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ContentslistsavailableatScienceDirect

Journal of Science and Medicine in Sport

j ou rn a l h om ep a ge :w w w . e l s e v i e r . c o m / l o c a t e / j s a m s

Review

Human performance research for military operations in extreme cold environments ^夽

Wendy Sullivan-Kwantes

^a,∗

, Francois Haman

^b

, Boris R.M. Kingma

^c

, Svein Martini

^d

, Emilie Gautier-Wong

^e

, Kong Y. Chen

^f

, Karl E. Friedl

^g

aDRDC-Toronto,DefenceResearchandDevelopmentCanada,Ontario,Canada

bUniversityofOttawa,Ottawa,Ontario,Canada

cTNO,TheNetherlandsOrganizationforAppliedSciences,Soesterberg,TheNetherlands

dNorwegianDefenceResearchEstablishment,Kjeller,Norway

eLesVoltigeursdeQuébec,35CanadianBrigadeGroup,QuebecCity,Quebec,Canada

fNIDDK,NationalInstitutesofHealth,Bethesda,Maryland,USA

gUSArmyResearchInstituteofEnvironmentalMedicine,Natick,Massachusetts,USA

a rt i c l e i nf o

Articlehistory:

Received6July2020

Receivedinrevisedform6October2020 Accepted17November2020

Availableonline15December2020

Keywords:

Arctic

humanperformance militarypersonnel cold

thermogenesis

personalequipment–coldweather loadcarriage,metabolicregulation

a b s t ra c t

Objectives:SoldierperformanceintheArcticdependsonplanningandtraining,protectiveequipment, andhumanphysiologicallimits.Thepurposeofthisreviewwastohighlightthespanofcurrentresearch onenhancingsoldiereffectivenessinextremecoldandaustereenvironments.

Methods:ThepracticesofseasonedsoldierswhotrainintheArcticandcold-dwellingnativesinform performancestrategies.Weprovideexamplesofresearchandtechnologythatbuildontheseconcepts.

Results:Examplesofcurrentperformanceresearchincludeevaluationofequipmentandtacticssuchas thebioenergeticsofloadcarriageoversnowinNorwegianexercises;Canadianﬁeldmonitoringofhand temperaturesandfreezingcoldinjuriesforbetterprotectionofmanualdexterity;andDutchpredictive modelingofcold-wetworktolerances.Healthyyoungmencanrespondtocoldwithasubstantialther- mogenicresponsebasedonUSandCanadianstudiesonbrownadiposetissueandothermechanisms ofnon-shiveringthermogenesis;thepotentialadvantageofgreaterfatinsulationisoffsetinobeseunﬁt subjectsbyasmallerthermogenicresponse.Currentphysiologicalstudiesareaddressingpreviously unansweredproblemsofcoldacclimationprocedures,thermogenicenhancementandregulation,and modulationofsympatheticactivation,allofwhichmayfurtherenhancecoldsurvivalandexpandthe performanceenvelope.

Conclusion:ThereisaninseparablebehavioralcomponenttosoldierperformanceintheArctic,andeven thebestequipmentdoesnotbeneﬁtsoldierswhohavenottrainedintheactualenvironment.Training inexperiencedsoldierstoperformancelimitsmaybehelpedwithpersonalmonitoringtechnologiesand predictivemodels.

).

1. Introduction

“ThelinebetweensurvivingandoperatingissubtleintheArctic.

Withonlyasmallmisstep,aunitorindividualcantransition fromﬁghtingforatacticalgoaltoﬁghtingforsurvival.”

夽Thispaperrepresentsasynthesisofpresentationsanddiscussioninathematic sessiononArcticoperations.ThisisalsoaworkproductofNATOpanel310which metatthesameconference,withoverlappingmembershipinthissession.

∗Correspondingauthor.

E-mailaddress:[email protected] (W.Sullivan-Kwantes).

ObservationsrecordedafterajointCanada-US“ExerciseGuer- riereNordique”CaptainNathanFry¹

TheArcticregion,sharedbyCanada,UnitedStates,Denmark, Norway,Sweden,Iceland,FinlandandRussia²isoneoftheextreme environmentsonearth.Thisregionhassomeofthecoldesttem- peratures,strongwinds,fog,iceblizzards,crevassesanddepending ontheseason,24hoflightordarkness.Inadditiontothechalleng- ingenvironment,theArcticcircleisalargeareaencompassing∼6

%oftheearthandover 21millionkm².Inthisareabeyondthe treeline,infrastructureisextremelylimitedrequiringairtransport inresponsetoemergencies(medical,industrial,tourism,natural

https://doi.org/10.1016/j.jsams.2020.11.010

creativecommons.org/licenses/by-nc-nd/4.0/).

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disasterorsearchandrescue).^2,3Dependingonthetypeofevent, emergencyresponsecantakeanywherefromhourstodaysoreven longerduetogeographiclocation,weather,terrain,andavailability ofaircraftandresponsepersonnel.^1,4,5

InterestintheArcticisincreasingduetoglobalwarmingand themeltingofthearcticiceresultingintheopeningofnewocean routes,increasingaccesstomineralandoilreservesandexploita- tionofotherpotentialArcticresources.^2,6 Agrowinginterestin theArcticregionincreasesthelikelihoodthatitwillbemorefre- quented andcontested.Thisheightenstheneedforaneffective Arcticresponsecapability forconﬂicts,emergencies,safety,and environmentalconcerns.Soldiersmustbehighlyspecialized,well trained,wellequippedandself-sufﬁcienttooperatesuccessfully and survive in this remote and extreme environment. Military preparedness in recent years has focused on hot climates but NorwayandCanadahavenowestablisheddedicatedArcticunits thatareestablishingtactics,techniquesandproceduresforArctic operations.¹

Forthisbriefreview,wechosesixdeepdivetopicsthatspan theproblemsetforhumanperformanceinArcticOperations.We bringforthperspectivesofsoldierswhotrainintheArctic,military performanceresearchersworkingonimprovedmaterielandtac- ticsforarcticoperations,andﬁeldandlaboratory-basedintegrative physiology investigationsfocused oncold weatherperformance problemsolving.Inaddition,wediscussourrespectivelaborato- ries’effortsinunderstandinghowtheextremecoldaffectshuman performance,culminatinginadiscussionthathighlightstheimpor- tanceoftreatinghumanperformanceastheproductofmultiple factorsworkingasanintegratedsystemthatcanpermitaperson tothriveintheextremecoldorleavesthemvulnerabletoinjury.

2. Preparationandﬁeldtrainingexercises:whatarethe issues?

Inthe1940s,asetofmilitaryexercisesintheCanadianArctic highlightedadvancesaswellascontinuingchallenges anddefi- cienciesin equipment,training, communication,and tactics.^1,7,8 Many of theproblems identifiedare continuingissuesfor mili- taryoperationsintheArctictoday.Improvedpersonalprotective equipment(PPE)andtraininginitsproperusestillleadsthelistof challenges.Coldweatherprotectiveclothingisstillcumbersome, andthehobblingeffectofbulkyandrestrictiveclothingincreases energycostsforthesoldiersbyatleast10%.^9,10Thisisfurthercom- plicatedwhenpersonalprotectiveequipmentforCBRNEorbody armorisalsorequired.¹¹Handsandfeetareparticularlyvulnera- bletoperformanceimpairmentandinjuryinthecold.^12–16Soldiers wantabetterhandprotectionsystemthatprovidesthermalpro- tectionbutpermitsgreaterdexterity;largemittensalonedonot suffice.¹³Protectionofthehandsinextremecoldhasbeenthesub- jectofmanystudiesbutcontrolofperipheralvasoconstrictionis stillelusive.Norwegianstudiesofcold-inducedvasoconstriction indicatelargevariabilityinreperfusionofthefingersinhealthynor- malsoldiers.¹⁷Thismaybeapredictorofrisk,asSwedishsoldiers slowesttorecoverperfusionoftheirfingerswereathigherriskfor CWIduring15monthsoffieldtraining,¹⁸whilecoldpressortests measuringmoresystemicresponsesdidnotpredictCWIinSwedish cadetsandCanadiansoldiers.^19,20Moistureis aspecialproblem inthecold,increasingrisksofhypothermiaandseriousmedical threats suchas trench foot,produced byextended exposureto dampcoldconditionsinaboot.²¹Cold-wetissuescanbemitigated bybetterequipmentdesignbut,mostimportantly,involvetrain- ingtopreventtheseproblems,suchasworkpacingandproper self-management.Keyresearchandtechnologyproblemsrelated toequipmentandtrainingaresummarizedinTable1.

Table1

ResearchandtechnologywishlistforhumanperformanceenhancementinArctic operations.

1 Increasedhandprotection,enablingmanual

dexterityandreducingfreezingcoldinjuryrisk (includingmethodstoincreaseperipheral bloodﬂowtowarmextremities)

2 Sweatmanagementsystem/fabricsinclothing

andboots

3 Biologically-basedthermogenicstrategiesto preventhypothermia(e.g.,effectiveprotocol forcoldacclimatization;dietarysupplements thatincreasenonshiveringthermogenesis)

4 Wearablephysiologicalmonitoringthat

functionsinextremecold

5 Non-foggogglesandbetterfaceprotection

6 Virtualrealityteamtrainingforsafe

indoctrinationofnovicesoldierstoArctic operationsandtheconceptof“comfortably cold”

7 Protectiveshelterswithlowthermal

signaturesandnewheater/stovetechnology forrapidandefﬁcientwarming,heatingwater, anddryingofclothing(withreductionofﬁre andCOrisk)

8 Medicaltreatmentprotocolsandmedical

equipmentfortraumaticcareintheArctic

Arcticmilitarytrainingismulti-facetedandconsistsofteaching basicArcticsurvivaltechniques,creatingasurvivalplan,wearing andmaintainingpersonalprotectiveequipment,buildingshelters, guidanceonhowtokeepwarmaswellastoavoidoverheating, avoidingcoldinjuries,ﬁrst-aid,andbeingpreparedforthepsy- chological and mental challenges of being in the Arctic. Arctic warriorsrequirearesetofexpectationswithamentalpreparation tobe“comfortablycold,”andanunderstandingofthedifference betweendiscomfortandcriticalsignalsthatrequirepromptcorrec- tiveaction.ForCanadianArctictroops,thesearepartofastandard trainingcurriculumwhichisrequiredbeforedeployingtotheArc- tic.Preparationfor exercisessuchasGuerrierNordique(Nordic Warrior)requireequipmentshakedowns,tentsetup,stovecraft, andextensivehandsontraining.Decadesoflessonslearnedhave taughtthemilitarythat“theaverageuntrainedsoldierisworse thanuselessinharshArcticconditions.”¹Thisobservationwasre- enforcedinarecentArcticexercisewhereunpreparedsoldiershad tobeevacuatedduetomultipleinjuriesandinabilitytooperatein theextremecoldintheﬁrstdaysoftheexerciseresultinginaban- doningtheexercisetofocusonbasictraining([nameredacted], unpublishedobservations, 2019).Unlikealmostanyotherenvi- ronmentintowhichsoldiersmaybesent,theArcticisunforgiving andunpreparedsoldierswillnotbeabletoperformandmaynot survive.

Leadership is required to ensure that proper preparation is not sacriﬁced in a hasty attempt to meet the operational objectives—doing soin the unpredictable Arcticconditions can quicklyresult inmission failure,injury, or death. Leadership is alsocriticaltoenforceappropriatebehaviors(e.g.buddychecksfor frostbiteandpromptreportingofemergentproblems);anemerg- ingproblemmustbedealtwithonthespottopreventamuchmore seriousissuethirtyminuteslater.²²Inthiscontext,pre-deployment preparationsalsorequirethetransmissionofknowledgeasthey relatetothesociology,psychologyandbiology ofcold weather operations.²²Forexample,bodysize,ethnicity,andsexmayaffect performance,increasethevulnerabilitytoinjuryandaffectdeci- sionsaboutequipmentandtrainingrequirements.

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Ifthereisonethingthathasbeenlearnedfrommultiplemil- itaryArctictraining exercises,itisthatsoldiersrequiretraining fromexperts,especiallyexperiencednativecold-dwellerssuchas theInuitintheCanadianRangers,andpersonalexperiencetolearn howtoeffectivelyusetheirPPEintheextremecold.¹Thistraining willgivethemtheknowledgeandunderstandingonhowthecloth- inginteractswiththeirownphysiology,especiallyduringperiods ofhighactivityorlowactivitywhereclothinglayeringisextremely importanttoavoideithersweatingorgettingtoocold.Thistrain- ingwillalsoinformthemonwhatpiecesofpersonalprotectivekit worksbetterforthemassomepeoplerequiremoreprotectivekit thanothersordifferentstyleofkit.Itisonlythroughmulti-day experienceintheﬁeldintheArcticthatthelevelofexperience required tosafelyand successfully operateintheArcticcanbe gained.²³

3. Assessmentofcurrentperformanceproblemsandrisks forcoldweatherinjury

SuccessfullyoperatingintheArcticrequirescomplexplanning, trainingandsuperiorPPEequipmenttohelppreventcold-weather injuries(CWI)astemperaturesinthewintercanreachaslowas

−60^◦C¹²resultinginbareskinfreezinginlessthanoneminute.²⁴ These factorsarecritical enablersfor operatingand performing theirdutiesoptimallyassoldiersarefacedwithvaryingcoldcondi- tionswithchanginglevelsofactivity(fromridingsnowmobilesto buildingsnowsheltersorconstructingtents)requiringonetodress inmultiplelayerswhichcanbeshedoraddedtoavoidsweatingor freezing,bothwhichcouldresultinaCWI.²⁵

In onestudyof 215USand Canadiansoldiersina jointArc- tic exercise at -21 ^◦C (with windchill of -44 ^◦C) that included airborneoperations,theincidenceofself-reportedfrostbitewas 17%,and withphysicianassessment,ashighas21%.²⁶ Sullivan- Kwantes andMoesconductedasubsequentprojectrelatingPPE selectionandrisksofCWI,comparingissuedandnon-issuedPPE wornbyCanadianArmedForces(CAF)intheArctic.Atotalof511 militarypersonnelcompletedasurveytodocumentthetype of clothingselectedandwornbydeployedCAFmembers.Surveysalso includedself-reportedofﬁcialdiagnosisof aCWI (hypothermia, frostnip, andfrostbite)bythemedical ofﬁcerduringtheopera- tion(unpublishedobservations).TheseCWIsurveysrecordedthe injury, equipment worn duringthe time of theinjury, and the mechanismofinjury.Resultsfromthisstudyindicatedthatahigh proportionofparticipantsreportedusingnon-issuedlinergloves (72%),mittens(56%),hat(47%),socks(42%),thermalunderwear (40%), boots (39%) and goggles (42%) either alone or in com- bination withissued kit. The predominantcontributors toCWI werewetclothing,fatigue,dehydration,clothingitemsnotprovid- ingadequateprotection,snowmobileriding,andremovinghand protectionforincreaseddexterity forcertaintasks.Insum,CWI incidencesoccurforseveralunrelatedreasons,andtotheextent thattheissuedequipmentmaybeatfault,soldierscommonlyaug- menttheirkitwithstorebought,andpresumablyuntested,kit.²⁷ Thishighlightstheneedforcommonstandardsofprotectivecloth- ing.

Itisimportanttonotethatinjurieslikefrostnip,frostbite,non- freezingcoldinjuries,andhypothermiacanhaveimmediateimpact onthecapabilityofaunitandcanresultinlongtermhealthimpli- cationsorapermanentdisabilityforthesoldier^3,4,28Trainingto recognizethewarningsignsofimpendingCWIiskeytoprevention withincreasedphysicalactivitysuchas,changingdampclothing, addingprotectiveequipment,ormovementtoheatedshelter.High ratesofCWIandunder-reportingofCWIiscommonincurrentArc- ticexercises.^28,29Reasonsforunderreportingvariedandincluded

pride,notrealizingtheyhadaninjury,andintentionallyhidingthe injurytoavoidbeingpulledfromoperations.¹⁷

Oneconsiderationforthefutureistoincludetemperatureand moisturesensing in someof the protective equipmentto both alert novices toprompt action required as wellas to continue togaininsightsintohowtoimprovetheclothing,gloves,boots, andfaceprotection.Wearablesensingcouldmatureintorealtime physiologicalmonitoringfeedback toindividualsand leadersto preventCWIandsustainperformance.³⁰ Developmentofphysi- ologicalmonitoringtechnologiesthatwilloperateinextremecold ﬁeldconditionsand willnot hampersafetyandperformance of theindividualspresentsaspecialsetofchallenges.Thisispartof thelessonslearnedoverseveralyearsofexperimentswithArctic troopsmonitoredforgloveandboottemperatures,wherebatter- iesfailed,wiresfrozeand broke,andthere wasa riskof injury fromdirectskincontactbycoldmetalsensors([Nameredacted], unpublishedobservations,2018).

Specializedclothingdesignedaroundtheneedsoftheextreme coldweatherwarfighterwillbeaperformanceenhancertosoldiers whohavetrainedtouseittomaximumadvantage.AnewU.S.Cold TemperatureandArcticProtectiveSystem(CTAPS)hasbeendevel- opedbased,inpart,onsoldieruserfeedbackfollowingUSNational GuardparticipationintheCanadianGuerrierNordique2019exer- cise;simplifiedlayers,considerationtomoistureproblems,hood design, and external pocket access arevital newfeatures. This improvesonthedesignoftheExtendedColdWeatherClothing SystemIII(ECWCSIII)implementedforcoldweatheroperationsin 2007whichisexcellentforcoldweatherbutdoesnotfullyserve Arcticneeds.BiophysicalcomparisonsofUSArmyandMarine,Nor- wegian,andCanadiancoldweatherensemblessuggestanarrow rangeofstrategiesinpreferreddesigns,preferreduseofnatural fabrics,andcommonrequirementsacrosscountries.³¹Thecurrent majorchallengeincoldweatherclothingismanagementofsweat andexternal moisture;thiswashighlightedin acomparison of threegenerationsofFinnishcoldweatheruniformswherecom- fortisimprovedwiththerightcombinationoffabricsandlayersto reducefrictionandenhancemoisturetransferintheinnerlayers, andalsoprotectfromexternalwetness.³²Anewbootsystemfor Arcticoperations(-20to−60^◦C)movesawayfromtheoldvapor barrierdesignandusesnewinsulationtechnology,addressinga criticalneedinfootprotectionasdiscussedinthecold-wetmod- elingportionofthispaper.Nativecoldregiondwellerssuchasthe InuitandSaamipeoplerelyheavilyonanimalfurforoptimalArc- ticprotection,includinggroundclothsundersleepingbagsandin mittens.Asanexample,wolverinefurruffsonhoodshelpcreatea protectivemicroenvironmentaroundthefacesofarnotduplicated byartificialfabrics,andthereisanimportantsciencebasistosup- portthistraditionalknowledge;acoldfacecausesvasoconstriction ofthehands.^33,34

4. Mobilityandloadcarriageissues

MobilityintheArcticcanbeparticularlychallenging,andmove- mentisparticularlyhamperedbysnowrequiringvaryinglevelsof energyexpendituredependingontheterrain.³⁵ThevariousArc- ticcountriesdifferintemperaturesandterrainanddependingon themilitaryoperation,militarymovementoverlandmayconsist ofsnowvehicles,suchassnowmobilesorsoldierstravelingonskis orsnowshoes;bothrequirespecialtrainingandequipmenttobe successful.Designscanbeoptimized forspeciﬁcconditionsand operations;forexample,snowshoechoicecanbebasedonsnow conditions,intendedmovement,andloadswithdifferentenergy costs.³⁶InCanadianArcticexercises,heavyequipment,fuel,and frozeniceforwateristransportedintraditionalInuitkomatiksleds.

InNorway,thedismountedsoldiersinthesquadusuallyuseback-

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packsfortheequipmentandsuppliesthattheyneedtotransport andsometimesapulkthatthesoldierspullbehindthem,alternat- ingthetaskbetweenteammembers.Thepulkisashorterversion oftheahkiousedbysomearmies;bothareaformofsledwithout runners.Inaﬂatlandscape,itisanadvantagetohavepulkcom- paredtoabackpacks.⁵However,inmorehillyterrainthisisnotso obvious.Therefore,itisofimportancetounderstandhowthesol- diersinasquadmanagetomovethroughmorehillyterrainwitha pulkeachandwithnobackpacks.Herewesummarizetheresults ofapreviouslyreportedtest.³⁷

Eightsoldiersfromtwodifferentsquads,fourfromeachmade twodifferentgroups.Theyparticipatedinthetwodaysstudyand Group1wasskiingwithpulktheﬁrstdayandwithbackpackthe nextandGroup2theotherwayaround(Fig.1).Previouslythey hadbeenonaﬁeldexerciseover6days,mainlyskiingwithpack- ing.TheyfollowedatrackinhillyterrainalongtheRussianborder inNorwayclosetoElvenes.Theweatherconditionsweregoodwith temperatureabout-6to−8^◦C,windbelow5m/sandnoprecipi- tation.Heartrate,coretemperature,andskintemperature(chest) weremeasured(EquivitalHidalgoLtd)andalsodistance,average speed,weightofpulk/backpacksandverticalmovement.Soldiers werealsosurveyedabouttheirpreference.

Averagespeedforthetwogroupswas1.7and1.0km/h(pulk) and2.3and1.4km/h(backpack)withcorrespondingmeanweights of47±7(±SD)and34±5kg(pulk)and28±3and21±2kg(backpack).Themeanverticalmovementforthetwogroupswithpulk were410muphill(ascent)and340mdownhill(descent)andthe correspondingresultsforbackpackwere435mand375m,respectively.Averageheartratescombinedforbothgroups(n=8)were increasedfrom107±5(backpack)to117±9(pulk)(pairedt-test, p<0.05).Averagecoretemperaturecombinedforbothgroupswere 37.7^◦C+0.2(n=6,pulk)and37.6^◦C±0.2(n=7,backpacks)(N.S).

Figure1showsanexampleofheartrateandcoretemperatureover thetwodaysforonesoldier.Chestskintemperaturewassignif- icantlowerforpulkcomparetobackpacks(∼3^◦C)(p<0.05).The resultfromthesurvey(questionnaire)showedthat67%ofthesol- dierspreferredthepulk,33%preferredacombinationofpulkand backpacks,andnobodywantedasolutiononlybasedonbackpacks.

Basedonthemonitoringintheﬁeldinthisstudythesoldiers withthepulkcouldtakeabout50%moreweightwiththemcom- paredtobackpacks;however,speeddecreasesby∼25-30%.Based onheartrateresponses,physicalloadincreasedby∼10%.Chestskin temperaturedecreasedby∼3^◦Cprobablyduetoincreasedventila- tionintheclothing.Itwaseasiertoopenuptheliningoftheupper body whenwalkingwithapulkcomparedtoabackpacks.Most soldierspreferthepulk.ForArcticoperations,somekindofsled (ie.,pulk,ahkio,orkomatik)isusefulandefﬁcientfortransporting loads.

5. Thecold-wetproblem–thebaneofarcticwarriors

Managementofsweatin coldenvironmentsisa keytrained behaviorimportanttopreventionofcoldinjury.³⁸InArcticoper- ations, this involves smart pacing of physical work, managing clothinglayers,and payingattentiontodampsocksandgloves.

Betterunderstandingofcoldwettolerancelimitsisimportantfor predictivemodelsofcoldtoleranceformilitaryplanningorreal timeperformancelimits.Thisisproducedintheformofpredictive modelsthatcanbeusedbymilitaryplannersorinrealtime.One exampleofusefulcoldpredictivemodels,isthewindchillindex whichiswidelyusedforguidanceinoperationalsettingsonrisk forfrostbiteofexposedskin.^39,40Formoreelaborateguidance,heat balancemodelscanbeusedthatrequireinputonclothinglevel, activitylevelandenvironmentalconditionstoprovideoutputon durationlimittoperformanceloss,durationlimittohypothermia,

aswellastherequiredclothinginsulationtopreventhypothermia orsweating.^41,42Ingeneralguidelinesareaimedatkeepingcloth- ingdry–orassumeclothingisdry.⁴³However,wetclothingcan greatlyincreasetheimpactofcoldweatherconditions.⁴⁴Moreover, duringmilitaryoperationsclothingcangetwetbybothinternaland externalfactorssuchassweating,rainfallorwadingthroughwater.

Therefore,itcanbeoperationallyrelevanttohaveanindicationof theimpactofwetvs.dryclothingonsafedurationofexposure.

AttheICSPP2020meeting,theWetEquivalentairTemperature (WET)wasproposed.⁴⁵WETisthesteadystateeffectiveairtemper- aturefordryclothingwhichresultsinequalheatlosstoexposureto theactualairtemperatureforwetclothing.Withrespecttointer- pretationitiscomparabletoa‘feels-like’-indexthatcanbeusedas inputinothermodelsasaproxytoestimatetheeffectofwetcloth- ing.Thedryandwetdurationlimitsprovideabandwidthofsafe operationtime. For instance,existingguideline IREQ-ISO-11079 providesinsightonrequiredinsulationandsafeexposureduration (DLE)butassumestheclothingisdry.ThewetDLEcanbeestimated bysubstitutingairtemperaturewiththeWET-index.Forinstance, aguardstandinginan18^◦C,50%relativehumidity,1.0m/swind environment,theDLEisgreaterthan8hours.Howeverwetcloth- ingcancauseanequivalentheatlossasifairtemperaturewere WET=8^◦C,andthatsigniﬁcantlyreducesthewetDLEto2hours.

WETiscalculatedbyadaptingtheheatbalancemodeldescribed inParsons.^46,47 Wetnessofclothinginﬂuencestheheatbalance inthreeways:clothinginsulationdecreasesbyreplacingairwith water;evaporationfromskintissuebecomesnegligibleifthereis noairmediumforevaporationtotakeplace;and clothingtem- peraturedecreasesdue toincreasedevaporationattheclothing surface.

Toaccountfortheseeffects,clothingwetness(wcl)isusedas analogoustoskin wetness.⁴⁴ Dry clothingassumesthevalue 0 (wcl=0)andwet(wcl=1).Basedonanempiricalstudywetcloth- inginsulation(wcl=1)islinearlyinterpolatedwith40%reduction indryclothinginsulationforfullywetclothing.⁴⁶Evaporationof skintissueisscaledwithclothingwetness(Esk_wet_clothes=(1-wcl) Esk_dry_clothes).Evaporationfromwetclothingisformulatedanalo- goustoevaporationofsweatfromtheskin,usingwclinsteadof wsk.Fig.2showsanindividualexampleofmeasuredmeanskin temperature(4positionsISO,neck,scapula,abdomenandcalf)and predictedmeanskintemperatureofthemodelforthreelevelsof clothingwetness.TheindividualisdressedinBDUandstandingin a19^◦C,40%relativehumidity,0.1m/sairspeedenvironment.

Theapplicationofthis conceptualbiophysicalmodel toArc- ticconditionsneedstobeexplorednext.Rapidfreezingofsweat withintheclothingaddsyetanotherlevelofmodelingcomplexity.

Anothermodelingchallengeistopredictriskoftrenchfootinjury fromtheinteractionoftemperature,water,anddurationofskin exposureinaboot.⁴⁸

6. Howimportantisbodymorphologyandcompositionin insulation?

Abasicunderstandingofhowhumanthermalphysiologymay differaccordingto bodymorphology and compositionis useful inestimating theperformanceof soldiersoperatingin coldcli- mates.Bodysizeandshapeinfluencedbyadaptationtohotand coldenvironmentshavebeenthesubjectofmanypublishedstud- ies,includingmany centered onthe Bergmannand Allen rules that cold-dwellers have largerand rounder bodies topreserve heat.⁴⁹Withfurtherstudy,theserelationshipsaremorecompli- cated,influenced in modern timesby other factors.⁵⁰ Body fat insulationfollowsmorecomplicatedrulesaswell;specifically,Inu- its,havebeenreportedtohavelowerbodyfatcontentinrelation tobodysize.⁵¹Innon-humanspeciessuchaselephantseals,ahigh

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Fig.1. Norwegiansoldierswithpulks(leftpanel)andbackpacks(rightpanel).Typicalheartrate(bluedots)andcoretemperature(reddots)responseduringthetwoday studyisshownforonesoldier(toppanel).

Fig.2. Measuredmeanskintemperature(solidline),andsimulatedmeanskintemperature(dashedline)forA)0%clothingwetness,B.25%clothingwetness,C.50%clothing wetness,D.50%clothingwetnesswith12%increaseinmetabolicheatproductiontoreﬂectcold-inducedthermogenesis(e.g.,muscletension,brownadiposetissue,orother).

Measuredwatertemperatureis17^◦C.

fat contentappearstoincludeanimportantsubcutaneousinsu- lativerolefor protectionincoldenvironments.⁵² Onecouldask

“do higherbody fat troopshavea similarperformance edge as Arcticwarriors?”Inarecentlypublishedstudy,Brychtaandcol- leagues investigated thequestionwitha 13-day protocolin an environmentally-controlled setting.⁵³ Using a whole-roomindi- rectcalorimeter,restingenergyexpenditurewasmeasuredin12 leanand9obesehealthy,young(25±6yearsold),lightlyclothed (0.36 clo), fasted menon each day during a 5hexposure toa

differentconstantambienttemperature,randomizedbetween16- 31^◦C.Skinandcorebodytemperature,muscleactivity,andother physiologicalresponseswerealsomeasuredaswellassubjective scalesofthermalcomfort.Thevolumeandactivityofbrownadi- posetissue(BAT)wasalsomeasuredwith18F-Fluorodeoxyglucose positron emission tomography-computed tomography (PET-CT) after exposure to an individualized coldest tolerable ambient temperature.

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Fig.3. A.Restingenergyexpenditurenormalizedtobasalmetabolicrate(BMR)asafunctionofambienttemperaturewithgroupbranched-regressionmodelsfor12lean men(black)and9obesemen(red).B-C.Lowercriticaltemperatureiswarmerinleanmenthanobesemen(B),butbothgroupshavesimilarcoldesttolerabletemperature beforeovertshivering(C).D.Nonvoluntarymuscleactivity,asquantitatedbytherootmeansquared(RMS)surfaceelectromyogram(EMG)averagedfromfourmusclegroups (PectoralisMajor,Trapezius,BicepsBrachii,andRectusFemoris)duringmotionlessperiods,attheindicatedambienttemperatureandexpressedasapercentageofaverage EMGactivityatambienttemperature>28oC.E.Linearregressionmodelsoftheweightedmeanskintemperatureasafunctionofambienttemperatureweresimilarforlean andobesemen.F.Brownadiposetissuevolumewasgreaterforleanthanobesemen.Source:Brychtaetal.⁵³

Thisstudydemonstratedthatallsubjectshada temperature rangewhererestingenergyexpenditurewasminimal,deﬁnedas thebasalmetabolicrate(BMR,Fig.3A).Thetemperaturebelow which cold-induced thermogenesisoccurred, the lower critical temperature, wascolderin obesethan leanmen(21.1±1.7 vs.

22.9±1.2 ^◦C, p=0.03, Fig. 3B) but both groups reached simi- lar coldest tolerable temperatures (20.3±1.6 vs. 21.2±1.5 ^◦C, p=0.2,Fig. 3C).At thecoldesttolerable temperature,the addi- tionalheatproducedaboveBMRfordefendingcoretemperature was less in obese than lean men (125±146kcal/day or 6±7%

of the BMR vs. 300±218kcal/day or 17±11%, p=0.01). Mus- cle electrical activity conﬁrmed that shivering initiated at the lowercriticaltemperatureforbothleanandobesemen(Fig.3D).

Coreandweighted-meanskintemperatures(Fig.3E)variedsim- ilarlywithambienttemperatureinbothgroups,butobesemen had cooler proximal and warmer peripheral skintemperatures in thecold.Cold-activated BATvolumewaslowerin theobese vs. lean men (130±121 vs 334±162ml, p=0.03, Fig. 3F), but didnotcorrelatewiththeindividualcold-inducedthermogenesis.ThissuggeststhatBATactivationwasnotacriticalfactorin thedifferencesinheatproductionbetweenobeseandnonobese subjects.

Theﬁndingsthatobesemendidnottoleratecoldertempera- turesbetterthanleanmensuggeststhatanyadditionalinsulation providedbygreaterbodyfatwasoffsetbylesscold-inducedthermogenesis in the obese men; this did not, however, examine placement of thefatbetweenvisceral andsubcutaneous distri- butions.Itfurtherrevealedthatmuscleactivitybegantoincrease atthelowercriticaltemperature,andshiveringbecameovertat temperaturesonly1-1.8^◦Ccolderinnon-acclimatizedmenduring 5hoursofexposure.

7. Physiological(andbehavioral)acclimationisactually possible

WhenScholanderreportedhisphysiological studiesof cold- dwelling natives (Saami) in the 1950s, he described a “semi- tropical”microenvironmentinthestudyparticipantsbecauseof theirprotectiveclothing.⁵⁴But,inotherstudiesofhumansexposed tocolditbecameclearthattherearehumanstrategiestoadjust chronically(acclimate)tocold exposure, notably a risein nonshivering thermogenesis and an improvement in physiological insulation.⁵⁵Innon-exercisingindividuals,totalcold-inducedheat productionisobtainedfromtheactivationofnonshiveringthermo- genesisandshiveringthermogenesis.Hypothesesaboutincreased subcutaneousinsulation inextremecold waterswimmers have beentested in Ama and Haenyeo diving women. The adaptive differenceswerenotinbodyfatbutinthemetabolicresponses, withincreasedthermogenesisandahypothermictolerance,with toleratedoraltemperaturestypicallybelow35^◦Cand aslowas 30^◦C,lowerthanclassicstudiesofnakedaboriginesandchannel swimmers.^56,57

Inadults,shivering isbyfarthegreatestcontributor ofheat andactivatesalmost80%oftotalmusclemass.⁵⁸Whileshivering isimportanttoproduceheat,itremainsparticularlyuncomfort- ableandaffectsthermalcomfortandworkcapacitybyimpairing motor skills and coordination.⁵⁹ Over the last decades, many researchershaveattemptedtoidentifythebestwaytoincrease nonshivering thermogenesis in order to decrease the relative contributionof shiveringand muscle recruitment.Using a cold acclimationprotocolcomprisedof31days,8hperdayat∼12^◦C, Davisreportedan80%decreaseinshiveringintensityanda15%

decreaseintotalheatproductioncomparedtomenacclimatized to summer conditions.⁶⁰ Almost half a century later, Blondin and his colleaguesshowed that shivering couldbe reduced by 20% when non cold acclimatized were exposed to10^◦C at 2h

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Fig.4. Thermogenicresponsestoacutecoldexposurebeforeandaftercoldacclimation.Changesinrateofmetabolicheatproduction(kJ/min)(A),shiveringintensity[%

maximumvoluntarycontraction(MVC)](B),totalheatproduction(kJ·150min)(C),andtotalshivering(%MVC·150min)(D)duringanacutecoldexposureatameanskin temperatureof26^◦C.Percentchangeinheatproduction(B)andshiveringintensity(F)frombeforetoaftercoldacclimationforeachparticipant.Source:Gordonetal.⁵⁹

per day and 5 days a week for 4 weeks.⁶¹ In thelatter study, it was alsoshown that BAT volume and thermogenic capacity couldbeincreasedby45%and182%,respectively.Thisconfirmed that BATactivationand thestimulation ofotherNST processes could modulate shivering activity in humans. However, Young suggested that the greatest cold acclimation effects could be obtainedusinguncompensablecoldexposurewherecoretemper- ature isreducedrepeatedly.⁶² Totestthis hypothesisand inan attempttooptimizecoldtoleranceresponseinashorteramount of time, Gordonexposed non cold-acclimatizedmen to7 con- secutive days to 14^◦C water for 1h or until core temperature decreasetoamaximumof35.5^◦C.⁵⁹Resultsconfirmedthatsub- stantial improvement in cold tolerance and a 36% decrease in shiveringintensity(Fig.4).Clearly,completingsuchacoldaccli- mationprotocolrequiresgreatmotivationbuttheoutcomecould beextremelybeneficialformissionsuccessandpotentially,cold survival. Inaddition,this wouldexpand theenvelopeoftolera- blecoldinmilitaryoperationsbutespeciallyforextremecoldin Arcticenvironments,takingadvantageofcoldacclimationmecha- nisms.

In addition tophysiologically induced acclimatization,other strategies to promote nonshivering thermogenesis (and reduce the relative contribution of shivering to total heat produc- tion)havebeeninvestigated.Pharmacologicalenhancementhas shown thermogenic beneﬁts, notably the synergistic effects of caffeine(adenosine antagonist) and ephedrine (beta adrenergic agonist).^63,64 Pharmacologically-induced increase in brownadiposetissue appearstobepossible,asdemonstrated ina recent studywithonemonthofhighdosemirabegronadministration.⁶⁵ Themechanismisbeingfurtherinvestigated,withevidencethat inhumanstheeffectofthisbeta-3adrenergicagonistisactually occurringthroughcrossoveractivationofmoreconventionalbeta- 2adrenergicreceptors.⁶⁶WhetherornotBATcontentcanmake anyimportantdifferencetocoldweatherperformanceforhumans isyettobedetermined.Comparisonoftheeffectoncoldinduced thermogenicresponsesthatmightbeproducedbycoldacclima- tizationstrategiesandbypharmacologicalenhancementremains animportantquestion.Othersuggestivebut yetunprovencon- ceptsforresearchexplorationincludedietaryandtrainedvoluntary controlofthermogenesis.Dietarysupplementsemergingfromlab

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studiesandotherssuggestedfromobservationsofnativepractices (e.g.,consumptionofsealliverhighinretinoicacid)remaintobe testedforperformanceenhancingbeneﬁtstoArcticwarriors.Vol- untary controlofthermogenesisaspracticed byTibetanmonks (“Tum-yo”yoga)isanareaofinvestigationthatwouldrequirean understandingofthebrainregionsinvolvedinordertodevelop biofeedbacktargetsforasoldiervirtualrealitytrainingtool,inorder totrainvoluntarycontrolofthermogenesisthathasbeenmastered byTibetansonlythroughyearsofpractice.⁶⁷

8. Conclusion

The Canadian Rangershave an Inuktitut word for operating in theArctic:ihuma.Ihumatranslatesintowisdom,reasonand knowledgeandistakentomeanachievementofindividualcom- petenceinArcticsurvivalalongwiththeabilitytoleadothersin theArctic.^1,68Thevariousperspectivespresentedinthisthematic sessionduringtheInternationalCongressonSoldierPhysicalPer- formance 2020 addresschallenges faced bythe militaryin the Arcticand,together,bringusclosertooptimizinghumanperfor- mance inextremeconditions. However,afullerappreciation of thefactorsthat contributetohumanperformance, survivaland thrivingduringmilitaryoperationsdependonourabilitytointe- gratethoseperspectivestocreateaholisticunderstandingofhow thehuman,theenvironment,training,leadershipandtheequip- ment interacttoeitherfacilitatethesuccessofmissions orput them atriskforfailure.Payingattentiontolessonslearnedand usingascientiﬁcapproachtounderstandingthechallengesofArc- tic operationsgoalongwaytogenerateknowledgethroughan applicationofmeasurementsandreason.Thelastpiece,wisdom, includesanunderstandingthatothers havevaluableknowledge andpracticesthat,whilenotnecessarilybasedinscience,havecen- turiesofproveneffectivenessbythosewhoapplythem.Thelessons learned andpracticalguidancefromtheInuitandotherindige- nouspeoplewholiveintheArctichavesigniﬁcantplaceamongthe equipment,techniques,andtacticsourmilitarieshavedeveloped tohandleextremetemperatures.

Disclaimer

Theviewsandopinionspresentedinthismanuscriptaresolely thoseoftheauthorsanddonotnecessarilyrepresentanyofﬁcial policyorpositionoftherepresentedagencies.

Acknowledgement

Authorswouldliketothanktheorganizersand attendeesof theICSPP2020Conference.FHwassupportedbytheNaturalSci- encesandEngineeringResearchCouncilofCanada;BRMKhasbeen supportedbyMinisterievanDefensie,DenHaag,TheNetherlands, underprogrammeV19175thgenstressors;KEFissupportedunder the“MetabolicLimitsofExtremePerformers”STresearchinitiative, USArmyMedicalResearchandDevelopmentCommand.

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