applied sciences

Article An Evidence Basis for Future Equestrian Lateral Crush Certification Tests

Thomas A. Connor 1,2,3, J. Michio Clark 1,4, Pieter Brama 5, Matt Stewart 2, Aisling Ní Annaidh 1 and Michael D. Gilchrist 1,*

1 School of Mechanical & Materials Engineering, University College Dublin, Belfield, 4 Dublin, Ireland; [email protected] (T.A.C.); mclark@vectorscientific.com (J.M.C.); [email protected] (A.N.A.) 2 COMFG Ltd. (Charles Owen), Royal Works, Croesfoel Ind. Park, Wrexham LL14 4BJ, UK; [email protected] 3 R&D Consulting Engineers Ltd., Leeds LS17 6AF, UK 4 Vector Scientific Inc., Golden, CO 80403, USA 5 School of Veterinary Medicine, University College Dublin, Belfield, 4 Dublin, Ireland; [email protected] * Correspondence: [email protected]

 Received: 20 March 2020; Accepted: 3 April 2020; Published: 10 April 2020 

Abstract: The aim of this study is to determine what loads are likely to be applied to the head in the event of a falling onto it and to determine by how much a typical equestrian helmet reduces these loads. An instrumented headform was designed and built to measure applied dynamic loads from a falling horse. Two differently weighted equine cadavers were then dropped repeatedly from a height of 1 m (theoretical impact velocity of 4.43 m/s) onto both the un-helmeted and helmeted instrumented headforms to collect primary force–time history data. The highest mean peak loads applied to the headform by the lighter horse were measured at the bony sacral impact location (15.57 kN 1.11 SD). The lowest mean peak loads were measured at the relatively fleshier right ± hind quarter (7.91 kN 1.84 SD). For the heavier horse, highest mean peak loads applied to the ± headform were measured at the same bony sacral impact location (16.02 kN 0.83 SD), whilst lowest ± mean peak loads were measured at the more compliant left hind quarter (10.47 kN 1.08 SD). When ± compared with the un-helmeted mean values, a reduction of 29.7% was recorded for the sacral impact location and a reduction of 43.3% for the lumbosacral junction location for helmeted tests. Notably, all measured loads were within or exceeded the range of published data for the fracture of the adult lateral skull bone. Current helmet certification tests are not biofidelic and inadequately represent the loading conditions of real-world “lateral crush” accidents sustained in equestrian sports. This work presents the first ever evidence basis upon which any future changes to a certification standards test method might be established, thereby ensuring that such a test would be both useful, biofidelic, and could ensure the desired safety outcome.

Keywords: skull fracture; dynamic crush; lateral crush; roll over; head protection

1. Introduction Equestrian helmet certification tests are designed to ensure that a minimum performance and quality level is achieved in terms of helmet crashworthiness and structural integrity. As equestrian sports are high risk [1–5], with the primary type of accident involving a fall from the horse resulting in a head impact [6], it makes good sense that the main helmet functional test in the standards involves recreating some simplified impact conditions [7–9]. The next most significant test in most equestrian helmet standards is referred to as the lateral crush test, also referred to as the lateral deformation test or rigidity test. However,

Appl. Sci. 2020, 10, 2623; doi:10.3390/app10072623 www.mdpi.com/journal/applsci Appl. Sci. Sci. 20192020,, 910, x, 2623FOR PEER REVIEW 22 of of 12 11 deformation test or rigidity test. However, unlike impact tests, the origins of which are well dunlikeocumented impact in tests, the theliterature origins [10 of, which11], the are rationale well documented and evidence in the basis literature for the [10 crush,11], thetest rationaleare unclear and. Essentially,evidence basis this for particular the crush test areis formulated unclear. Essentially, as a quasi this-static particular test to test represent is formulated a horse as dynamically a quasi-static fallingtest to representagainst or a rolling horse dynamically over the head falling of a helm againsteted or . rolling over the head of a helmeted jockey. TheThe lateral crush test itselfitself isis relativelyrelatively simple.simple.A A helmethelmet is is placed placed between between two two metal metal plates plates and and is iscrushed crushed quasi-statically quasi-statically until until a peaka peak force force is is reached reached at at a specifieda specified loading loading rate rate (see (see Figure Figure1). 1) There. There is isno no headform headform in thein thehelmet. helmet. To pass To passthe test, the maximum test, maximum and residual and residual crush limits crush must limits not must be exceeded. not be exceededPeak loads. Peak are setloads to are be 800set to N be for 800 both N for PAS both 015 PAS [9] and 015 EN[9] and 1384 EN [8] 1384 and [ 10008] and N 1000 for the N for Snell the E2016 Snell E2016standard standard [12]. In [12] all. In cases, all cases, the maximum the maximum permitted permitted crush crush is 30 is mm 30 mm and a thend residualthe residual crush crush may may not notexceed exceed 10 mm. 10 mm.

Figure 1. LateralLateral crush crush test. test.

InIn discussions discussions with with engineers engineers working working within within the standards the standards industry industry and with and standards with standardscommittee committeemembers, itmembers, is understood it is understood that the lateral that the crush lateral tests crush are usedtests toare ensure used to that ensure the helmetthat the is helmet ‘not too is ‘notsoft’ too and soft’ that and the structurethat the structure of the helmet of the has helmet some has ‘stabilizing some ‘stabilizing effect’. It effect’ is not. intendedIt is not intended to simulate to simulatea real-world a real accident.-world However, accident. there However, has been there no quantification has been no of quantification what constitutes of what a helmet constitutes that is ‘too a helmetsoft’, particularly that is ‘too if its soft’, impact particularly performance if is its sufficient. impact Additionally, performance in is discussions sufficient. with Additionally, the equestrian in discussionscommunity, with it is clear the thatequestrian the lateral community, crush test it is is believed clear that to represent the lateral a horse crush falling test is onto believed a helmet. to representIndeed, the a horse most recentfalling revisiononto a helmet of the. EN1384Indeed, standardthe most recent was to revision increase of the the peak EN1384 force standard that could was be tosustained increase from the peak 630 Nforce to 800 that N. could That be decision sustained was from taken 630 on N the to basis800 N that. That it should decision improve was taken helmet on theperformance basis that init should the event improve of a horse helmet falling performance onto a rider’s in the head. event However, of a horse there falling is no onto evidence a rider’s that head this. However,change would there have is no any evidence influence that on this helmet change performance. would have any influence on helmet performance. EquestriansEquestrians have a a high high risk risk of of head head injury injury [13] [13] and and the the majority majority of of professional professional jockey jockey fatalities fatalities areare as as a a result result of of head head injury injury sustained sustained from from a a fall fall.. Additionally, Additionally, reported rates of concussion or mild mild traumatictraumatic brain injuryinjury (mTBI)(mTBI) areare higher higher for for equestrians equestrians than than those those in in boxing boxing and and American American football football [6]. [6]However,. However, when when compared compared with with the number the number of falls of andfalls headand head impacts, impacts, crush crush injuries, injuries, particularly particularly to the tohead, the head, appear appear to be rare to be with rare few with reported few reported in the literaturein the literatur [14]. Nevertheless,e [14]. Nevertheless in some, in situations some situations such as suchcross-country as cross-country riding or riding , or eventing, a horse cana horse somersault can somersault during aduring jump anda jump land and on land the rider. on the In rider such. Incases such the cases injuries the can injuries be catastrophic can be catastrophicand sometimes and fatal sometimes [14]. There fatal may [ be14] merit. There in introducing may be merit a more in introducingrealistic crush a more test to realistic the standards crush test if the to the objective standards of the if testthe objective is to improve of the helmet test is performanceto improve helmet while performancebeing dynamically while crushed. being dynamically However, there crushed are no. However, primary empirical there are data no primary on which empirical to base such data a test.on whichIt is not to known base such what a typicaltest. It loadsis not areknown applied what to typical the rider’s loads head are during applied such to the an accidentrider’s head and itduring is not suchknown an byaccident how much and ait typicalis not known equestrian by how helmet much might a typical reduce equestrian these loads. helmet might reduce these loadsThe. aim of this present paper is to address this deficit directly by determining what loads are likelyThe to beaim applied of this topresent the rider’s paper head is to inaddress the event this ofdeficit a horse directly falling by onto determin it, anding to what investigate loads are the likely to be applied to the rider’s head in the event of a horse falling onto it, and to investigate the Appl. Sci. 2020, 10, 2623 3 of 11 Appl. Sci. 2019, 9, x FOR PEER REVIEW 3 of 12 extentextent toto whichwhich aa typicaltypical equestrianequestrian helmethelmet reducesreduces thesethese loads.loads. ItIt isis hopedhoped thatthat thesethese primaryprimary datadata willwill helphelp toto informinform andand createcreatean anevidence evidencebasis basis for for future future standard standard lateral lateral crush crush tests. tests.

2.2. MaterialsMaterials andand MethodsMethods ThisThis study study has has two two main main parts. parts. First,First, an an instrumented instrumented headform headform was was designed designed and and built built to to measuremeasure appliedapplied dynamicdynamic loadsloads fromfrom aa fallingfalling horse.horse. Second,Second, equineequine cadaverscadavers werewere droppeddropped ontoonto bothboth un-helmetedun-helmeted andand helmetedhelmeted instrumentedinstrumented headformsheadforms andand thethe associatedassociated force–timeforce–time historyhistory datadata werewere collected.collected.

2.1.2.1. InstrumentedInstrumented HeadformHeadform ToTo measure measure the the lateral lateral forces forces applied applied by the by falling the horse, falling the horse, external the geometry external of geometry the EN960:2006 of the standardEN960:2006 headform standard was headform used to createwas used a CAD to create model a (seeCAD Figure model2). (see The Figure headform 2). The was headform modelled was on sizemodelled J (the averageon size J male). (the average A Makerbot male). Z18 A 3DMakerbot printer Z18 was 3D used printer to physically was used print to physically the headform print from the thisheadform CAD model, from this similar CAD to [model15]. To, makesimilar the to printed [15]. To poly-lactic-acid make the printed (PLA) poly size-lactic J headform,-acid (PLA Monsterfil) size J 1.75headform, mm PLA Monsterfil filament 1.75 was mm printed PLA at filament 100% density was printed in 0.2 mmat 100% layers. density The printin 0.2had mm a layers two-shell. The outerprint surfacehad a two and-shell a linear outer infill. surface The headformand a linear was infill. fitted The with headform a Kistler was uniaxial fitted loadwith cell a Kistler rated touniaxial 70 kN load and datacell rated acquisition to 70 kN was and by data means acquisition of a Kistler was single by means channel of a laboratoryKistler single amplifier. channel Datalaboratory were filteredamplifier to. ISOData 6487 were [16 filtered]. to ISO 6487 [16].

FigureFigure 2.2. ((aa)) SplitSplit headformheadform explodedexploded view.view. (1) 1) Printed left-left- andand right-handright-hand sides,sides, (2)2) u uni-axelni-axel load cell, (3)3) l loadoad distribution plates, 4) (4) mounting mounting bolts bolts.. (b (b) )Split Split headform headform assembly. assembly.

2.2.2.2. EquineEquine CadaverCadaver DropDrop TestsTests ForFor thethe dropdrop tests,tests, twotwo freshfresh equineequine cadaverscadavers werewere used,used, oneone 343343 kgkg femalefemale (horse(horse 1)1) andand oneone 370370 kgkg malemale (horse (horse 2). 2). Both Both animals animals had had been been euthanised euthanised for for reasons reasons unrelated unrelated to theto the present present study study on theon the day day of testing. of testing. Full F ethicalull ethical exemption exemption was was approved approved (AREC-E-17-09). (AREC-E-17-09). TheThe equineequine cadaverscadavers werewere droppeddropped fromfrom aa heightheight ofof 1.21.2 mm ontoonto thethe instrumentedinstrumented JJ headformheadform whichwhich waswas positionedpositioned onon aa rigidrigid concreteconcrete surface.surface. TheThe dropdrop heightheight waswas chosenchosen followingfollowing analysisanalysis ofof real-worldreal-world equestrian equestrian accident accident video video footage, footage such, such as describedas described by Connorby Connor et al., et [al.,13] [ and13] and Clark Clark et al., et [17 al.,]. The[17]. concrete The concrete surface surface had been had chosen been aschosen it was as essentially it was essentially rigid and rigid served and to eliminateserved to surfaceeliminate variability. surface variability.A Manatu forklift and lifting beam was used to lift the cadavers to the drop height (see Figure3). A quickA Manatu release forklift hook clamp and lifting was used beam to was drop used the to cadaver. lift the Forcadavers the un-helmeted to the drop tests,height four (see impact Figure locations3). A quick were release chosen hook on eachclamp horse, was theused left to hind drop quarter, the cadaver. the right For hind the un quarter,-helmeted lumbosacral tests, four vertebrae, impact andlocations the sacral were vertebrae chosen on (see each Figure horse,4). These the left impact hind locations quarter, were the righ chosent hind based quarter, on the lumb analysisosacral of videovertebrae footage, and of the horse sacral falls vertebrae and they (see also Figure represented 4). These the largestimpact area locations of the were animal chosen that is based not covered on the byanalysis a saddle, of video apart fromfootage the of head horse and falls neck. and For they each also horse, represented 3 drops werethe largest carried area out of per the impact animal location. that is Thenot helmetcovered model by a saddl usede, was apart a commonly from the head available and neck 57 cm. For jockey each stylehorse, equestrian 3 drops were helmet, carried certified out per to impact location. The helmet model used was a commonly available 57 cm jockey style equestrian helmet, certified to ASTM F1163-15 [7], EN 1384 [8] and PAS015 [9]. and Ideally, other equestrian Appl. Sci. 2020, 10, 2623 4 of 11

Appl. Sci. 20192019,, 99,, xx FORFOR PEERPEER REVIEWREVIEW 44 of 1212 ASTM F1163-15 [7], EN 1384 [8] and PAS015 [9]. and Ideally, other equestrian helmet models would helmetalso have models been would tested. also However, have been due tested to availability.. However, and due price to availability constraints, and this price common constraints, and widely this commonused helmet and model widely was used chosen helmet as a good model representative was chosen of as commercially a good representative available of commercially for the 50th availablepercentile helmets male sized for head.the 50th Helmeted percentile tests male were sized carried head. out Helmeted on the lumbosacral tests were carried vertebrae out junction on the lumbosacralandlumbosacral the sacral vertebraevertebrae vertebrae junctionjunction locations, andand the asthethese sacralsacral were vertebraevertebrae found locations,locations, to be the asas most thesethese stable werewere locationsfoundfound toto bebe in thethe terms mostmost of stabledroptest locations repeatability. in terms Additionally, of drop test repeatability only the heavier.. Additionally, male animal only was the used heavier for the male helmeted animal tests,was usedas the for number thethe helmetedhelmeted of helmets teststests available,, as the number was limited of helmets to six. available The force–time was limited data were to six. recorded The force for– eachtimetime datadrop were test. Inrecorded total, 30 for drop each tests drop were test carried.. InIn total,total, out: 30 24drop un-helmeted tests were testscarried and out 6 helmeted:: 24 un--helmeted tests. Means tests and standard 6 helmeted deviations tests.. Means were calculated and standard for repeated deviations tests were at each calcula impacttedted forlocation for repeated repeated and for tests tests all at at impact each each impactlocationsimpact locationlocation combined. andand Peak forfor allall load impactimpact data werelocationslocations analysed combined.combined. statistically PeakPeak using loadload adatadata one-way werewere ANOVA analysedanalysed with statisticallystatistically post-hoc usingt-tests a (α one= 0.05)--way between ANOVA helmeted with post and--hoc un-helmeted t--teststests (α(α == tests0.05)0.05) and betweenbetween between helmetedhelmeted impact andand locations unun--helmeted to determine tests andstatistically between significant impact locations differences. to determine statistically significant differences.

Figure 3. CadaverCadaver horse horse drop drop test set up.

Figure 4. ((a)) Left Left hind hind quarter quarter impact impact location.location. ( (b)) S Sacralacral vertebrae impact location.location. ( (cc)) L Lumbosacralumbosacralsacral vertebrae impact location. (( d)) Right Right hind hind quarter quarter impact impact location. location.

3. Results Results ForceForce–time–timetime data data w werewere successfully collected for all 30 drop tests with the split headform headform proving toto be be reliable reliable and and repeatable repeatable.repeatable.. 3.1. Horse 1 Drop Tests 3.1. Horse 1 Drop Tests Horse 1 was the lighter of the two , weighting 343 kg. The most repeatable data were collected Horse 1 was the lighter of the two horses, weighting 343 kg.. The most repeatable data were from the lumbosacral junction and sacral vertebrae impact locations (see Figure5b,c). Both left and right collected from the lumbosacral junction and sacral vertebrae impact locations (see Figure 5b,c).. Both hind quarter locations were less reliable as the headform was pushed out of position in some impacts (see leftleft andand rightright hindhind quarterquarter locationslocations werewere lessless reliablereliable asas thethe headformheadform waswas pushedpushed outout ofof positionposition inin Figure5a,d). The highest mean peak loads applied to the headform were measured at the sacral impact some impacts (see Figure 5a,d).. The highest mean peak loads applied to the headform were measured location (15.57 kN). The lowest mean peak loads were measured at the right hind quarter (7.91 kN). at the sacral impact location (15.57 kN).. The lowest mean peak loads were measured at the right hind Table1 below summarises the peak force for each test, time to peak load for each test, means quarter (7.91 kN). and standard deviations for each impact location, and means and standard deviations for all impact locations on horse 1. Time to peak load is presented, as this is within the dynamic loading phase of the Appl. Sci. 2020, 10, 2623 5 of 11 impact. At some stage the load transitions from being dynamic to essentially static and so it is difficult to determine the full dynamic duration of the impact. The highest mean peak loads applied to the headform by horse 1 were measured at the sacral impact location (15.57 kN 1.11 SD). The lowest ± mean peak loads were measured at the right hind quarter (7.91 kN 1.84 SD). Appl. Sci. 2019, 9, x FOR PEER REVIEW ± 5 of 12

FigureFigure 5. Un-helmeted 5. Un-helmeted drop drop test test force–time force–time history history plots plots for for horse horse 1. (1.a) ( Lefta) Left hind hind quarter quarter impact impact location. (b) Lumbosacrallocation. (b) Lumb vertebraeosacral junction vertebrae impact junction location. impact (location.c) Sacral (c vertebrae) Sacral vertebrae impact impact location. location. (d) Right (d) hind quarterRight impact hind quarter location. impact location.

Table 1 below summarises the peak force for each test, time to peak load for each test, means and standard deviations forTable each 1. impactUn-helmeted location drop, and test means results and for standard horse 1. deviations for all impact locations on horse 1. Time to peak load is presented, as this is within the dynamic loading phase of Test no. Impact Location Peak Force (kN) Time to Peak (ms) the impact. At some stage the load transitions from being dynamic to essentially static and so it is difficult to determine1 the full Left dynamic Hind Quarter duration of the impact. 11.65 The highest mean 17.83 peak loads applied to the headform 2by horse 1 Left were Hind measured Quarter at the sacral 15.96 impact location (15.57 22.44 kN ± 1.11 SD). The 3 Left Hind Quarter 13.61 28 lowest mean peak loads were measuredMean at the right hind 13.74 quarter (7.91 kN ± 1.84 22.76 SD). SD 2.16 5.09 Table 1. Un-helmeted drop test results for horse 1. 1 Lumbosacral Junction 6.53 17.48 Test no. 2Impact Lumbosacral LocationJunction Peak 8.28 Force (kN) 70.04Time to Peak (ms) 1 3Left Hind Lumbosacral Quarter Junction 10.1511.65 68.72 17.83 2 Left Hind QuarterMean 8.3215.96 52.08 22.44 SD 1.81 29.97 3 Left Hind Quarter 13.61 28 1Mean Sacral Vertebrae 16.8513.74 21.08 22.76 2 SacralSD Vertebrae 14.892.16 25.16 5.09 3 Sacral Vertebrae 14.98 23.32 Mean 15.57 23.19 1 Lumbosacral JunctionSD 1.116.53 2.02 17.48 2 Lumbosacral Junction 8.28 70.04 1 Right Hind Quarter 7.11 31.72 3 2Lumbosacral Right HindJunction Quarter 10.0110.15 20.76 68.72 3 RightMean Hind Quarter 6.68.32 28.28 52.08 SD Mean 7.911.81 26.92 29.97 SD 1.84 5.61 1 SacralMean Vertebrae (All Locations) 11.3916.85 31.24 21.08 2 SacralSD Vertebrae (All Locations) 3.8014.89 18.31 25.16 3 Sacral Vertebrae 14.98 23.32 Mean 15.57 23.19 Appl. Sci. 2019, 9, x FOR PEER REVIEW 6 of 12

SD 1.11 2.02

1 Right Hind Quarter 7.11 31.72 2 Right Hind Quarter 10.01 20.76 3 Right Hind Quarter 6.6 28.28 Mean 7.91 26.92 SD 1.84 5.61

Mean (All Locations) 11.39 31.24 Appl. Sci. 2020, 10, 2623 6 of 11 SD (All Locations) 3.80 18.31

3.2.3.2. Horse 2 Drop Tests HorseHorse 22 waswas the heavier horse, weighting 370370 kg.kg. Again, the most repeatable data werewere collected fromfrom thethe lumbosacrallumbosacral junctionjunction andand sacralsacral vertebraevertebrae impactimpact locationslocations (see(see FigureFigure6 6b,b,c).c). However,However, left left hindhind quarter quarter data data also also showed showed good good repeatability repeatability (see Figure(see Figure6a). The 6a) highest. The highest mean peak mean loads peak applied loads toapplied the headform to the headform were measured were measured at the sacral at the impact sacral location impact (16.02 location kN). (16.02 The lowest kN). The mean lowest peak loadsmean werepeak measuredloads were at measured the left hind at the quarter left hind (10.47 qua kN).rter (10.47 Overall, kN) mean. Overall, peak mean loads peak for all loads impact for all locations impact werelocations 1.52 were kN greater 1.52 kN (11.8%) greater for (11.8%) horse 2for impacts horse compared2 impacts compared with horse with 1 impacts. horse 1 impacts.

FigureFigure 6.6. Un-helmetedUn-helmeted drop test force–timeforce–time history plotsplots forfor horsehorse 2.2. (a) LeftLeft hindhind quarterquarter impactimpact location.location. (b) LumbosacralLumbosacral junction vertebrae impactimpact location.location. (c) Sacral vertebraevertebrae impactimpact location.location. (d) RightRight hindhind quarterquarter impactimpact location.location. Table2 below shows peak force for each test, time to peak load for each test, means and standard Table 2 below shows peak force for each test, time to peak load for each test, means and standard deviations for each impact location, and means and standard deviations for all impact locations on deviations for each impact location, and means and standard deviations for all impact locations on horse 2. horse 2. Table 2. Un-helmeted drop test results for horse 2.

Test no. Impact Location Peak Force (kN) Time to Peak (ms) 1 Left Hind Quarter 9.89 18.28 2 Left Hind Quarter 11.7 33.48 3 Left Hind Quarter 9.8 35.44 Mean 10.47 29.07 SD 1.08 9.39 1 Lumbosacral Junction 12.89 61.04 2 Lumbosacral Junction 13.64 64.6 3 Lumbosacral Junction 12.27 64.88 Mean 12.93 63.51 SD 0.69 2.14 1 Sacral Vertebrae 15.81 14.12 2 Sacral Vertebrae 16.94 40.36 3 Sacral Vertebrae 15.31 44.32 Mean 16.02 32.93 SD 0.83 16.41 Appl. Sci. 2020, 10, 2623 7 of 11

Table 2. Cont.

Test no. Impact Location Peak Force (kN) Time to Peak (ms) 1 Right Hind Quarter 8.74 13.52 2 Right Hind Quarter 15.66 31.93 3 Right Hind Quarter 12.25 39.84 Mean 12.25 28.43 SD 3.46 13.5 Mean (All Locations) 12.91 38.48 SD (All Locations) 2.65 18.16

3.3. Horse 2 Helmeted Drop Tests Figure7 shows that the helmeted tests were more repeatable in general when compared to the un-helmeted tests, with good agreement between each test. As with the un-helmeted impacts, the sacral impact location transferred the highest peak loads to the headform, with a mean of 11.26 kN, although hind quarter locations were not tested in this case. Table3 below shows the peak force for each test, the time to peak load for each test, as well as the means and standard deviations for each impact Appl.location Sci. 2019 and, 9, x means FOR PEER and REVIEW standard deviations for all impact locations on horse 2 (helmeted tests).8 of 12

FigureFigure 7. 7.HelmetedHelmeted drop drop test test force force–time–time history history plots plots for for horse horse 2. 2.(a ()a Lumb) Lumbosacralosacral junction junction vertebrae vertebrae impactimpact location. location. (b ()b Sacral) Sacral vertebrae vertebrae impact impact location. location. Table 3. Helmeted drop test results for horse 2. Table 3. Helmeted drop test results for horse 2. Test no. Impact Location Peak Force (kN) Time to Peak (ms) Test no. Impact Location Peak Force (kN) Time to Peak (ms) 1 Sacral Vertebrae 11.53 56.68 1 Sacral2 Vertebrae Sacral Vertebrae 11.53 10.05 54.6 56.68 2 Sacral3 Vertebrae Sacral Vertebrae 10.05 12.2 57.64 54.6 3 Sacral VertebraeMean 11.2612.2 56.31 57.64 Mean SD11.26 1.1 1.55 56.31 1SD Lumbosacral Junction 7.341.1 78.8 1.55 2 Lumbosacral Junction 6.66 71.56 1 Lumbosacral3 Lumbosacral Junction Junction7.34 7.97 76.4 78.8 Mean 7.33 75.59 2 Lumbosacral JunctionSD 6.66 0.66 3.69 71.56 3 Lumbosacral Junction 7.97 76.4 Mean (All Locations) 9.29 65.95 MeanSD (All Locations)7.33 2.30 10.86 75.59 SD 0.66 3.69

Mean (All Locations) 9.29 65.95 SD (All Locations) 2.30 10.86

3.4. Helmeted vs. Un-helmeted vs. Impact Location Tables 4 and 5 below show statistical comparison results of both un-helmeted and helmeted impacts and pooled data comparisons for all impact locations.

Table 4. Helmeted vs. un-helmeted peak load comparisons for both combined and individual impact locations. p-Values in bold indicate statistically significant differences. Note: These values are calculated using un-helmeted data from horse 2 only as this was the horse used for helmeted impacts.

Helmeted vs. Un-Helmeted Impacts p-Value % Difference

Combined Locations <0.001 43.60 Lumbosacral Junction <0.001 55.40 Sacral Vertebrae <0.05 36.73

Table 5. Pooled peak load comparisons for all impact locations. p-Values in bold indicate statistically significant differences. LHQ and RHQ denote left and right hind quarters, respectively.

Impact Location Comparison p-value % Difference Appl. Sci. 2020, 10, 2623 8 of 11

3.4. Helmeted vs. Un-helmeted vs. Impact Location Tables4 and5 below show statistical comparison results of both un-helmeted and helmeted impacts and pooled data comparisons for all impact locations.

Table 4. Helmeted vs. un-helmeted peak load comparisons for both combined and individual impact locations. p-Values in bold indicate statistically significant differences. Note: These values are calculated using un-helmeted data from horse 2 only as this was the horse used for helmeted impacts.

Helmeted vs. Un-Helmeted Impacts p-Value % Difference Combined Locations <0.001 43.60 Lumbosacral Junction <0.001 55.40 Sacral Vertebrae <0.05 36.73

Table 5. Pooled peak load comparisons for all impact locations. p-Values in bold indicate statistically significant differences. LHQ and RHQ denote left and right hind quarters, respectively.

Impact Location Comparison p-Value % Difference LHQ vs. RHQ >0.005 18.41 LHQ vs. Lumbosacral Junction >0.005 12.98 LHQ vs. Sacral Vertebrae <0.005 26.49 Lumbosacral Junction vs. Sacral Vertebrae <0.001 39.13 Lumbosacral Junction vs. RHQ >0.005 5.46 Sacral Vertebrae vs. RHQ <0.005 44.36

4. Discussion

4.1. Horse Impact Data This study successfully collected quantitative force data from a horse falling onto a surrogate headform. To the best of the authors’ knowledge, this is the first time that such data has been collected and presented. In general, the repeatability of the force–time curves is good, despite movement of the headform in some cases. Similarities in the shape of the loading curves can be seen for each impact location and standard deviations about the mean values are small. Statistically significant differences (p < 0.05) in peak loads were observed between the left hind quarter and sacral vertebrae, the lumbosacral junction and the sacral vertebrae, and between the sacral vertebrae and the right hind quarter (see Table5). The sacral vertebrae impact location applied the highest peak loads to the headform in all cases. This is likely due to the stiff, bony nature of this location with minimal soft tissue covering. Additionally, the location is at the centre of the animal and, during the impact, there was minimal movement of the headform, which ensured that it properly registered most of the associated load. Quite the opposite was true of the hind quarter impact locations. There was much less headform stability here as initial contact between the horse and the headform could cause the headform to be pushed away from the horse. The lumbosacral junction vertebrae impact location proved to be very stable and shows a very different loading curve when compared to the other impact locations. A steep initial rise is followed by a much smaller slope, and in some cases a plateau. Due to the nature of this location, this two-stage loading phase occurs when the initial contact between the horse and the headform is first made and then, as the impact continues, the front and rear of the animal come into contact with the ground, effectively reducing the effective mass applied to the headform. Additionally, during each impact, the horse rotated as the legs came to rest on the concrete floor. It is possible that the rotation of the animal caused some additional instability of the headform. Horse mass appears to be a factor. A 7.3% increase in the mass of the falling horse resulted in, on average, an 11.8% increase in the peak load applied to the headform. The horses used in this study were not large and would typically fall into the pony category [18]. Much heavier animals are ridden Appl. Sci. 2020, 10, 2623 9 of 11 by equestrians and most sport horses weigh around 500–600 kg. A similar drop test with an animal of such a larger mass would result in a significant increase in loads applied to the headform.

4.2. Helmet Effects Helmeted drop tests were only carried out on the sacral and lumbosacral junction vertebrae impact locations. Statistically significant differences (p < 0.05) were observed between helmeted and un-helmeted impacts for both impact locations (see Table4). When compared with the un-helmeted mean values for these locations, a reduction of 29.7% can be seen for the sacral impact location and 43.3% for the lumbosacral junction location. The helmet significantly reduces peak loads applied to the headform. However, mean peak loads at the sacral and lumbosacral junction impact locations are still high at 11.26 kN and 9.29 kN, respectively.

4.3. Skull Fracture Force levels required for lateral skull fracture vary significantly. Mean values reported in the literature range from 3.5 kN to 12.4 kN [19–25]. However, these forces vary with the surface area of the impactor and the impact velocity [20]. It should also be noted that skull fracture occurs at much lower loads for children [26]. Regardless, the measured loads in every test presented in this study fall within or exceed this range. This includes the helmeted tests. Although the helmet can significantly reduce the load applied to the head, it cannot protect against skull fractures in a severe scenario such as this. However, this type of dynamic loading could be seen as an extreme case and it is quite possible that the helmet would provide significant protection in cases where the horse merely rolls onto the head rather than falls onto it. More research is required to determine if this is the case for such a quasi-static loading scenario.

4.4. Limitations The headform used was instrumented with a uniaxial load cell. This was chosen as a robust solution, given the particular experimental conditions. However, a triaxial system would have provided more information, particularly in cases where the headform moved during the impact. The headform itself could be considered a rigid body and, therefore, would not respond as a human head would. However, such a robust headform was required given the extreme nature of the tests carried out. The horses used in this study may not have represented the heavier adult sport horses and thoroughbreds and, therefore, might underestimate the actual risks occurring in equestrian sports. Nevertheless, this category of a lighter horse potentially reflects the most vulnerable in equestrian sports, namely children riding ponies. The impact locations were chosen because it was believed that they would ensure the most repeatable results. It is not known if these locations commonly come into contact with a rider’s head during impact but, at present, this is unknown for any location. The study was limited by the number of data points that could be collected and therefore, any statistical analyses were limited by the small data set. More data are required to have confidence in statistical differences presented in this paper.

4.5. Future Work More work is required to investigate loads applied to the head by horses of greater mass, different impact locations, different drop heights and to understand what influence might be associated with a saddle. Additionally, a more biofidelic headform that takes into consideration the scalp [27] and skull may provide more insight. Such experiments are difficult and expensive to arrange. It is suggested that the data presented in this present study could be used to validate a finite element or multibody mathematical model, allowing alternative scenarios to be investigated. The current standard lateral crush test method should be evaluated to see if there is any relationship between this quasi-static test and dynamic crush. It may be possible to adapt current tests as a cost-effective measure to ensure better helmet performance. Appl. Sci. 2020, 10, 2623 10 of 11

Current tests do not come close to simulating the loading conditions of real-world accidents in a biofidelic manner and any future changes to a standard test method should have a firm evidence basis to ensure the test is both useful and can lead to the desired safety outcome.

5. Conclusions The data show that the force applied to the head from a falling horse can exceed the lateral skull fracture tolerances, even if a helmet is used. The highest mean peak load applied to the headform was 16.02 kN 0.83 SD. Peak loads were reduced by as much as 43.3% for helmeted tests. However, ± all measured loads were within or exceeded the range of published data for the fracture of an adult lateral skull bone. If the current standard lateral crush test continues to be used, some clarification on the rationale for this test would be useful. However, if the equestrian and standards communities believe that the lateral crush test used in certification standards is intended to provide some protection against a horse falling onto a rider’s head, the data presented in this study should be taken into consideration. It should be used as a basis for further experimental and numerical work on which to develop new test methods, as no other data set yet exists.

Author Contributions: Conceptualization, T.A.C., P.B., A.N.A. and M.D.G.; methodology and validation, T.A.C., J.M.C., P.B. and M.S.; analysis, T.A.C., J.M.C., A.N.A. and M.D.G., resources, project administration and funding acquisition, P.B., M.S., A.N.A. and M.D.G.; writing—original draft preparation, T.A.C.; writing—review and editing, all authors. All authors have read and agreed to the published version of the manuscript. Funding: This project received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No. 642662. The funding body had no role in the design of this study or in the collection, analysis or interpretation of the data. Conflicts of Interest: The authors declare no conflict of interest. Declaration of Interest Statement: No benefits in any form have been or will be received from a commercial party related directly or indirectly to the subject of this manuscript.

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