regulations for them to be allowedwithin the football leagues. Thecurrent test methods evaluate linear impacts, but lack evaluations of oblique impactswhich are believed to cause concussions. Several test methods have been suggested,but little is known regarding how they compare.Objective: The purpose of this studywas to compare three different testmethods forevaluating helmet performance, utilizing finite element simulation. Three differenthelmet models were used for comparison, evaluating head kinematics. The helmetmodels were additionally ranked from best to worst based on their performances.Method: Three test methods, linear impactor, 45◦ angled linear impactor, and adrop test onto a 45◦ angled plate were simulated with three different open sourcehelmet models. Simulations were conducted with one impact velocity at threeimpact locations. The influence of the interaction between helmet and head was alsoevaluated by altering the friction coefficient.Results: The test methods showed different results depending on helmet models,impact locations, and kinematics evaluated. Similarly, rankings of the helmetswere varied based on methods and impact location. Little difference was observedafter lowering the friction coefficient in majority of cases. The linear and angularacceleration for the drop side impact were mostly affected.Conclusion: Further evaluations of the test methods and comparison to realimpacts is required to evaluate what method resembles head impacts best. Loweredfriction coefficient had an effect for the drop impacts, but minor effect for other testmethods.
Keywords
Concussion, Football Helmets, Test methods, FE simulations.
ii |
Sammanfattning
Introduktion: Hjärnskakningar inom amerikansk fotboll är ett stort problem medhög rapporterad skadeincidens. Effekten av bra hjälmdesign har visat sig minskaantalet skador, som t.ex. skallfrakturer. Men de har inte visat sig lika effektivamot attminska risken för hjärnskakning. Hjälmarna behöver bli godkända enligt standarderför att de ska kunna användas under matcher. Dagens testmetoder utvärderars meden linjär impaktor men inte sneda islag, som trors orsaka hjärnskakningar. Olikatestmetoder har föreslagits men lite är känt hur de skiljer sig mellan varandra.Syfte: Syftet med studien var att jämföra tre olika hjälmtestmetoder med finiteelement simuleringar. Tre olika hjälmar ingick i jämförelsen där huvudkinematikenutvärderades. Hjälmarna rankades också från den bästa till den sämsta baserat påresultatet från hjälmtesterna.Metod: Tre testmetoder (linjär impaktor, 45◦ vinklad linjär impaktor, och dropptestmot en 45◦ vinklad platta) utvärderades för tre olika hjälmar. Simuleringargenomfördes med en islagshastighet och tre olika islagspunkter. Interaktionenmellan hjälm och huvud utvärderades också genom att variera friktionskoefficienten.Resultat: Testmetoderna visade olika resultat beroende på hjälmmodell,islagspunkt och vilken kinematik som utvärderades. Likaså varierade rankingenav hjälmar med testmetod och islagspunkt. Minskning av friktionskoefficientengav små förändringar i majoriten av islagen. Linjära accelerationen ochrotationsaccelerationen för dropptesten med sidoislaget hade störst inverkan.Slutsatser: Ytterligare utvärderingar av testmetoder och jämförelse med riktigaislagssituationer för att utvärdera vilkenmetod som bäst återskapar huvudislag inomamerikansk fotboll behövs. Lägre friktionskoefficient hade effekt i dropptestet mensmå effekt för de två andra testerna.
Nyckelord
Hjärnskakning, Amerikansk fotbollshjälm, Testmetoder, Finite elementsimuleringar.
| iii
Acknowledgements
Firstly, I would like to particularly thank my supervisor Madelen Fahlstedt for allthe help and guidance throughout the process, and allowing me to work with suchan interesting topic. Thank you for all the helpful meetings, and for always takingyour time to help me whenever I ran into problems and answering my questions withpatience and positivity.
Secondly, I would like to thank my reviewer Svein Kleiven for the good andconstructive criticism.
Special thanks to my supervision group for all the helpful comments, criticism andsupport throughout the process. I would also like to thank all my fellow thesisstudents in the Neuronic group for the support
Last but not least, I would like to thank my family and friends for all the supportthroughout my studies and for always believing in me.
This study utilized models licensed from Biomechanics Consulting and Research, LC(Biocore), models derived therefrom, or both. The development of those modelswas made possible by a grant from Football Research, Inc. (FRI) and the NationalFootball League, with input from the NFLPA. The views expressed are solely thoseof the author and do not represent those of Biocore, FRI, or any of its affiliates orfunding sources [1].
iv | TABLE OF CONTENTS
Table of Contents1 Introduction 1
2 Method 42.1 Simulations of Test Methods . . . . . . . . . . . . . . . . . . . . . 42.2 Setup and Modeling of the Impactors . . . . . . . . . . . . . . . . 5
2.2.1 Linear Impactor . . . . . . . . . . . . . . . . . . . . . . . . 52.2.2 Angled Linear Impactor . . . . . . . . . . . . . . . . . . . . 62.2.3 Drop Test . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72.2.4 Modifications of the Open Source Models . . . . . . . . . . 82.2.5 Adjustments of Friction Coefficient . . . . . . . . . . . . . 92.2.6 Data Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . 9
3 Results 113.1 Simulations of Test Methods . . . . . . . . . . . . . . . . . . . . . 113.2 Adjusted Friction Coefficient Between Head and Helmet . . . . . 20
4 Discussion 264.1 Resultant Kinematics and Injury Criteria . . . . . . . . . . . . . . 26
4.1.1 Simulations of Test Methods and Helmet Rankings . . . . 264.1.2 Injury Criteria . . . . . . . . . . . . . . . . . . . . . . . . . 274.1.3 Reduced Friction Coefficient Between Head and Helmet . 29
4.2 Limitations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 294.3 Future work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
5 Conclusions 32
References 33
Appendices 33
LIST OF FIGURES | v
List of Figures
1.1 Demonstration of a drop test and linear impactor, both showingside impact. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.2 Demonstration of the drop test on to an angled surface. . . . . . 22.1 Demonstration of the FE helmet models, showing the front view
and cross section of the side view for each helmet. . . . . . . . . . 42.2 Impact locations with linear impactor. . . . . . . . . . . . . . . . 62.3 Side impact with the linear impactor. Red arrow showing the
direction of the impactor, or in the negative x-direction . . . . . . 62.4 Impact locations with angled linear impactor. . . . . . . . . . . . 72.5 Side impact with the angled linear impactor. Red arrow showing
the direction of the impactor, or in the negative x-direction . . . . 72.6 Impact locations for drop test. . . . . . . . . . . . . . . . . . . . . 82.7 Side impact of the drop test onto the angled plate. Red arrow
showing the direction of the helmet, or in the negative z-direction 83.1 Peak resultant linear accelerations for the linear impactor (LI),
angled linear impactor (ALI) and drop test (DI), for each impactlocation, comparing and ranking the different helmets. . . . . . . 12
3.2 Peak resultant angular accelerations for the linear impactor(LI), angled linear impactor (ALI) and drop test (DI), for eachimpact location, comparing and ranking the different helmets. . 14
3.3 Peak resultant angular velocity for the linear impactor (LI),angled linear impactor (ALI) and drop test (DI), for each impactlocation, comparing and ranking the different helmets. . . . . . . 16
3.4 HIC15 values for the linear impactor (LI), angled linearimpactor (ALI) and drop test (DI), for each impact location,comparing and ranking the different helmets. . . . . . . . . . . . 18
3.5 BrIC values for the linear impactor (LI), angled linear impactor(ALI) and drop test (DI), for each impact location, comparingand ranking the different helmets. . . . . . . . . . . . . . . . . . . 19
3.6 CM values for the linear impactor (LI), angled linear impactor(ALI) and drop test (DI), comparing and ranking the differenthelmets. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
3.7 Peak resultant values for theRiddell helmetmodel after loweringthe friction coefficient between the helmet and the head. Showingresults from all test methods, linear impactor (LI), angled linearimpactor (ALI), and drop test (DI), for frontal (A) and sideimpact (C). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
vi | LIST OF TABLES
3.8 Peak resultant values for the Vicis helmet model after loweringthe friction coefficient between the helmet and the head. Showingresults from all test methods, linear impactor (LI), angled linearimpactor (ALI), and drop test (DI), for frontal (A) and sideimpact (C). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
3.9 Peak resultant values for the Schutt helmet model after loweringthe friction coefficient between the helmet and the head. Showingresults from all test methods, linear impactor (LI), angled linearimpactor (ALI), and drop test (DI), for frontal (A) and sideimpact (C). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
3.10 HIC15 and BrIC values after lowering the friction coefficientbetween the helmet and the head. Showing results from all testmethods, linear impactor (LI), angled linear impactor (ALI), anddrop test (DI). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
3.11 CM values for the linear impactor (LI), angled linear impactor(ALI) and drop test (DI), comparing the different helmets. Thefriction coefficient (fc) for the Riddell and Vicis helmet waschanged form 0.5 to 0.1, and for the Schutt helmet form 0.2 to0.1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
List of Tables
2.1 Simulations performed for each helmet model. . . . . . . . . . . . 52.2 Material properties of the plate used for the drop test. . . . . . . . 82.3 Modifications made of the open source helmet models. . . . . . . 94.1 Probability of MTBI based
on linear acceleration, angular acceleration and HIC15. Criteriadeveloped based on reconstructions of helmet-to-helmet impactsrecorded during NFL games [30]. . . . . . . . . . . . . . . . . . . 28
4.2 CORA scores of the helmetmodels for the frontal and side impactsat 9.3 m/s, and overall score for the linear impactor from thehelmet model manuals [36–38]. The side impact for the Vicishelmet was not evaluated. . . . . . . . . . . . . . . . . . . . . . . . 30
LIST OF TABLES | vii
Abbreviations
NFL National Football League
NOCSAE The National Operating Committee on Standards for AthleticEquipment
HIII Hybrid III
CEN European Committee for Standards
FE Finite Element
COG Center of Gravity
CFC Channel Frequency Class
HIC Head Injury Criteria
BrIC Brain Injury Criteria
CM Combined Metric
LI Linear Impactor
ALI Angled Linear Impactor
DI Drop Impact/Test
mTBI mild Traumatic Brain Injury
AIS Abbreviation Injury Scale
CORA CORelation and Analysis
NATION National Athletic Treatment, Injury and Outcomes Network
NCAA National Collegiate Athletic Association
LOC Loss of Consciousness
SI Severity Index
DAMAGE Diffuse Axonal Multi-Axis General Evaluation
STAR Summation of Tests for the Analysis of Risk
NFLPA National Football League Players Association
THUMS Total Human Model for Safety
1. INTRODUCTION | 1
1 Introduction
Highly reported injury rates and possible long term effects of concussions resultingfrom head impacts in American Football are of amajor concern [2–5]. More than 0.6concussions were reported in the National Football League (NFL) per game duringpre-,post-, and regular season from 2010-2014 [6, 7]. In addition, approximately0.6 concussions were reported per game during regular season from 2015-2018[8]. Because of the high incidence rates, the NFL initiated a project named theEngineering Roadmap [9]. The aim of the project was to improve the protectiveequipment, and to gain an understanding of the biomechanics of head injuries inprofessional football.
Different helmets have been designed to protect the players from head injuries, sincethe first reported use of a helmet in a football game in 1893 [10]. Currently, helmetsare a mandatory protective equipment in games within football leagues, such as theNFL. Themodern football helmets have shown to be effective in protecting the playersagainst several head injuries, such as skull fractures which are believed to be causedby impacts resulting in translational kinematics. However, they are not as effectivein protecting against injuries such as concussion, which are believed to be caused byimpacts resulting in rotational kinematics (see Figure A.3 in Appendix A.3.1) [11–14].
Football helmets are subjected to tests to evaluate their performance and efficiency.That is done in order to assess the protective capability of the helmets, and to helpwith selection of appropriate helmets. The helmets designed are required to meetcertain standards and requirements to be allowed to be used within the leagues.Helmets that are to be used within the NFL and other American football leaguesare required to follow the standards and regulations developed by the NationalOperating Committee on Standards for Athletic Equipment (NOCSAE) [15]. Inthose standards, the helmets are to be subjected to a drop test onto a flat surface,and a linear impactor, where the helmets are impacted on different locations, andat different impact velocities [16–18]. Demonstration of a drop test and linearimpactor can be seen in Figure 1.1. As the NOCSAE standards only evaluate bypass/fail criteria, the helmets that are to be used within the NFL are subjected toadditional laboratory tests to evaluate and rank their performance [19]. Based onthose evaluations, several helmets have been banned from the league due to poorperformance. More information regarding the additional laboratory tests can be seenin Appendix A.4.2.
The drop test performed in theNOCSAE standards has been shown not to be effectivein evaluating the protective ability for concussions [20]. The test method utilizingthe linear impactor was proposed to be added to the NOCSAE standard by Pellmanet al. [20], to evaluate the performance in reducing the risk of concussions. Thelinear impactor was designed to simulate head-to-head impacts, but those impactshave shown to be one of themost common causing concussions in American Football[21].
2 | 1. INTRODUCTION
(a) Drop test (b) Linear impactor
Figure 1.1: Demonstration of a drop test and linear impactor, both showing sideimpact.
The test method with the linear impactor includes a Hybrid III (HIII) neck. Thatallows for rotational motion of the headform after impact, and rotational kinematicscan bemeasured. The drop test does not include theHIII neck, and does not allow forrotational motion of the head. Therefore, the drop test is not effective in evaluatingthe risk of concussion. Another method utilizing a drop test and evaluates obliqueimpacts has been developed for testing of bicycle helmets [22, 23]. That method hasbeen suggested by the European Standard Committee (CEN) for testing of helmets.It includes a drop test where a helmeted headform is subjected to a free fall ontoan angled surface instead of a flat surface. Demonstration of the test can be seenin Figure 1.2. The linear impactor allows for measurement of rotational kinematics,however in current standards it only evaluates linear impacts. By adjusting the linearimpactor, making it impact the helmets with an angle, oblique impacts might beevaluated.
Figure 1.2: Demonstration of the drop test on to an angled surface.
1. INTRODUCTION | 3
As described, several test methods have been suggested for evaluation of helmets.However, little is known about how they differwhen it comes to comparing the helmetperformance. The aim of this project was to analyze three different test methods, thelinear impactor, angled linear impactor, and the angled drop test (according to CEN,see Figure 1.2) with finite element (FE) simulations. Three different helmet modelswere used to both evaluate the ranking between helmets and if the helmet designaffected the different test methods, by evaluating the head kinematics.
4 | 2. METHOD
2 Method
Simulations were setup and analyzed using LS-PrePost® revision 4.6 [24], and ranusing LS-Dyna® revision 9.1 [25].
First, simulations of three test methods, a linear impactor, linear impactor with 45◦
angled surface, and a drop test onto a 45◦ angled plate, impacting a helmeted HIIIcrash test dummy, were conducted. Then, the simulations for two impact locationswere ran again for all test methods, where the friction coefficient between the headand helmet was adjusted.
2.1 Simulations of Test Methods
Open sourcemodels of three helmetmodels, version 1 of theRiddell Revolution SpeedClassic (model R41179), Schutt Air XP Pro (model 789102), and Vicis Zero1 (model01) from Biocore [1] were used. Demonstration of the helmets can be seen in Figure2.1. These open source models included the helmets fitted on Hybrid III crash testdummies, and a linear impactor, see Figure 1.1b [26].
(a) Riddell Revolution Speed Classic modelR41179
(b) Schutt Air XP Pro model 789102
(c) Vicis Zero1 model 01
Figure 2.1: Demonstration of the FE helmet models, showing the front view andcross section of the side view for each helmet.
Total of 9 simulations were conducted for each helmet model. Conductingsimulations at one impact velocity and at three impact locations, with the linearimpactor, the linear impactor angled by 45◦, and the drop test onto a 45◦ angled plate.
2. METHOD | 5
The impact locations and velocity were selected based on the literature, see AppendixA.6, where themost commonhead-to-head impactswere shown to be frontal and sideimpacts. Two of the impact locations selected were pre set in the open sourcemodels,one frontal impact and one side impact, labeled as A and C respectively in the opensource models. The labeling of the pre set impact locations are in accordance to theEvaluation by the NFL, see Figure A.7 in Appendix A.4.2. The third impact selectedwas an adjusted side impact which was selected based on the study by Pellman etal.[27], labeled here as C adjusted. The simulation matrix for each helmet model canbe seen in Table 2.1.
Table 2.1: Simulations performed for each helmet model.
Impact Velocity [m/s] Impact Location
Linear Impactor9.3 A (Frontal impact)9.3 C (Side impact)9.3 C adjusted (Side adjusted)
Angled LinearImpactor
9.3 A9.3 C9.3 C adjusted
Drop Test9.3 A9.3 C9.3 C adjusted
2.2 Setup and Modeling of the Impactors
2.2.1 Linear Impactor
A linear impactor was included in the open source models, and was used withoutchanges for the simulations of the linear impactor. As previously mentioned, impactlocations A and C, or frontal and side respectively, were pre set in the model. Hence,for the side adjusted impact the impactorwas positioned accordingly. Demonstrationof the impact locations and the set-up of the simulations can be seen in Figure 2.2.Figure 2.3 shows a side impact with the linear impactor, demonstrating the impactdirection of the impactor which is in the negative x-direction.
6 | 2. METHOD
(a) Impact Location A (b) Impact Location C (c) Impact Location Cadjusted
Figure 2.2: Impact locations with linear impactor.
Figure 2.3: Side impact with the linear impactor. Red arrow showing the direction ofthe impactor, or in the negative x-direction
2.2.2 Angled Linear Impactor
For the angled linear impact, the linear impactor was rotated 45◦ around the z-axis, and translated to impact the helmet at approximate the same impact locations.Demonstration of the impact locations and the set-up of the simulations can be seenin Figure 2.4. Next, the impact velocity was applied in the same direction as forthe linear impactor. As the ram and the backplate became disconnected duringimpact, constrains were created between the backplate and the ram (constrainedrigid bodies). Figure 2.5 shows the side impact with the angled linear impactor,demonstrating the impact direction of the impactor which is in the negative x-direction.
2. METHOD | 7
(a) Impact Location A (b) Impact Location C (c) Impact Location C adjusted
Figure 2.4: Impact locations with angled linear impactor.
(a) Frontal view (b) Top view
Figure 2.5: Side impact with the angled linear impactor. Red arrow showing thedirection of the impactor, or in the negative x-direction
2.2.3 Drop Test
For the drop impact, an 45◦ angled plate was included with the helmeted models,and positioned to impact approximately the same locations as with the linear andangled linear impactors. Demonstration of the impact locations and the set-up of thesimulations can be seen in Figure 2.6. The plate was modeled with shell and solidelements, and the material properties can be seen in Table 2.2. Finally, the frictioncoefficient between the helmet and impacting plate was set as 0.5. Figure 2.7 showsa side impact where the helmet is dropped on the impacting plate, demonstrating theimpact direction of the helmet which is the negative z-direction.
8 | 2. METHOD
Table 2.2: Material properties of the plate used for the drop test.
Element type Mass Density[kg/m3]
Young’smodulus [GPa]
Poisson’s ratio
Solid 1500 2050 0.31Shell 1500 30 0.25
(a) Impact Location A (b) Impact Location C (c) Impact Location C adjusted
Figure 2.6: Impact locations for drop test.
Figure 2.7: Side impact of the drop test onto the angled plate. Red arrow showing thedirection of the helmet, or in the negative z-direction
2.2.4 Modifications of the Open Source Models
Modifications were made on the Vicis helmet model due to lack of stability of themodel. A top and bottom surface was modeled on the forehead pad with shellelements, and connectedwith a single surface contact. The Riddell and Schutt helmetmodels were used without changes. The modifications made and material propertiescan be seen in Table 2.3.
2. METHOD | 9
Table 2.3: Modifications made of the open source helmet models.
Helmet model Modifications DescriptionRiddell N/A N/ASchutt N/A N/AVicis Top and bottom
surface created on theforehead pad
Shell elementsMaterial properties:Mass Density: 306 kg/m3
Young’s Modulus: 1.4 MPaPoissons’ Ration: 0.3Single surface contact
2.2.5 Adjustments of Friction Coefficient
The friction coefficient (fc) between the helmet andheadwas lowered for eachhelmet,to represent a low-friction surface between the head and the helmet. The value forthe Riddell and Vicis helmets were changed from 0.5 to 0.1, but for the Schutt helmetit was changed from 0.2 to 0.1. The simulations for all test methods were run for thefrontal (A) and side (C) impact at 9.3 m/s impact velocity.
2.2.6 Data Analysis
The kinematics were extracted to Matlab version R2018a for further analysis.Kinematicsmeasured of the first 30ms of the simulations at the head center of gravity(COG) were analyzed. The kinematics evaluated included the linear accelerations,rotational accelerations, and rotational velocities in x, y and z directions, as well asthe resultants for each impact. Before the analysis the data was filtered with ChannelFrequency Class (CFC) 180. The peak values were accessed comparing different testmethods, different impact locations, and the different helmet models. The HeadInjury Criteria (HIC15) and Brain Injury Criteria (BrIC) were then calculated foreach impact locations, for each test method. Finally, the Combined Metric (CM) wascalculated for each test method, averaging over the three impact locations, and usedfor overall ranking of helmets. HIC15 was calculated as follows:
HIC = max{(t2 − t1)[1
(t2 − t1)
∫ t2
t1
a(t)dt]2.5} (2.1)
where a is the resultant linear acceleration, and t1 and t2 are two time points duringthe acceleration pulse, with maximum difference of 15 ms [2]. BrIC was calculated asfollows:
BrIC =
√(
ωx
66.25)2 + (
ωy
56.45)2 + (
ωz
42.87)2 (2.2)
where ωx, ωy, and ωz are maximum angular velocities in the x, y and z direction,respectively [28]. The CM was calculated averaging over the different impact
10 | 2. METHOD
locations for each test method as follows:
CM =1
n
∑(αpeak
αave
+ωpeak
ωave
+HIC15
HIC15ave) (2.3)
where the n is the number of different testing conditions, αpeak and ωpeak are the peakrotational acceleration and velocity, respectively, and HIC15 as described above [29].The αave, ωave, and HIC15ave are normalizing values, which are grand average valuesof all testing conditions for all helmet models [29]. More information regarding theCM can be seen in Appendix A.3.2
3. RESULTS | 11
3 Results
Results of peak linear acceleration, angular acceleration and angular velocity, aswell as calculated HIC15, BrIC and CM values will be presented for all simulations.Performance of the helmets for each test method, linear impact (LI), angled linearimpact (ALI), and drop impact (DI), will be compared. In addition, ranking ofthe helmets based on the kinematics evaluated, HIC15, BrIC, as well as the overallperformance based on the CM values will be presented.
3.1 Simulations of Test Methods
Peak resultant linear acceleration for the frontal impact, side impact and sideadjusted impact, comparing the three test method for each helmet can be seen inFigures 3.1a, 3.1c, and 3.1e. Ranking of the helmets from best (I) to worst (III), basedon the peak linear acceleration values can be seen in Figures 3.1b, 3.1d, and 3.1f. Thedifferent test methods for the frontal impact resulted in similar ranges of values. Forthe side and side adjusted impacts however, there is a noticeable difference betweenthe different test methods. The drop test results in values noticeably higher than theothermethods, and the linear impactor results in higher values than the angled linearimpactor, except for the side adjusted impact for the Vicis helmet. Overall, the linearimpactor and angled linear impactor show similar trends in all impact locations,but the drop test results in a noticeable higher values for the side and side adjustedimpacts compared to the frontal impact. The rankings of the helmets, which helmetresulted in the lower peak linear acceleration, are dependent on impact location andtestmethod, but overall the Vicis helmet performs the best, and the Riddell the worst.Linear acceleration over the 30msperiod for all helmets and testmethods can be seenin Figure B.1 in Appendix B for the different impact locations.
12 | 3. RESULTS
(a) Resultant linear acceleration for frontal impacts (b) Ranking of helmets from best (I)to worst (III) based on peak linearacceleration for frontal impacts
(c) Resultant linear acceleration for side impacts (d) Ranking of helmets from best (I)to worst (III) based on peak linearacceleration for side impacts
(e) Resultant linear acceleration for side adjustedimpacts
(f) Ranking of helmets from best (I)to worst (III) based on peak linearacceleration for side adjusted impacts
Figure 3.1: Peak resultant linear accelerations for the linear impactor (LI), angledlinear impactor (ALI) and drop test (DI), for each impact location, comparing andranking the different helmets.
3. RESULTS | 13
Results of the peak angular acceleration, comparing the helmets, test methods, andimpact location can be seen in Figure 3.2. Angular acceleration over the 30 msperiod for all helmets and test methods can be seen in Figure B.2 in Appendix Bfor the different impact locations. Similar trends can be seen with the peak angularacceleration as with the linear acceleration. There is a noticeable difference betweenthe test methods, especially with the side and side adjusted impact. The drop testdoes not show as apparent difference compared to the other test methods for the sideand side adjusted impacts as with the linear acceleration. Rather, it is dependent onthe helmet models and impact location what test method results in the highest orlowest values. As an example, for the Riddell helmet, for the side impact the droptest shows noticeably the highest value, but for the side adjusted impact, the linearimpactor results in a value slightly lower than the drop test. For that impact, theangled linear impactor still results in a value noticeably lower than both the drop testand linear impactor. The rankings of the helmets are dependent on impact locationand test method. Nevertheless, the Schutt helmet performs best in all test methodsin the frontal impact and Riddell the worst. For the side adjusted impact, the Vicisperforms the best and the Riddell the worst for the linear impactor and drop test, butfor the angled linear impactor, the Riddell helmet performs the best.
14 | 3. RESULTS
(a) Resultant angular acceleration for frontal impacts (b) Ranking of helmets from best (I)to worst (III) based on peak angularacceleration for frontal impacts
(c) Resultant angular acceleration for side impacts (d) Ranking of helmets from best (I)to worst (III) based on peak angularacceleration for side impacts
(e) Resultant angular acceleration for side adjustedimpacts
(f) Ranking of helmets from best (I)to worst (III) based on peak angularacceleration for side adjusted impacts
Figure 3.2: Peak resultant angular accelerations for the linear impactor (LI), angledlinear impactor (ALI) and drop test (DI), for each impact location, comparing andranking the different helmets.
3. RESULTS | 15
The peak resultant angular velocity can be seen in Figure 3.3, and angular velocityover the 30 ms period for all helmets and test methods can be seen in Figure B.3 inAppendix B for the different impact locations. The values for the linear impactor andangled linear impactor are within similar ranges for all impact locations and helmets.For the drop test, the values are dependent on the impact location, with noticeablylower values for the side and side adjusted impacts for the Schutt helmet. Again, theranking of the helmets is different depending on test method and impact location.As an example, the Schutt helmet performs best with all test methods for the sideadjusted impact, with the linear impactor and drop test for the frontal impact, andthe drop test for the side impact. However it performs worst in all other impacts, orwith the angled linear impactor in frontal and side impact locations, and the linearimpactor for the side impact.
Peak values of the linear acceleration, angular acceleration and angular velocity in x,y and z directions for each impact location and test method can be seen in Table B.1for the Riddell helmet, Table B.2 for the Schutt helmet, and Table B.3 for the Vicishelmet in Appendix B.
16 | 3. RESULTS
(a) Resultant angular velocity for frontal impacts (b) Ranking of helmets from best (I) toworst (III) based on peak angular velocityfor frontal impacts
(c) Resultant angular velocity for side impacts (d) Ranking of helmets from best (I) toworst (III) based on peak angular velocityfor side impacts
(e) Resultant angular velocity for side adjusted impacts (f) Ranking of helmets from best (I) toworst (III) based on peak angular velocityfor side adjusted impacts
Figure 3.3: Peak resultant angular velocity for the linear impactor (LI), angledlinear impactor (ALI) and drop test (DI), for each impact location, comparing andranking the different helmets.
3. RESULTS | 17
Ranking of the helmets based on calculated HIC15 can be seen Figure 3.4, and theBrIC values in Figure 3.5. The HIC15 shows the same trend between different impactlocations for all test methods. The drop test showed noticeably the highest values forall impact locations and helmets, and the angled linear impactor the lowest values.
For BrIC, again there are differences comparing each test method. The drop testresults in the lowest values for all helmets in the side and side adjusted impacts, withnoticeable differences compared to the other testmethods. For the frontal impact, thedrop test results in the lowest value for the Schutt helmet. However, for the Riddelland Vicis helmet, the drop test results in slightly higher values then the other testmethods. For the rankings of the helmets, again they are interchangeable dependingon impact location and test method.
18 | 3. RESULTS
(a) HIC15 for frontal impacts (b) Ranking of helmets from best (I) toworst (III) based on HIC15 for frontalimpacts
(c) HIC15 for side impacts (d) Ranking of helmets from best (I)to worst (III) based on HIC15 for sideimpacts
(e) HIC15 for side adjusted impacts (f) Ranking of helmets from best (I)to worst (III) based on HIC15 for sideadjusted impacts
Figure 3.4: HIC15 values for the linear impactor (LI), angled linear impactor (ALI)and drop test (DI), for each impact location, comparing and ranking the differenthelmets.
3. RESULTS | 19
(a) BrIC for frontal impacts (b) Ranking of helmets from best (I)to worst (III) based on BrIC for frontalimpacts
(c) BrIC for side impacts (d) Ranking of helmets from best (I) toworst (III) based on BrIC for side impacts
(e) BrIC for side adjusted impacts (f) Ranking of helmets from best (I)to worst (III) based on BrIC for sideadjusted impacts
Figure 3.5: BrIC values for the linear impactor (LI), angled linear impactor (ALI)and drop test (DI), for each impact location, comparing and ranking the differenthelmets.
20 | 3. RESULTS
The CM can be seen in Figure 3.6. The CM shows that the drop test results inthe highest values, and the angled linear impactor the lowest for all test methods.Comparing the helmets, the rankings were different depending on test method. Forthe linear and angled linear impactor, the values are within small range, but for thedrop test the difference comparing the helmets is more noticeable. The Vicis helmetperformed best in all test methods. The Schutt performed the worst with the linearand angled linear impactor, but the Riddell the worst for the drop test.
(a) CM values for all impacts and test methods (b) Ranking of helmets from best (I) toworst (III) based on CM values
Figure 3.6: CM values for the linear impactor (LI), angled linear impactor (ALI)and drop test (DI), comparing and ranking the different helmets.
3.2 Adjusted Friction Coefficient Between Head and Helmet
Peak resultant kinematics before and after lowering the friction coefficient betweenthe head and helmet can be seen in Figure 3.7 for the Riddell helmet, Figure 3.8 forthe Vicis helmet, and Figure 3.9 for the Schutt helmet. The results from reducing thefriction coefficient did not show noticeable differences in majority of the cases. Inseveral cases the kinematic values reduced, but in others they increased.
For the Riddell helmet, the most differences were for the drop side impact in angularacceleration and linear acceleration. For that impact, the angular accelerationdropped by about 45% (see Figure 3.7b), and the linear acceleration reduced by about12% (see Figure 3.7a). For the frontal impact, there were small changes for the linearacceleration. The angular acceleration reduced for the angled linear impactor anddrop test, but increased for the linear impactor. For the angular velocity, it wasreduced for the linear impactor, but increased for the angled linear impactor anddrop test.
3. RESULTS | 21
The changes for the Vicis helmet showed similar trend as the Riddell helmet, howeverwith smaller differences. Again, themost differenceswas a reduction for the drop sideimpact, where the angular acceleration was reduced by 18%.
The Schutt helmet showed even less changes after reducing the friction coefficient.For that helmet, the drop side impact did not decrease in angular acceleration, butincreased slightly, or by about 5%.
(a) Peak resultant linear acceleration (b) Peak resultant angular acceleration
(c) Peak resultant angular velocity
Figure 3.7: Peak resultant values for the Riddell helmet model after lowering thefriction coefficient between the helmet and the head. Showing results from all testmethods, linear impactor (LI), angled linear impactor (ALI), and drop test (DI), forfrontal (A) and side impact (C).
22 | 3. RESULTS
(a) Peak resultant linear acceleration (b) Peak resultant angular acceleration
(c) Peak resultant angular velocity
Figure 3.8: Peak resultant values for the Vicis helmet model after lowering thefriction coefficient between the helmet and the head. Showing results from all testmethods, linear impactor (LI), angled linear impactor (ALI), and drop test (DI), forfrontal (A) and side impact (C).
3. RESULTS | 23
(a) Peak resultant linear acceleration (b) Peak resultant angular acceleration
(c) Peak resultant angular velocity
Figure 3.9: Peak resultant values for the Schutt helmet model after lowering thefriction coefficient between the helmet and the head. Showing results from all testmethods, linear impactor (LI), angled linear impactor (ALI), and drop test (DI), forfrontal (A) and side impact (C).
The calculatedHIC15 andBrIC values canbe seen inFigure 3.10, and theCM inFigure3.11 for all helmets. As with the linear acceleration, theHIC shows themost reductionin drop side impact, especially for the Riddell helmet. The other test methods andimpact locations show minor changes. The BrIC shows similar results as with theangular velocity for the respective helmet, except for the linear frontal impact forthe Riddell helmet. For that impact the BrIC increases after lowering the frictioncoefficient.
24 | 3. RESULTS
(a) Riddell HIC (b) Riddell BrIC
(c) Vicis HIC (d) Vicis BrIC
(e) Schutt HIC (f) Schutt BrIC
Figure 3.10: HIC15 and BrIC values after lowering the friction coefficient betweenthe helmet and the head. Showing results from all test methods, linear impactor(LI), angled linear impactor (ALI), and drop test (DI).
3. RESULTS | 25
For the CM, the overall changes are minimum for the linear impactor and angledlinear impactor, but are more noticeable for the drop test. The biggest change isfor the Riddell helmet, where it is reduced by about 17% after reducing the frictioncoefficient.
Figure 3.11: CM values for the linear impactor (LI), angled linear impactor (ALI)and drop test (DI), comparing the different helmets. The friction coefficient (fc) forthe Riddell and Vicis helmet was changed form 0.5 to 0.1, and for the Schutt helmetform 0.2 to 0.1
26 | 4. DISCUSSION
4 Discussion
The aim of this project was to compare three different test methods for evaluating theprotective capability of helmets in American Football with finite element simulation.Three different open source helmetmodels were used for evaluation, and they rankedbased on resultant kinematics and injury criteria. Simulations were ran for threeimpact locations and one impact velocity, whichwere selected based on the literature.The pre set impact locations of frontal (A) and side (C) impacts were selected basedon the reason that majority of helmet-to-helmet concussion cases have resulted fromfrontal or side impacts, and at an average velocity of approximately 9.3 m/s. Theside adjusted impact location was chosen for evaluation, to assess if it would showdifferent results than the pre set side impact. The location of the side adjustedimpact was selected from a study by Pellman et al. [27], as that impact location wasshown to be one of the common impacts in the NFL. That study was selected as itdemonstrated in a clear way where the impacts were located. Other and more recentstudies considered showed side impacts where the impact location could have beenover a large area.
For further evaluation of the helmets, the friction coefficient between the head andthe helmet was reduced to evaluate if that would result in reduced peak rotationalkinematics.
4.1 Resultant Kinematics and Injury Criteria
4.1.1 Simulations of Test Methods and Helmet Rankings
The results of the test methods were different depending on the impact location,and the helmet models. For the frontal impacts, the peak linear and rotationalaccelerations showed relatively similar results between the test methods, with theexception of angular acceleration of the Riddell helmet. In that case there wasa clear difference between the test method resulting in the lowest value and thehighest, which were the linear impactor and the drop test respectively. However,more differences were noted between the test methods for the side and side adjustedimpacts. For the linear acceleration, the drop test resulted in extensively highervalues than the other methods for all helmet models. But for the angular velocity,the drop test resulted in the lowest values for that impact locations. For the angularacceleration, there was no single method that resulted in higher values than other.Rather, it was interchangeable between helmet models and impact locations.
For the linear acceleration, the Vicis helmet ranked the best for majority of the testmethods and impact locations, with exception of two caseswhere it ranked the secondbest. A possible explanation is that the Vicis helmet has a lower Young’s Modulusthan the other helmet models. The Young’s modulus of the Vicis model was set as5.5 MPa, compared to 15.65 MPa and 18.36 MPa for the Riddell and Schutt modelsrespectively. Hence, the shell of the helmet can deformmore during an impact. Other
4. DISCUSSION | 27
possible explanations of the noticeable differences comparing the drop test and thelinear and angled linear impactors are the modeling differences of the drop plate andthe impactors. The drop plate is modeled with a higher Young’s Modulus than thelinear impactor. In addition, the friction coefficient between the surface of the plateand the helmet was set as 0.5, but for the linear impactor it is set as 0.1. The reasonfor the higher friction coefficient for the drop plate was that if kept lower, the helmetsslid down the plate after impact.
Another possible explanation of the difference of kinematics between test methodsis that for the drop test, the helmet is in a free fall, but for the linear and angledlinear impactor, the headform is restricted with a HIII neck. The HIII neck mightbe restricting the motion of the head to some degree. Nevertheless, the free fallof the headform might result in unrealistic movements of the head as there are norestrictions of motions.
The difference between the rotational kinematics of the helmets can also be explainedby the geometry of the helmets, and the locations of impact. As an example, bycomparing different time steps of the simulations of the different helmets for linearfrontal impact, it can be seen that after the impact with the Schutt helmet, theheadform does not rotate as much as with the Riddell and Vicis helmet. This canbe seen in timesteps 10-20ms in Figure B.4 for the Riddell helmet, Figure B.5 for theSchutt helmet, and Figure B.6 for the Vicis helmet in Appendix B.
4.1.2 Injury Criteria
The HIC15 values resemble the results of the linear acceleration for majority of cases.However, for some of the cases the ranking of the helmets change. For majority ofcases, the HIC15 values are the highest for the drop tests, and lowest for the angledlinear impactor with the exception for the frontal impact with the Riddell helmet. Forthat case, the value for the linear impactor is slightly higher than for the drop test.The values for the linear and angled linear impactor are within the range from 307to 655, and for the frontal drop impact from 498 to 746. However, for the side andside adjusted drop impacts, they range from 1110 to 2308. An injury criteria, showingprobability of mild traumatic brain injury (mTBI) based on reconstruction of helmet-to-helmet impacts recorded during games in theNFL has been developed by Zhang etal.[30], evaluating the risk based onHIC15, and linear and angular acceleration. Thatcriteria and can be seen in Table 4.1. Majority of the HIC15 values resulting from thetest methods, for all impact locations are higher than the value of 80% probability ofmTBI (369), with an exception of few casesmeasuredwith the angled linear impactor.As an additional comparison, a HIC of 1000 has been evaluated to have a 16% riskof serious brain injury [31]. The results from the drop test, side and side adjustedimpacts are all over that limit. Those excessive values can not be seen comparing thelinear and angular accelerations, where only few cases exceed the 80% risk. However,the selected impact velocity is an average of impacts resulting in concussions.
28 | 4. DISCUSSION
Table 4.1: Probability of MTBI based on linear acceleration, angular acceleration andHIC15. Criteria developed based on reconstructions of helmet-to-helmet impactsrecorded during NFL games [30].
Probabilityof mTBI
LinearAcceleration [g]
AngularAcceleration [rad/s2]
HIC15
25% 66 4600 15150% 82 5900 25080% 106 7900 369
The BrIC values, calculated based on angular velocity, is in accordance to the resultsof the angular velocity. Hence, it shows different results depending on test method,impact location and helmet model, with noticeably lower values from the drop testsfor the side and side adjusted impacts for the Riddell and Schutt helmet. Accordingto the injury risk curve presented by Takhounts et al. [28], a BrIC value betweenapproximately 0.65 to 0.7 would have a 90% risk of a minor head injury, includingmTBI, according to the Abbreviation Injury Scale (AIS) 1[13]. A BrIC value 1 wouldhave a 20% risk of head injuries of AIS 4-5, or severe and critical injuries. Majority ofthe BrIC values for the test methods are below 1. However, for the frontal impact, allthe values for the linear impactor, as well as the angled linear impactor for the Vicishelmet, and the drop test for the Vicis andRiddell helmets are above 1. Majority of theBrIC values are above 0.65 to 0.7, resulting in a 90% risk of mTBI, with the exceptionof the Schutt and Riddell helmets for the drop test in side and side adjusted impacts.However, as previously mentioned, the combination of selected impact velocity andlocations are taken from previous reported concussion cases.
Finally, the CM values, combining the linear acceleration (or HIC15), angularacceleration and angular velocity, resulted in the highest values for the drop testsand the lowest for the angled linear impactor. Again, the ranking of the helmets aredepending on the test method used. The ranking of helmet in the NFL is performedwith this method, and with a linear impactor. In this study, the ranking for thelinear impactor resulted as follows: the Vicis helmet ranked the best, then the Riddellhelmet, and the Schutt helmet preformed the worst. The rankings are the same withthe angled linear impactor, but with lower values. When looked at the drop test, theranking is however not the same. For the drop test, the Vicis helmet still performs thebest, but the Schutt helmet the second best. According to the rankings fromNFL, themodel of the Schutt helmet used in this study has been banned from the league dueto poor performance [32]. As a comparison, the same model of the Vicis helmet wasranked the top performing helmet. As mentioned, the Vicis helmet performed thebest with the linear and angled linear impactors, and Schutt the worst. Nevertheless,the values are within a very small range, and the Schutt helmet was ranked first orsecond for several cases of the kinematics. As an example, it was ranked first for alltest methods for frontal angular acceleration (see Figure 3.2a-3.2b).
4. DISCUSSION | 29
4.1.3 Reduced Friction Coefficient Between Head and Helmet
The friction can be different depending on the fit of the helmet, and if the user hasa lot of hair or is bold [33]. In addition, the HIII dummies have a much higherfriction coefficient than the scalp. It has been shown that different friction coefficientsbetween the head and the helmet can affect the peak kinematics of oblique impacts[34, 35]. The friction coefficient between the head and the helmet was reduced tovery low value, or 0.1, to represent a low friction, sliding layer, and to access if thatwould have an affect on the peak angular kinematics.
When adjusting the friction coefficient between the head and the helmet, it wasnoticed that the coefficient for the Schutt helmet was set differently than the for theRiddell and the Vicis helmet. The friction coefficient was set low, or at 0.2, for theSchutt helmet, which might have affected the peak values in the simulations of thedifferent test methods. Hence, for the Schutt helmet, it was expected that the resultswould show less changes after adjusting it from 0.2 to 0.1, than after changing thecoefficient from 0.5 as with the other helmet models.
Comparing the kinematics of the Riddell helmet, it can be seen that the largestchange was for the drop side impact, where the linear and angular accelerationreduced. Changes were also observed of angular acceleration and velocity for theangled linear impactor and drop impact for the frontal impact. There, the angularacceleration reduced, but the angular velocity increased. Other impacts resulted inless changes. Similar trends could be seen with the Vicis helmets, where the mostnoticeable change were for the drop side impact for all kinematics. As expected,the Schutt helmet resulted in minimum changes for all kinematics. For the overallperformance rankings evaluated with the CM, comparing before and after adjustingthe friction coefficient again the most changes can be seen for the drop test with allhelmet models. The other test methods show minimum changes.
4.2 Limitations
Only one impact velocity was used for evaluation and 2-3 impact velocities. Byevaluatingmore impact locations and velocities it could increase the credibility of theresults. As could be seen, kinematics of the side and side adjusted impacts showedsimilar trends, but the frontal impact showed different trends both for the results oftest methods and helmet rankings.
The adjustment of angled linear impactor selected impacts the helmets with the edgeof the impactor. By adjusting it in a different way, by impacting with a larger surfaceof the impactor it might resemble the drop test better. For the drop test, the frictionbetween the helmet and the plate had to be increased to 0.5 from 0.1, as the helmetslid down the plate after impacting with a lower friction.
The open sourcemodels used lacked stability in several cases, and adjustments had tobe made. The Vicis model had to be adjusted for it to run without error terminations
30 | 4. DISCUSSION
for every case. Nevertheless, the changesmadewere evaluated to haveminor effect onthe results. The helmet models have been assessed with the CORelation and Analysis(CORA) for several cases, comparing results of simulations and experimental test,and relating how well they resemble [36–38]. However, the linear acceleration andangular velocity were only evaluated over a 30 ms time window from the start of theimpact, and not the angular acceleration. In addition, not all models were assessedfor the frontal (A) and side (C) impacts at 9.3 m/s. The CORA scores have valuesbetween 0 and 1, where 0 represent that the two results are completely different, and1 represents that they are identical. The CORA scores for the helmet models, for thefrontal and side impacts at 9.3 m/s, and overall scores with the linear impactor canbe seen in Table 4.2.
Table 4.2: CORA scores of the helmet models for the frontal and side impacts at 9.3m/s, and overall score for the linear impactor from the helmet model manuals [36–38]. The side impact for the Vicis helmet was not evaluated.
Helmet ImpactHead LinearAcceleration
Head AngularVelocity
RiddellFrontal (A)@ 9.3m/s 0.6 0.66Side (C) @ 9.3 m/s 0.84 0.74Overall 0.76
SchuttFrontal (A)@ 9.3m/s 0.76 0.698Side (C) @ 9.3 m/S 0.813 0.688Overall 0.772
VicisFrontal (A)@ 9.3m/s 0.45 0.66Side (c) @ 9.3 m/s N/A N/AOverall 0.73
There were limitations of the injury criteria used. The BrIC criteria takes intoaccount rotational motion of the head, the rotational velocity, which is thought to beimportant to evaluate the risk of concussion. However, it was developed for frontalimpacts only. The main limitation of this criteria is that it was developed based ontests conducted on animals [28]. The CM was only calculated based on two andthree testing conditions, or two and three different impact locations and one impactvelocity. In theNFLhelmet rankings, the CM is calculatedwith eight impact locationsand at three impact velocities utilizing only the linear impactor [19].
4.3 Future work
More impact locations and impact speeds could be tested and compared. Forevaluations of the angled linear impactor, other adjustments of the impactor could betested, e.g. to access if the rotation of the impactorwould affect the results. That couldbe done by rotating the impactor around a different axis, and/or evaluating whateffect it would have to rotate it by more or less than 45◦. In addition, adjustments
4. DISCUSSION | 31
of the impactor where it impacts the helmets with a larger surface, instead of with theedge of the impactor, could be conducted and assessed.
Adjustments of the helmet models should be conducted to increase stability. Inaddition, they should be further validated, both for the linear impactor, e.g. byvalidating the angular acceleration, as well as validations with the other testmethods.
Finally, the results of the different test methods should be further evaluated,comparing to and evaluating real impacts to assess what test methodmost resemblesthose impacts.
32 | 5. CONCLUSIONS
5 Conclusions
The aim of the project was to compare three different test methods for evaluationof testing performance of American Football helmets. The test methods werecompared with three different helmet models, and the helmets ranked models. Forfurther evaluations, the friction coefficient between the helmet and the head wasreduced.
The different test methods showed different results depending on the helmets andimpact locations. They showed different results depending on what kinematics andinjury criteria were evaluated. In addition, the ranking of the helmets were differentdepending on what test method was evaluated and showed minimum differences.However, overall the Vicis helmet performed best.
The effect of reducing the friction coefficient between the helmet and the head did notresult in a noticeable difference inmajority of the cases. However, noticeable changeswere seen in reduction of linear and angular acceleration for the drop side impacts forthe Riddell and Vicis helmets. The Schutt helmet had lower pre set friction coefficientand showed less changes. After changing the friction coefficients, the overall rankingsshowed noticeable changes for the drop test only. Hence, the friction coefficientbetween the helmet andhead appears to have an effect for the drop test, butminimumeffect for the linear and angled linear impactor.
REFERENCES | 33
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[25] LS-DYNA. URL: http://www.lstc.com/products/ls-dyna.
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38 | REFERENCES
Appendices
REFERENCES | 39
Appendix - Table of Contents
A State of The Art 40A.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40A.2 Head Related Injuries . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40A.3 Head Injury Mechanism and Injury Criteria . . . . . . . . . . . . . . . . 42
A.3.1 Head Injury Biomechanics . . . . . . . . . . . . . . . . . . . . . 42A.3.2 Injury Criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
A.4 Performance Standards andEvaluations ofHelmets in American Foot-ball . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45A.4.1 The National Operating Committee on Standards for Athletic
Equipment (NOCSAE) . . . . . . . . . . . . . . . . . . . . . . . 46A.4.2 Evaluations by the National Football League (NFL) . . . . . . . 48A.4.3 STAR Rating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48A.4.4 Test Method Proposed by European Standard Committee (CEN) 49
A.5 Finite Element Analysis of Head Injuries . . . . . . . . . . . . . . . . . 50A.5.1 Hybrid III Dummy model . . . . . . . . . . . . . . . . . . . . . . 50A.5.2 Helmet Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51A.5.3 Human Body Models . . . . . . . . . . . . . . . . . . . . . . . . 52
A.6 Previous Studies on Head Impacts in American Football . . . . . . . . 53
B Results from simulations 55
References 62
40 | A. STATE OF THE ART
A State of The Art
A.1 Introduction
Concussion, or mild Traumatic Brain Injuries (mTBI), are of a concern in AmericanFootball, with highly reported injury rates and possible long term effects [1–4]. Thereported injury rates of concussion per game in the National Football League (NFL)wasmore then 0.6 concussions per game during pre-, post- and regular season gamesfrom 2010-2011 to 2013-2014 [5, 6]. According to the latest reported injury datareleased by NFL, the rate of concussion was approximately 0.6 concussions per gameduring regular seasons in 2015-2018 [7]. The rate of concussion is not only of aconcern in the NFL, but in high school and collegian football as well. The injuryrate reported to the National Athletic Treatment, Injury and Outcomes Network(NATION) from134high schools during academic years from2011-2014, resulted in arate of 9.21 concussions per 10000 athlete exposure [8]. The rate in collegian football,reported to the National Collegiate Athletic Association (NCAA) in 2009-2014, was6.71 per 10000 athlete exposure [9]. With the increased awareness of the highlyreported concussion cases, many studies have been conducted to investigate whatis happening when concussions occur, by analyzing different head impacts.
Concussion, and other head related injuries, cannot be fully prevented. Howeverthey could be reduced with properly designed protective equipment, such as helmets.The first documented use of a helmet in American Football was a leather helmet in1893 [10, 11]. They were adopted due to significant morbidity and mortality withinthe sport. With the development of helmets, current helmets have shown to beeffective in reducing the risk of some head related injuries, such as skull fractures,but not as effective in reducing the risk of concussions [12–14]. In order to designproper equipment, the injury mechanisms need to be fully understood. In addition,to ensure the quality and performance of the helmets, effective regulations andstandards need to be in place, and be in accordance to the most recent evidences ofthe injury mechanisms. In the following chapters the current understanding of headrelated injuries, head injury biomechanics and criteria will be described, as well asthe current performance standards and evaluations of football helmets.
A.2 Head Related Injuries
Head related injuries can be categorized in several ways. They are often classifiedbased on if the injuries are to the skull or the brain, and then further categorized basedon type and severity of injury. Injuries classified as skull injuries include injuries tothe skull and soft tissues, or the scalp, while injuries to the brain include injuries to thecerebral cortex and the membranes between the brain and the skull, or the meninges(dura mater, arachnoid and pia mater) [1]. A cross-section of the head, showing theskull, meninges and part of the cerebral cortex can be seen in Figure A.1.
A. STATE OF THE ART | 41
Figure A.1: Cross-section of the head, showing the skull and meninges [15].
Brain injuries can then be further divided into focal injuries and diffuse injuries.Focal injuries are injuries to the brain where the damage is locally well-defined, suchas hematoma (epidural-, subdural- or intracerebral hematoma) or contusions, andoften appear either on the site of impact or at the opposite site of impact. Diffusebrain injuries are however not locally well-defined. They include injuries such asconcussion, swelling in the brain, and diffuse axonal injury [1]. This categorizationof head related injuries can bee seen in Figure A.2.
Head Injury
Skull Injury
Fracture
Facial skullSkull
Soft tissue injuries
LacerationContusion
Brain Injury
Focal
Hematoma
EpiduralSubdural
Intracerebral
Contusion
Diffuse
ConcussionSwelling
Diffuse axonal injury (DAI)
Figure A.2: Categorization of head related injuries. Modified from [1].
Injuries can be further categorized based on severity, as mild, moderate or severe.Concussion is a form of mild traumatic brain injury (mTBI), and is themost commontype of head related injuries, particularly in sports [1]. The symptoms of concussioncan be varied, hence they can be difficult to diagnose. The most common symptomsinclude: headache, difficulty concentrating, difficulty remembering, confusion,blurred vision, dizziness, nausea or vomiting, balance problems, and sensitivity tolight [11]. Concussions can even be further sub-classified as mild, moderate or severe[2]. Majority of concussions in sports are mild concussions which do not involveloss of consciousness (LOC) [16]. Moderate and severe concussions are however
42 | A. STATE OF THE ART
characterized by immediate LOC, where the severity can be classified based on thelength of LOC, and for how long the symptoms last [1, 2]. Even though concussionsare categorized as mild, they are of a major concern as they can have long term effect,and repeated concussions can lead to degeneration of the brain tissue [1]. Therefore,it is important that concussions are managed properly, and if a player sustains aconcussion, he/she should be removed from the game immediately [11].
A.3 Head Injury Mechanism and Injury Criteria
The mechanism of head injuries is complex, and has been investigated for decades[13]. It has been shown that head injuries are typically the result from either director indirect impacts to the head [17]. Injury biomechanics studies the effect of themechanical impacts on the biological system [13].
A.3.1 Head Injury Biomechanics
Different directions of impacts have been shown to result in different types ofinjuries. Pure radial impacts, results in translational motion, and cause primarilylinear accelerations of the head, while oblique impacts cause both translational androtationalmotion of the head and the brain [18]. Pure radial impacts are not relativelycommon in sports aswell as other accidents. An illustration of these impact direction,there relative kinematics and the types of injuries that are believed that can be theresult of them can be seen in Figure A.3.
As previously stated, concussion has different symptoms, and is often difficult todiagnose. Many studies have been conducted to try to predict the injury mechanismbehind it, and they provide conflicting evidence. Some studies state that translationalmotions, such as linear acceleration, are the primary factor, while others state thatrotational motions, such as rotational acceleration and rotational velocity, are theprimary factor [17]. The most recent publications state that it is the combinationof both translational and rotational motion that are the cause of concussion [1, 18–21].
A. STATE OF THE ART | 43
Figure A.3: Illustration of radial and oblique impacts, the resulting kinematics andtypes of injuries modified from [18]. The top row demonstrates radial impact and thebottom row oblique impact.
A.3.2 Injury Criteria
Several thresholds have been suggested such as for strain, linear acceleration,rotational acceleration, and the combination of linear and rotational acceleration toassess the risk of concussion, as reported by Patton et al. [22] and Elkin et al. [23].However, they further reported that there are several factors that have been shownto affect the risk, such as the impact location, i.e. at what site of the head the impactis, and what part of the brain is being evaluated.
Several criteria have been developed to try to predict head injuries. The differentcriteria evaluate different parameters to evaluate the risk of certain injuries. Asan example, some evaluate only either translational motion or rotational, whileothers evaluate both translational and rotational motion. Few of those injury criteriadeveloped for head injuries will be presented.
Severity Index (SI):The severity indexwas proposed in 1966 and is still being usedto evaluate head injury potential [13], and is as follows:
SI =
∫ T
0
A2.5dt (A.1)
44 | A. STATE OF THE ART
Where A is the instantaneous resultant linear acceleration, dt is the time incrementand 0 to T is the essential duration of the acceleration pulse.
Head Injury Criteria (HIC): The Head Injury Criteria (HIC) was proposed in1972, and is a modification of the SI [13]. It also takes into account the resultantlinear acceleration over time, and is as follows:
HIC = max{(t2 − t1)[1
(t2 − t1)
∫ t2
t1
a(t)dt]2.5} (A.2)
Where a is the resultant linear acceleration, and t1 and t2 are any two arbitrary timepoints during the acceleration pulse. Two common formulations ofHIC are theHIC15
and HIC36. They represent that the maximum difference between t1 and t2 is 15 msfor HIC15 and 36 ms for HIC36 [1].
Combined Metric (CM): The Combined Metric (CM) was developed by NFL, andevaluates risk of concussion by taking into account both peak linear and rotationalhead accelerations of the head during an impact, as well as rotational velocity [24].CM is averaged over 24 test conditions, testing 8 different impact sites and 3 differentimpact velocities. The different impact sites and impact velocities will be furtherdescribed in section A.4.2. The equation for CM is as follows:
CM =1
24
∑(αpeak
αave
+ωpeak
ωave
+HIC15
HIC15ave) (A.3)
where αpeak and ωpeak are the peak rotational acceleration and velocity, respectively,HIC15 is the head injury criteria described above. αave, ωave, and HIC15ave areaverage values of the rotational acceleration, rotational velocity and HIC for anygroup of helmets being compared [24]. The average values used in the years 2015-2017 can be seen in Table A.1.
Table A.1: The normalization values used for calculating CM in 2015-2017 [24].
Year αave(rad/s2) ωave(rad/s) HIC15ave
2015 4102 37.8 2092016 4127 38.0 2082017 4088 37.7 202
Brain Injury Criteria (BrIC): The Brain Injury Criteria was developed for bettercharacterization of traumatic brain injuries, and is as follows:
BrIC =
√(ωx
ωxC
)2 + (ωy
ωyC
)2 + (ωz
ωzC
)2 (A.4)
where ωx, ωy and ωz are maximum angular velocities in x, y and z directionrespectively, and ωxC , ωyC and ωzC are critical angular velocities in their respectivedirection [25]. The critical angular velocities can be seen in Table A.2.
A. STATE OF THE ART | 45
Table A.2: Critical angular velocities used for calculating BrIC [25].
Critical Max AngularVelocity
Average value [rad/s]
ωxC 66.25ωyC 56.45ωzC 42.87
Diffuse Axonal Multi-Axis General Evaluation (DAMAGE): DAMAGE is arecently developed criterion to predict themaximumbrain strain of head impacts dueto rotational motion about each axis of the head [26]. It is a complex second ordermechanical system constructed using a physical mass-spring-damper analogue. Itcan be represented as follows:
DAMAGE = βmaxt{|⇀
δ (t)|} (A.5)
⇀
δ = [δx(t)δy(t)δz(t)]T (A.6)
where⇀
δ is a vector that contains displacement time histories of three coupledmasses,and β is a scaling factor [26].
A.4 Performance Standards and Evaluations of Helmets inAmerican Football
Helmets have become a standard safety equipment in American Football in order toprotect the players when sustaining impacts to the head. Several different helmetshave been designed, and in order to evaluate their performance, different testmethods and standards have been developed.
The National Operating Committee on Standards for Athletic Equipment (NOCSAE)has developed performance standards for the helmets, as well as other athleticequipment, in order to enhance the performance of the equipment [27]. Severalregulatory bodies for sports in the United States require that the athletic equipmentcomplies to these standards, and has the certification for them to be allowed withinthe organizations.
TheNOCSAE standards are based on pass/fail criterion [28]. Hence, for being able tofurther evaluate the performance of helmets and compare them, othermethodologieshave been developed. One method is the Summation of Tests for the Analysis ofRisk (STAR) evaluation system, which ranks the performance of the helmets with1-5 stars. In addition to the requirement of complying to the NOCSAE standards,NFL has developed further performance evaluations of the helmets used within theleague.
46 | A. STATE OF THE ART
A.4.1 The National Operating Committee on Standards for Athletic Equipment(NOCSAE)
NOCSAE is an independent, nonprofit organization that develops performance andtest standards for athletic equipment based on research, which are regularly revised[27]. NOCSAE has developed standards both general, that are to apply for allprotective equipment, as well as for specific equipment such as the football helmets.The tests required to performwith the football helmets include a drop test and a linearimpactor, which are based on pass/fail criterion. Both test methods utilize a specialNOCSAE headform which the helmets are fitted on, and for the test with the linearimpactor, the NOCSAE headform is additionally mounted on to a neck of a HybridIII 50th percentile male dummy [28, 29]. Figure A.4 demonstrates a side impact withthe drop test and linear impactor.
(a) Drop test (b) Linear impactor
Figure A.4: Demonstration of a drop test and linear impactor, both showing sideimpact.
Drop testThe drop test utilizes a twin wire impactor, and the headform fitted on the carriageassembly [28]. The helmet to be tested is placed on the headform and dropped froma height where it should achieve acceptable free fall velocity. At impact, the linearacceleration is measured and used to calculate the Severity Index (SI). The pass/failcriterion for the drop test is 1200 SI, i.e. the helmet passes the test if it scores below1200 SI.
All helmet samples must be impacted at 7 locations (6 defined locations and 1random), at 2-4 different impact velocities [29]. All impact locations require tobe tested at ambient temperature (22◦C ± 2◦C) first, then 2-4 sites must be testedadditionally at a high temperature (39◦C ± 1◦C). The different impact velocities are3.46m/s, 4.23m/s, 4.88m/s and 5.46m/s, and the impact sites are front, side, frontboss, rear boss, rear, top, and random [29]. The impact locations can be seen inFigure A.5.
A. STATE OF THE ART | 47
(a) Side view (b) Front view
Figure A.5: Demonstration of approximate impact locations used in the NOCSAEstandards utilizing the drop test [29].
Linear impactorThe linear impactor is a pneumatic ram impactor, which impacts the helmetedNOCSAE headform and HIII neck, that is mounted onto a linear bearing table [30].The helmets are impacted at one impact velocity, 6.0 m/s, at six different impactlocations: side, rear boss NC, rear boss CG, rear, front boss, and front [29]. Thedifferent impact locations can be seen in Figure A.6. At the impact, the resultant peaklinear acceleration, SI, and resultant peak rotational acceleration are captured [30].For the helmets to pass the test the SI should not exceed 1200, and the rotationalacceleration should not exceed 6000 rad/s2 [29].
(a) Side view (b) Front view
Figure A.6: Demonstration of approximate impact locations used in the NOCSAEstandards utilizing the linear impactor [30].
48 | A. STATE OF THE ART
A.4.2 Evaluations by the National Football League (NFL)
As the NOCSAE certification is only based on pass/fail criterion, the NFL and NFLPlayers Association (NFLPA) in cooperation with biomechanical experts performadditional evaluation of the helmets [31]. They perform additional laboratory testingand an annual survey of the helmets the players use . The performance of the helmetsis then ranked frombest to worst and publish in order to help with choices of helmets.In 2018, the NFL prohibited players within the league to use several helmets for thefirst time, as they were evaluated to perform poorly [31].
In 2015-2017, the laboratory testing performed for the evaluation utilized a linearimpactor, a Hybrid III 50tĥ percentile male head which the helmets were fitted on[24]. According to the test protocol, each helmet was subjected to 24 impact tests,with 3 speeds at 8 impact locations (4 to the facemask and 4 to the shell). The impactspeeds were 5.5 m/s, 7.4 m/s and 9.3 m/s, and the impact locations can be seen inFigure A.7. Impacts A, A’, B, and UT are impacts to the facemask, and C, D, F, and Rare impacts to the helmet shell [24].
The helmets were then ranked based on calculation of CM (see Section A.3.2), wherethe lower values indicate better helmet performance.
(a) Side view (b) Front view
Figure A.7: Demonstration of approximate impact locations used inNFL rating [24,32].
A.4.3 STAR Rating
The Summation of Test for the Analysis of Risk (STAR) equation is a mechanism thatwas developed to evaluate the protective capability of football helmets, integratingexposure of the players head impact and risk of concussion [33]. The equation wasdeveloped based on datameasured during real impacts, and represents a range of on-field impacts [34]. The impacts were reconstructed utilizing a pendulum impactor,
A. STATE OF THE ART | 49
that impacted helmets fitted on a NOCSAE head and Hybrid III neck at four impactlocations and three impact velocities. The STAR equation is as follows:
STAR =4∑
L=1
3∑V=1
E(L, V ) ∗R(a, α) (A.7)
where E represents the head impact exposure as a function of the impact location (L)and velocity (V), and R represents the injury risk as a function of peak resultant headacceleration (a) and rotational acceleration (α), for each specific impact location andvelocity. The four impact locations are: front, front boss, side, and back, and thethree impact velocities are: 3.0, 4.6, and 6.1 m/s [34]. The impact locations can beseen in Figure A.8. Based on the results of the STAR value, the helmets are rankedwith number of stars ranging from 1-5, where 5 stars is the top score.
(a) Side view (b) Front view
FigureA.8: Demonstration of approximate impact locations used in the STARrating[34].
A.4.4 Test Method Proposed by European Standard Committee (CEN)
As previously mentioned, concussion is believed to be caused by an oblique impact,causing rotational motion of the head (see Section A.3.1) [35]. The drop tests utilizedin current standards and evaluations, where the helmets are dropped against a flatsurface, evaluate pure radial impacts, and not the oblique impacts. Hence, they arenot believed to be effective in evaluating the risk of concussion [18, 35]. Test methodevaluating oblique impacts has been developed for testing bicycle helmets, and hasbeen proposed by the CEN to be added for evaluations of helmets [35, 36]. Thatmethod utilizes a drop test, where a helmeted headform is dropped in a free fall ontoan angled anvil. Demonstration of the drop test can be seen in Figure A.9.
50 | A. STATE OF THE ART
Figure A.9: Demonstration of the drop test on to an angled anvil.
A.5 Finite Element Analysis of Head Injuries
The Finite Element Method (FEM) has been utilized to investigate the brain injurymechanisms and predict injuries from impacts to the head. FEM is a numericalmethod where the behaviour of complex physical systems can be simulated withcomputational modeling [37]. The geometry of the physical system is first modeledby breaking it down into small elements and nodes, which are then assembled andan approximate solution found by solving a system of equations. The solution isdependent on how the geometry is modeled, selection of type of elements, as well asspecification ofmaterial properties, boundary-, initial-, and loading conditions.
Several FE models have been developed and utilized in research for analysis andprediction of injuries, such as models of anthropomorphic test devices (crash testdummies), human body models and helmet models. Each model has differentcharacteristics, and thereforemight result in varied results. Several studies have beenpublished evaluating head/brain response of impacts inAmericanFootball, includingevaluation of concussion risk such as by evaluating deformation of the brain duringan impact, and the strain in the brain [14, 22, 38, 39]. However, few studies havebeen published including a model of American football helmet.
A.5.1 Hybrid III Dummy model
Hybrid III crash test dummies are frequently used for evaluation of injurymechanisms, including reconstruction of impacts and in performance standards forevaluations of football helmets. Commercially available models have been developedof theHybrid III crash test dummies, e.g. fromHumenatics Innovation Solution [40]and used in research.
Recently, an open-source Hybrid III, head and neckmodel was developed as a part ofNFL’s project named the Engineering Roadmap [41]. The headmodel was composed
A. STATE OF THE ART | 51
of three parts including the deformable skin rubber, a rigid skull, and a head mount,and the neckmodel of ten parts, representing the neck butyl rubber, aluminum discs,neck cable, occipital-condyle pin joint, and nodding block. The head and neck modelcan be seen in Figure A.10.
Figure A.10: Demomstration of FE Hybrid III head and neck model.
This open-source model additionally includes a model of the NOCSAE headform, thelinear impactor, pendulum impactor, and a drop test, based on the current testingmethods for American Football helmets [41]. The Hybrid III and impactor modelswere optimized and validated by performing a series of experiments and simulationsby Giudice et al. [41]. For each impact response, a composite ISO (cISO) score wascalculated, resulting in a score between 0 and 1, where 0 indicates no similarity and 1indicates that the response is identical. The results showed a good correlation of thehead and neck kinematics, with a mean cISO scores for each of each impact between0.65 and 0.81.
A.5.2 Helmet Models
Open-source FEmodels for four helmets, frequently used by players within the NFL,have been created as a part of NFL’s Engineering Roadmap [42]. FE models weredeveloped of the Vicis Zero1 (model 01), Shutt Air XP Pro (model 789102) , RiddellRevolution Speed Classic (model R41179), and the Xenith X2E helmets, based on thedesign and material characteristics of the respective helmet [42, 43]. The differenthelmet models can be seen in Figure A.11, demonstrating a front view and a crosssection of the side view for each helmet. By developing models for various helmets,they could possibly help predict how the different helmets react to real-world impacts,analyzing how the helmets protect the head, as well as give information for futureimprovements in helmet design [42].
The development of the helmetmodels included creating the geometry of the helmets,material characterization utilizing material testing, and model validation [44]. The
52 | A. STATE OF THE ART
helmets have different design and characteristics, but modern football helmetsusually include five predominant sub-components: outer shell, energy-absorbingelements, comfort pads, facemask, and a strap system [44]. FE models of eachcomponent of the helmet was first generated and validated, before assembling thecomponents to generate the full helmet system [44–46]. The helmets were fittedon a open-source model of the Hybrid III dummies, presented in section A.5.1, andNOCSAE headforms.
(a) Riddell Revolution Speed Classic (b) Schutt Air XP Pro
(c) Vicis Zero1 (d) Xenith X2E
Figure A.11: Demonstration of the FE helmet models, showing the front view andcross section of the side view for each helmet.
A.5.3 Human Body Models
Different FE human body models have been developed to try to represent thecomplex human body in detail. Due to the complexity of the human body, thedevelopment of these models take years, and many simplifications have to be made[47]. However, they provide important information and give the opportunity tostudy injury mechanisms that are difficult to evaluate otherwise, such as brain tissueanalysis of impacts for evaluation of concussion [14, 23].
The Total Human Model for Safety (THUMS) is an example of those models, whichwas developed by Toyota Motor Corporation and Toyota Central R&D, and as thename implies, is a model of the whole body. The model includes the outer shape,as well as bones, muscles, ligaments, tendons and internal organs [48]. Anotherexample is a detailed head and neckmodel, the KTH FE human head and neckmodelwhich was developed at KTH Royal Institute of Technology, to investigate varioushead impact injuries [49]. In a study by Alvarez et al. [50], these two models were
A. STATE OF THE ART | 53
combined and referred to as the THUMS/KTH v1.4 model, where the head and neckof the THUMS model version 1.4 have been replaced with the KTH head and neckmodel. They stated that as the KTH model has a more detailed representation of thehead and neck, that by combining the models it would result in a better anatomicalrepresentation, especially of the brain, and a better control of muscle activationparameters.
A.6 Previous Studies on Head Impacts in American Football
With increased awareness of highly reported rates of concussion in AmericanFootball, many studies have been conducted to investigate head impacts that occur inthe sport. As an example, series of articles on concussion in professional football hasbeen published in Neurosurgery as a result of a clinical and biomechanical researchstudy initiated by NFL in 1994, performing analysis of on-field data from 1996-2001[51].
Video analysis of head impacts in NFL, and laboratory reconstructions of the on-field impacts have been conducted in several studies. The analysis of head impactsresulting in concussion include evaluations of impact locations, impact directions,impact velocities, impact types (including if the impacts are from other players, e.g.head-to-head impacts, or shoulder-to-head impact, or to the ground), and if playersin certain player positions are at more risk than others. Pellman et al. [20, 52],Clark et al. [5], and Lessley et al. [53] studied head impacts that occurred duringthe seasons from 1996-2001, 2013-2014, and 2015-2017, respectively. Pellman etal. [52] performed video analysis of 182 severe helmet impacts, evaluating theimpact speed and performing a detailed analysis of impact location and direction.In addition, reconstructions were performed of 31 impacts with helmeted Hybrid IIIdummies. Clark et al. [5] analyzed videos of 429 impacts, evaluating anticipationstatus, closing distance, impact location on the injured player’s helmet, and impacttype. Lessley et al. [53] analyzed 332 concussion cases, performing analysis includinghelmet impact locations, impact type, player activity and player position. The resultby Pellman et al. [52] showed that the impact velocity of concussed struck playerswas on average between 6.1 m/s and 9.8 m/s, and was based on impact location.Twenty-nine% of impacts were on the facemask, and the rest on the helmet shell.Sixty-one% of the impacts involved another players helmet, followed by helmet-to-body, and helmet-to-ground impacts. Majority of helmet-to-helmet and helmet-to-body impacts occurred on the side of the struck players helmet, while majority ofhelmet-to-ground occurred to the rear part of the helmet. For the striking players,majority of impacts were to the front of the helmet shell, but they seldomly resultedin reported concussions. The results by Clark et al. [5] showed that types of impactwere approximately evenly distributed between helmet-to-helmet (37.8 %), helmet-to-body (32.4%) and helmet-to-ground (29.8%) impacts, and with impact locationsbroadly distributed over the helmet of the injured player (face mask, front, back, left,right, or crown). Total impacts to the side (left and right) counted for approximatelyhalf of all impacts [5]. Contradictory to those results, Lessley et al. [53] showed that
54 | A. STATE OF THE ART
the source of concussion in nearly half of all impacts were caused by helmet-to-bodyimpacts, followed by helmet-to-helmet impacts, and lowest proportion of helmet-to-ground impacts. However, in all of three studies, the majority of head impacts of thestruck players were to the side of the helmets.
Studies have also been conducted evaluating the test methods used in theperformance standards. Hernandez et al. [54] investigated how closely the twin-wire drop test, similar as used in the NOCSAE standard, models the head rotationin impacts in American Football. The study included field measurements of 421head impacts, singular reconstructed on-field impact where two players engaged inhead-to-head contact, and a laboratory testing, utilizing the drop test in accordanceto the STAR helmet rating protocol. The singular on-field impact was additionallyreconstructed utilizing the drop test. Peak translational acceleration, rotationalacceleration, rotational velocity, SI, Head Impact Power (HIP), and GeneralizedAcceleration Model of Brain Injury (GAMBIT) were then evaluated from all tests,and compared [54]. The results showed that the drop test was unable to producemajority of the rotational velocity data, and the direction of peak acceleration datawas dissimilar even though the amplitudes were similar. They further discussed thatthe results suggested that the drop test does not model human head rotation in headimpacts occurring in American Football and are resulting in mTBIs.
B. RESULTS FROM SIMULATIONS | 55
B Results from simulations
(a) Riddell frontal impact (b) Schutt frontal impact (c) Vicis frontal impact
(d) Riddell side impact (e) Schutt side impact (f) Vicis side impact
(g) Riddell side adjusted impact (h) Schutt side adjusted impact (i) Vicis side adjusted impact
Figure B.1: Resultant linear acceleration comparing the different test methods,linear impactor (LI), angled linear impactor (ALI) and drop test (DI), with thedifferent helmets for the frontal, side and side adjusted impacts.
56 | B. RESULTS FROM SIMULATIONS
(a) Riddell frontal impact (b) Schutt frontal impact (c) Vicis frontal impact
(d) Riddell side impact (e) Schutt side impact (f) Vicis side impact
(g) Riddell side adjusted impact (h) Schutt side adjusted impact (i) Vicis side adjusted impact
Figure B.2: Resultant angular acceleration comparing the different test methods,linear impactor (LI), angled linear impactor (ALI) and drop test (DI), with thedifferent helmets for the frontal, side and side adjusted impacts.
B. RESULTS FROM SIMULATIONS | 57
(a) Riddell frontal impact (b) Schutt frontal impact (c) Vicis frontal impact
(d) Riddell side impact (e) Schutt side impact (f) Vicis side impact
(g) Riddell side adjusted impact (h) Schutt side adjusted impact (i) Vicis side adjusted impact
Figure B.3: Resultant angular velocity comparing the different test methods, linearimpactor (LI), angled linear impactor (ALI) and drop test (DI), with the differenthelmets for the frontal, side and side adjusted impacts.
58 | B. RESULTS FROM SIMULATIONS
(a) 0 ms (b) 5 ms (c) 10 ms (d) 15 ms (e) 20 ms
Figure B.4: Different timesteps of the simulation of the linear impactor, frontalimpact with the Riddell helmet model.
(a) 0 ms (b) 5 ms (c) 10 ms (d) 15 ms (e) 20 ms
Figure B.5: Different timesteps of the simulation of the linear impactor, frontalimpact with the Schutt helmet model.
(a) 0 ms (b) 5 ms (c) 10 ms (d) 15 ms (e) 20 ms
Figure B.6: Different timesteps of the simulation of the linear impactor, frontalimpact with the Vicis helmet model.
B.RESU
LTSFR
OM
SIMULATIO
NS
|59
Table B.1: Peak kinematic values from simulations of the test methods, for each impact locations, with the Riddell RevolutionSpeed Classic (R41179) helmet model.
KinematicsLinear Impactor 45◦ Angled Linear
ImpactorDrop test
Frontal Side Side Adjusted Frontal Side Side Adjusted Frontal Side Side Adjusted
Acceleration:X [g] 61.1 7.8 12.9 58.9 26.0 24.4 82.6 23.6 24.9Y [g] 55.9 121.3 121.4 50.7 91.7 88.1 49.8 213.5 142.2Z [g] 87.3 55.3 90.2 72.8 71.5 86.8 59.7 124.3 155.0Rotational Acceleration:X [rad/s2] 4881.2 6243.0 8080.9 4185.4 4596.4 5223.5 6055.3 8407.2 8335.8Y [rad/s2] 5889.0 1474.4 2440.7 7216.7 1352.7 1752.9 8347.3 3783.2 4301.6Z [rad/s2] 7021.6 2344.5 2759.6 6199.0 4166.8 2325.5 2910.5 3757.8 3578.6Rotational Velocity:X [rad/s] 35.2 48.2 49.9 23.1 51.5 52.5 31.8 36.8 34.6Y [rad/s] 36.3 10.3 22.5 42.5 13.8 13.1 58.8 8.1 10.5Z [rad/s] 33.9 12.7 17.6 21.1 20.5 17.0 14.3 6.9 5.6
60|
B.RESU
LTSFR
OM
SIMULATIO
NS
Table B.2: Peak kinematic values from simulations of the test methods, for each impact locations, with the Schutt Air XP Pro(789102) helmet model.
KinematicsLinear Impactor 45◦ Angled Linear
ImpactorDrop test
Frontal Side Side Adjusted Frontal Side Side Adjusted Frontal Side Side Adjusted
Acceleration:X [g] 79.6 13.5 11.7 67.5 26.9 12.6 77.8 5.3 20.3Y [g] 54.8 128.1 119.7 42.1 87.5 95.4 46.7 195.1 175.8Z [g] 70.8 76.2 84.1 55.7 74.1 73.9 18.5 75.6 101.1Rotational Acceleration:X [rad/s2] 2618.5 6536.3 8020.9 2944.8 5265.6 6894.8 3068.5 4864.8 5310.1Y [rad/s2] 4815.6 1670.5 1690.8 3825.8 1581.6 1632.4 4389.7 1671.6 2366.9Z [rad/s2] 3286.5 4844.6 2735.7 2180.4 6453.8 3738.1 1952.1 2965.6 4188.9Rotational Velocity:X [rad/s] 25.8 51.5 47.1 27.4 52.0 46.7 21.0 15.1 14.2Y [rad/s] 48.8 15.5 18.3 44.7 15.3 13.9 38.0 4.2 7.4Z [rad/s] 15.1 15.3 16.4 17.0 35.1 20.2 17.5 14.1 9.9
B.RESU
LTSFR
OM
SIMULATIO
NS
|61
Table B.3: Peak kinematic values from simulations of the test methods, for each impact locations, with the Vicis Zero1 helmetmodel.
KinematicsLinear Impactor 45◦ Angled Linear
ImpactorDrop test
Frontal Side Side Adjusted Frontal Side Side Adjusted Frontal Side Side Adjusted
AccelerationX [g] 72.2 19.4 18.7 65.7 24.7 19.2 84.6 18.5 15.6Y [g] 52.0 94.6 81.8 45.8 72.5 90.0 41.6 154.8 118.4Z [g] 70.9 56.9 73.0 61.4 37.6 51.9 29.4 62.4 71.9Rotational AccelerationX [rad/s2] 4099.4 4556.5 4955.5 4365.8 5491.6 5226.2 4544.8 7121.9 5921.3Y [rad/s2] 5380.8 1069.6 1727.8 5322.7 1374.7 1418.1 5118.6 3027.7 2446.1Z [rad/s2] 2695.6 2786.6 2341.3 4032.3 3687.2 2904.2 3839.9 3403.3 2392.2Rotational VelocityX [rad/s] 34.7 48.3 52.5 30.0 44.6 46.0 24.1 40.5 44.0Y [rad/s] 43.9 6.6 8.5 35.6 9.1 14.0 57.2 12.1 10.7Z [rad/s] 26.7 14.3 23.5 34.6 20.0 19.1 19.3 14.2 12.1
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