Multivariate Statistical Assessment of Predictors of Firefighters' Muscular and Aerobic Work Capacity

26 Pages·2015·2.91 MB·English

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Citation for the original published paper (version of record):
Lindberg, A., Oksa, J., Antti, H., Malm, C. (2015)
Multivariate Statistical Assessment of Predictors of Firefighters' Muscular and Aerobic Work
Capacity.
PLoS ONE, 10(3)
http://dx.doi.org/10.1371/journal.pone.0118945
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RESEARCHARTICLE
Multivariate Statistical Assessment of
’
Predictors of Firefighters Muscular and
Aerobic Work Capacity
Ann-SofieLindberg1,2*,JuhaOksa3,HenrikAntti4,ChristerMalm1,2
1 SportsMedicineUnit,UmeåUniversity,Umeå,Sweden,2 Winternet,Boden,Sweden,3 PhysicalWork
Capacity-team,FinnishInstituteofOccupationalHealth,Oulu,Finland,4 DepartmentofChemistry,Umeå
University,Umeå,Sweden
* [email protected]
Abstract
Physicalcapacityhaspreviouslybeendeemedimportantforfirefightersphysicalworkca-
pacity,andaerobicfitness,muscularstrength,andmuscularendurancearethemostfre-
OPENACCESS
quentlyinvestigatedparametersofimportance.Traditionally,bivariateandmultivariate
Citation:LindbergA-S,OksaJ,AnttiH,MalmC linearregressionstatisticshavebeenusedtostudyrelationshipsbetweenphysicalcapaci-
(2015)MultivariateStatisticalAssessmentof
tiesandworkcapacitiesamongfirefighters.Analternativewaytohandledatasetsconsist-
PredictorsofFirefighters’MuscularandAerobicWork
ingofnumerouscorrelatedvariablesistousemultivariateprojectionanalyses,suchas
Capacity.PLoSONE10(3):e0118945.doi:10.1371/
journal.pone.0118945 OrthogonalProjectiontoLatentStructures.Thefirstaimofthepresentstudywastoevalu-
atethepredictionandpredictivepoweroffieldandlaboratorytests,respectively,onfire-
AcademicEditor:MariaFrancescaPiacentini,
UniversityofRome,ITALY fighters’physicalworkcapacityonselectedworktasks.Also,tostudyifvalidpredictions
couldbeachievedwithoutanthropometricdata.Thesecondaimwastoexternallyvalidate
Received:April12,2014
selectedmodels.Thethirdaimwastovalidateselectedmodelsonfirefighters’andoncivil-
Accepted:January8,2015
ians’.Atotalof38(26menand12women)+90(38menand52women)subjectswerein-
Published:March16,2015 cludedinthemodelsandtheexternalvalidation,respectively.Thebestprediction(R2)and
Copyright:©2015Lindbergetal.Thisisanopen predictivepower(Q2)ofStairs,Pulling,Demolition,Terrain,andRescueworkcapacitiesin-
accessarticledistributedunderthetermsofthe cludedfieldtests(R2=0.73to0.84,Q2=0.68to0.82).Thebestexternalvalidationwasfor
CreativeCommonsAttributionLicense,whichpermits
Stairsworkcapacity(R2=0.80)andworstforDemolitionworkcapacity(R2=0.40).Incon-
unrestricteduse,distribution,andreproductioninany
medium,providedtheoriginalauthorandsourceare clusion,fieldandlaboratorytestscouldequallywellpredictphysicalworkcapacitiesforfire-
credited. fightingworktasks,andmodelsexcludinganthropometricdatawerevalid.Thepredictive
DataAvailabilityStatement:Ethicalrestrictions powerwassatisfactoryforallincludedworktasksexceptDemolition.
preventpublicsharingofdata.Requestsfordatamay
besenttothecorrespondingauthor.
Funding:FundersoftheworkweretheSwedishCivil
ContingenciesAgency,https://www.msb.se/en/?
ResetTargetNavigation=true.Thefundershadnorole Introduction
instudydesign,datacollectionandanalysis,decision
Firefightershavevariedduties,includingoperativetasks(suchasfightingfiresandrescuing
topublish,orpreparationofthemanuscript.
peopleandanimals),serviceofmaterials,equipmentandvehicles,engageinfireprotective
CompetingInterests:ASLandCMareemployees
work,andwithinsomefireandrescueservices:givefirstaidincaseofmedicalemergencycalls.
atWinternet,whichisanon-profiteconomicco-
Workperformanceisabroadterm,hasdifferentmeaningswithindifferentdisciplines,and
operativepartnership,withnocommercialinterestin
datapublication.Theauthorshavenopartinthe includesseveraldimensions[1,2].Someofthedimensionsaffectingfirefighters’work
PLOSONE|DOI:10.1371/journal.pone.0118945 March16,2015 1/25
PredictionofFirefighters'PhysicalWorkCapacity
financialaspectsofWinternet.Thisdoesnotalterthe performancearetheuseoffireprotectiveclothing[3]andBreathingApparatus(BA)[4–7],er-
authors'adherencetothePLOSONEpolicieson gonomicsandheavyequipment[8–10],increasedbodytemperature[11],thetimepassingbe-
sharingdataandmaterials. tweenturn-outs[12],physicalworkcapacity[13–18],emotionalstress[19],cognitiveabilities
[20],work-organizationalandpsychosocialfactors,andperceivedwork-relatedstrainamong
firefighters[21,22].Consequently,physicalworkcapacityisonlyoneofseveraldimensionsaf-
fectingfirefighters’workperformance.Severalworktasks,e.g.carryingequipmentupstairs
andvictimrescueareratedasphysicallydemandingbyfirefighters[13,22–25],andthesework
tasksmaythereforesethighdemandsonphysicalworkcapacity.Theloadoffirefighters’protec-
tivegearreducesrelativemusclestrength,anaerobiccapacity,andmaximalaerobicpower
(VO :mL(cid:1)kg-1∙min-1)[26],theoreticallyreinforcestheimportanceofahighphysicalcapacity.
2max
Consequently,ahighaerobiccapacity(VO :L(cid:1)min-1)[7,17,27,28],VO (mL(cid:1)kg-1∙min-1)
2max 2max
[13,17,29,30],aswellasmusclestrengthandendurance[16,18,28,31–36]areoffundamentalim-
portanceforfirefighters’physicalworkcapacity.Alowphysicalworkcapacitymayreducethe
safetyoftheindividualfirefighter,thecolleagueorthevictim.
Inaccordancewiththegovernmentregulationforpermissiontoexecutesmokediving,
Swedishfirefighters’mustmeetphysicalrequirementsinapassorfailtest[37].Uponrecruit-
ment,additionalphysicaltestsareperformedforevaluationofphysicalworkcapacity.These
physicaltestsarecomposedbyeachindividualmunicipality,maydifferbetweenlocationsand
haveunknownscientificvalidity.InSweden,subjectsapplyingtoworkasafirefighterdonot
alwayshavefirefightingexperience.Consequently,thephysicaltestshavetobevalidbothfor
firefighters’andforcivilians’.Significantcorrelationsbetweenfirefighters’physicalworkca-
pacityandVO ,musclestrength/enduranceintheupper[28,31,32,35,36]andlower
2max
[31,32,34–36]bodyhavepreviouslybeenfound.However,thereisalackofstudiescombining
simulatedfirefightingworktaskswithlaboratoryandfield-testsforinvestigationofmuscle
strengthandenduranceoffirefighters[14,18].Usingsimplefieldtestshavebeenadvocatedfor
evaluationofphysicalworkcapacity[16,33],butusingacombinationoffieldandlaboratory
testsmayincreasethevalidityofexecutedtests.
Whenpre-employmenttestsareusedforselectionoffirefighters,itisimportantthatthese
testsarevalidtophysicalworkcapacity.Inordertorankandselectthephysicalteststhatbest
predictfirefighters’workcapacity,itisnecessarytoinitiallyincludealargenumberoftests.
Anthropometricdata,suchaspercentbodyfat[34,38]andbodymass[34]havepreviously
beenfoundbothtoaffect,andnottoaffectfirefighters’physicalworkcapacity.Becausebody
compositionmayormaynotaffectphysicalcapacity,usinganthropometricdataasselection
criterionsmaybediscriminative,andnegativelyselectapersonbasedon
incorrectassumptions.
Statisticalmethodspreviouslyusedfordevelopmentofrelevantphysicaltestsforfirefight-
ers´areessentiallybivariateandmultivariatelinearregression.Analternativetothesetradi-
tionalstatisticalmethodsismultivariate(MVA)statisticalorprojectionanalysis,mainlyused
withinchemometrics.Multivariatestatisticalanalysisisanappropriatestatisticalapproachto
increasethestatisticalpowerandfacilitateinterpretationwhenthedatasetconsistsoffewob-
servations(subjects)andmanycorrelatedvariables,andaviablecomplementtobivariatesta-
tistics[39].Inaddition,datacanbebothcross-validated(predictivepoweronthesamesubject
group)andexternallyvalidated(onanothersubjectgroup),givingadditionalinformationof
theselectedmodelsrobustness.
Inordertoassessfirefighters'physicalworkcapacity,thecombinedeffectofphysicalcapaci-
tiesneedstobetakenintoaccount.Toincreaseaccessibilityandreducecosts,field-testsrather
thanlaboratorytestsarepreferred.Thefirstaimofthepresentstudywastoevaluatethepre-
dictionandpredictivepoweroffieldandlaboratorytests,respectively,onfirefighters’physical
workcapacityonselectedworktasks.Alsotostudyifvalidmodelscouldbeachievedwhen
PLOSONE|DOI:10.1371/journal.pone.0118945 March16,2015 2/25
PredictionofFirefighters'PhysicalWorkCapacity
anthropometricdatawereexcluded.Thesecondaimwastoexternallyvalidateselectedmodels.
Thethirdaimwastovalidateselectedmodelsonfirefightersandcivilians.
Method
Studydesign
Thepresentstudyincludestwodatasets:onetraining-setandoneprediction-set.Thetraining-
setwasusedtoselectabatteryofphysicaltestsforevaluationoffirefighters’physicalworkca-
pacity.Variablesincludedwereobtainedfromtenlaboratorytests,fourteenfieldtests,three
anthropometricvariables,andfivesimulatedfirefightingworktasks,withatotalof99variables
(Table1).Thesephysicaltests/simulatedworktaskswereexecutedovertennon-consecutive,
randomizeddays,eachdayseparatedbyatleastonenon-testingday.Theinclusionofphysical
testswasbasedon[17,18]anattempttoimitatephysicaldemandswithinfirefightingwork
tasks,withrespecttomovement,equipmentweights,andworkduration.Worktaskswerein-
cludedbasedonapreviousstudyoffirefighters’ratedphysicaldemands[22],discussionswith
anexpertgroupwithintheSwedishCivilContingenciesAgency(SCCA),aswellasprevious
studiesoffirefighters’physicalworkcapacityofourresearchgroup[17,18].
Theprediction-setwasusedforexternalvalidation,andincludedsixfieldtests,threean-
thropometricvariables,andfivesimulatedfirefightingworktasks,withatotalof15variables
(Table1).Testswereexecutedoverthreeconsecutivedaysandextractedfromourpreviously
publishedstudies[17,18]andourgovernmentreport(Swedish)thatusedlinearmultivariate
regressionanalyses[40].
Table1.Physicaltestvariables,overview.
Variable-group Variablesincluded
Anthropometrics Bodyheight:m,Bodyweight:kg,BodyMassIndex:kg∙m−2
Laboratoryaerobic Submaximaltreadmillrunning:VariablesatOBLAandLT:Treadmill
speed,%HR ,%VO ,VO mL∙min−1VO2 :mL∙min−1and
max 2max 2 max
mL∙kg−1∙min−1,RER ,VE ,[La−]
max max bmax
Fieldaerobic Cyclingat200W:%HR ,Step-uptest:%HR andTreadmill
max max
walking:%HR ,Crawling30m:s,Running3000m:sands∙kg−1,
max
Rowing500m:s
Laboratorymusclestrength/ MaximalandenduranceShoulderflexionandextensionandKnee
endurance extensionandflexion:Nm,Nm%,W,W∙kg−1,J.EnduranceShoulder
pressandDeadlift:N,N%,W,W∙kg−1,J.EnduranceTrunkextensionand
flexion:Nm,Nm%,W,W∙kg−1,J.
Fieldmusclestrength/ Maximalhandgripstrength:kg.Situps,Squat,Benchpress,Upright
endurance barbellrowandBarbellshoulderpress:n.Handgripendurance:s
LaboratoryBalance Posturalstability:overall,mediallateral,andanteriorposteriorstability
index
Fieldmusclepower Standingbroadjump:m
Worktasks Stairs,Pulling,Demolition,Rescue,andTerrain:s.
Thetraining-setperformedallphysicaltests,theprediction-setperformedphysicaltestsinboldstyle.Some
physicaltestsincludedseveralvariables.LT:Lactatethreshold,OBLA:Onsetofbloodlactate
accumulation,[La-] :maximallactateaccumulationinblood,s:seconds,s∙kg−1:secondsscaledtokg
bmax
bodymass,N:Newton,N%:Nscaledtobodyweight,Nm:Newtonmeter,Nm%:Nmscaledtobody
mass,W:Watt,W∙kg−1:Wscaledtobodymass.J:Joule.WorktasksincludedwereCarryinghosebaskets
upstairs(Stairs),Hosepulling(Pulling),Demolitionatorafterafire(Demolition),Victimrescue(Rescue),
andCarryinghosebasketsoverterrain(Terrain).
doi:10.1371/journal.pone.0118945.t001
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PredictionofFirefighters'PhysicalWorkCapacity
Subjects
Training-set. Afterreceivingwrittenandverbalexplanationoftheprocedure,42subjects,
includingmalefull-timefirefighters,malepart-timefirefighters,andcivilianmenandwomen
withnoexperienceofworkingasfirefighters,volunteeredtoparticipate.Nofemalefirefighters
wereavailableforinclusion.Allsubjectsincludedinthetraining-setwerealsoincludedinpre-
viousstudiesrelatedtothecurrentstudy,evaluatingtheaerobicworkcapacity[17]andmuscle
strengthandendurance,anddynamicbalanceinfirefighters[18].Duetotheextensivedata,
publicationsareseparatedaccordingtoaims.
Prediction-set. Afterreceivingwrittenandverbalexplanationoftheprocedure,90sub-
jects,includingmaleandfemalefull-timefirefighters,maleandfemalepart-timefirefighters,
andcivilianmenandwomenwithnoexperienceofworkingasfirefighters,volunteeredtopar-
ticipate.SubjectswererecruitedfromtheFireandRescueServicesinSwedenandbynoticesat
localgyms.
Ethicsstatement. Allparticipantssignedaninformedconsent,statingtheirabilitytoexe-
cuteallpartsofthestudy,andabsenceofanyknowndiseasesaffectingphysicalperformance.
TheResearchEthicsCommitteeforNorthernSwedenatUmeåUniversityapprovedthestudy
onSeptember22,2009(Dnr09–046M)andthestudywasconductedinaccordancewiththe
WMADeclarationofHelsinki—EthicalPrinciplesforMedicalResearchInvolvingHuman
Subjects2008.
Descriptivedataandphysicaltests
Allsubjects’filledinahealthquestionnairestatingabsenceofanyknowndiseases.Subjects
werealwaysdressedinshorts/pants,t-shirt,andtrainingshoesduringthephysicaltests.Dur-
ingthesimulatedfirefightingworktasks,additionalclothingwasused.
Allphysicaltestsandsimulatedworktaskswereperformedatmaximalcapacity(e.g.maxi-
mumspeed,numberofrepetitionsorforce),unlessotherwisestated.Everytestdayincluding
laboratoryandfieldtestsstartedwithanappropriateandstandardized5to15minwarmup
procedure.Simulatedworktasktestswerenotprecededwithwarmupbecauseinarealtime
situationthereisnowarmupsessionpriorwork.Levelsetal.[41]foundnosignificantdiffer-
encesinfirefightingworktasksspeedinapre-warmingvs.thermoneutralgroup,duringinitial
work((cid:3)10min).
Subjectswereinstructedintheprocedureofallphysicaltestsandsimulatedfirefighting
worktasks,butnofamiliarizationswerescheduled.Anextensivedescriptionofthemethod
andequipmentusedinalltestshaspreviouslybeenpresented[17,18]andbelowasummaryof
thevariablesusedwithineachtestisprovided.
Afivetotenminutesrestseparatedphysicaltestsperformedbythetraining-set,allsubjects
hadequalrestingperiod.Fortheprediction-set,allphysicaltestswereseparatedbyfiveto240
minutesrest:fieldtestswereperformedinonedaywithhalfofthetestsinthemorningand
halfofthetestsintheafternoon.
Descriptivedata. Descriptivedataincludedsystolicanddiastolicarterialbloodpressure
(mmHg)(TriCUFF:AJMedical,Stockholm,Sweden),bodymass(kg),standingheight(m)
(SECAGmbH&Co.KG,Hamburg,Germany)andBodymassindex(BMI:kg∙m-2).
Laboratorytestsofmuscularstrength,muscularenduranceandbalance. Thetraining-
setperformedconcentricisokinetictestsofmaximalmuscleforceonaBiodexMulti-Jointsys-
tem3dynamometer(Biodexmedicalsystem,New-York,USA)[18](Table1).Totalwork(J)is
theamountofworkaccomplishedfortheentireset,averagepower(W)isameasureofanindi-
vidual´sabilitytoproducetorqueorforcewithinthetestsrangeofmotion(ROM)dividedby
time.Peaktorque(Nm)andforce(N)werealsoregistered,andalsoscaledtobodymass
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PredictionofFirefighters'PhysicalWorkCapacity
expressedaseither%(NandNm)orW∙kg-1[42,43].Frombilateraltests,thehighestindividual
performanceispresented.Enduranceshoulderpresswasperformedwithonesetof15repmax
atanangularspeedof240°∙sec-1[18].Endurancedeadlift(floortoknee)wasperformedwith
onesetof15repmaxatanangularspeedof240°∙sec-1[18].Maximalandenduranceshoulder
flexionandextensionwereperformedwithonesetofmaximalmuscleforce(5repmaxatan
angularspeedof60°∙sec-1)[18],followedbyonesetofmuscleendurance(15repmaxat180°∙sec-1)
[18].Maximalandendurancekneeextensionandflexionwasperformedwithonesetofmaximal
muscleforce(5repmaxatanangularspeedof60°∙sec-1),followedbyonesetofmuscleendurance
(30repmaxat180°∙sec-1)[18].Endurancetrunkflexionandextensionwasperformedwithoneset
of15repmaxtrunkflexionandextensionat60°∙sec-1[18].
Thetraining-setexecutedalaboratorytestofDynamicStabilityontheBiodexBalancesys-
temSD(Biodexmedicalsystem,New-York,USA)[18](Table1).
Fieldtestsofmuscularstrengthandendurance. Benches,barbells,asmithmachine(Pre-
cor,CLFitness,Sweden),dumbbells,andfreeweights(CasallSportAB.Sweden)wereused.
Theweightofbarbells,dumbbells,andfreeweightswerecontrolledwiththepreviouslypre-
sentedSECAscale.Ametronome(KorgMA-30metronome:KorgandMoore,Marburg,Ger-
many)wasusedfortestsperformedatapre-definedspeed.Testswerestoppediftherequired
paceorrangeofmotioncouldnotbefollowed,despitethreeverbalencouragementsforcorrec-
tion.Onlycorrectlyperformedexerciseswerecounted.Allincludedfieldtestsarepresentedin
Table1.Testsofmuscleendurancewereexecutedwithequalbarbellweightsirrespectivelyof
subject’ssex,ageorbodyweight,becauseinarealtimesituationtheweightoftheprotective
gearandequipmentareequalforallfirefighters.
Musclestrength:Boththetraining-setandprediction-setperformedatestofMaximal
handgripstrength(Grip-D:EleikosportAB,Halmstad).Thehighestperformancefromthree
trialsoneachhandwasregistered[18].
Muscleendurance:Thetraining-setperformedaSit-upstestwithastandardizedlifting
heightandspeed[18,44],Endurancehandgriptest,holdinga27.0kgdumbbellineachhand,a
SquattestusingaSmithmachine(22.0kgbarbell)andstandardizedspeed,andanBarbell
shoulderpresstestwitha7.5kgEZ-barbellperformedatastandardizedspeed[18].Boththe
training-setandprediction-setperformedaBenchpresstestatastandardizedspeed,usinga
30kgbarbellandanUprightbarbellrowtestwitha7.5kgEZ-barbellperformedatastandard-
izedspeed[18].
Musclepower:Boththetraining-setandpredictions-setperformedaStandingbroadjump
test,thebestjumpoutofthreeperformedwasregistered[18].
Laboratoryaerobicfitnesstests. Laboratoryaerobictests[17]wereperformedonatread-
mill.Continuousmeasurementsofheartrate(HR)(PolarheartratemonitorS810;PolarElec-
troOy,Kempele,Finland),oxygenconsumption(VO ),ventilation(V )andrespiratory
2 E
exchangeratio(RER)weremade(JaegerOxyconPro;CareFusion,SanDiego,USA)with
HansRudolphaccessories(HansRudolphInc.,Shawnee,Kansas,USA).Fingertipbloodlactate
wasanalyzedwithBiosen5130(EKF-diagnostic,GmbH,Barleben,Germany).Thetraining-set
performedaSubmaximaltreadmillrunningtestinordertodeterminelactatethreshold(LT)
[45,46]andOnsetofbloodlactateaccumulation(OBLA)[46,47]andaMaximaltreadmillrun-
ningtesttomeasureVO [17](Table1).
2max
Fieldaerobicfitnesstests. Fieldaerobictestswereperformedindoors.Thetraining-set
performedasubmaximal6minCycling(Ergomedic,839E;MonarkExerciseAB,Vansbro,
Sweden)testat200Wusingastandardizecadence,andamaximal30mCrawlingtest[17].In
addition,dressedinpersonalprotectivegearincludingBA(thetotalweightofclothingand
equipmentwas24±0.5kg)theyperformedbothasubmaximal6minStep-testatastandard-
izedspeed,andasubmaximal6minTreadmillwalkingtestat4.5km∙h-1and8°incline[17,37].
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PredictionofFirefighters'PhysicalWorkCapacity
Boththetraining-setandprediction-setperformedamaximal3000mRunninganda500m
Rowing(ConceptII:Concept2.Inc.,Morrisville,USA)test[17](Table1).
Simulatedworktasks. Boththetraining-setandprediction-setperformedaworktask
courseincludingCarryinghosebasketsupstairs(Stairs),Hosepulling(Pulling),Demolitionat
orafterafire(Demolition),andVictimrescue(Rescue)[17,18].Thesetaskswereperformedin
sequencewithtwominutesofactiverest(aimedformovingbetweenthestations)betweeneach
worktask.Subjectsweredressedinafireemergencyjacket,gloves,andBA(19.0±0.5kg).The
Pullingworktaskwasslightlydifferentforthetraining-setandtheprediction-set.Thetraining-
setuseda70mmdiameterrope,andtheprediction-setuseda70mmdiameterwater-filled
hose.Thepullingresistancewasequal,asverifiedbyslowlypullingtherope/hose.Inaddition,
bothsubjectgroupsperformedaCarryinghosebasketsoverterrain(Terrain)worktaskwitha
totaldistanceof1600m[17,18](Table1).
Statistics
Descriptivecomparisonsbetweenthetraining-setandtheprediction-setwerecarriedoutwith
StatisticalPackagefortheSocialSciences(SPSS)version20.0(IBMCorp,Armok,NY,USA),
usingOneWayANOVA.Physicaltestcapacitycomparisonsbetweenthetraining-setandthe
prediction-setwereexecutedwithindependentt-testforparametricdata,andwiththeKrus-
kal-Wallistestfornon-parametricdata.Datawasconsideredasnormallydistributedifatleast
twoofthreeparameterswereachieved:skewnessandkurtosisrangedwithin±2.58ofstandard
error,theShapiro-Wilk´stestwas>0.05andtheQ-QPlotwerenormallydistributed,visually
inspected[48].BivariatecorrelationsofworktaskperformanceswereanalyzedwithPearson
product-momentcorrelationcoefficient(r).
MultivariatestatisticalanalyseswereexecutedinSIMCAversion13.0(MKSUmetricsAB,
Umeå,Sweden).SIMCAprovidesoptionspatternrecognitionatseverallevels.Alldatawas
meancenteredandscaledtounitvariance(UV)priortoanalysis.Variablesthatwereconsid-
eredasskewedwerelog transformedtonormality.Tosimplifyinterpretationofthemodels,
10
variableswithalowperformancetimeequalinghighperformancewereconverted:1/variable.
Thesevariableswere:Running3000m(sands∙kg-1),Cycling,Treadmillwalking,andStep-up
(%HR ),Rowing500m,Crawling,Stairs,Pulling,Rescue,andTerrain(s),andDynamic
max
balance.
Firstmodeling. Theinitialmultivariatemodelingwasperformedwithdatafromthetrain-
ing-set.Thetotalnumberofvariablesobtainedfromthelaboratoryandfieldtests,andanthro-
pometricswere94(X=94)andthetotalnumbersofvariablesobtainedfromsimulatedwork
taskswerefive(Y=5)(Table1).Foranoverviewofdata,anunsupervisedPrincipalCompo-
nentAnalysis(PCA)wasperformedandforpredictionoftheoverallfitofthemodel(R2=1,
themodelexplains100%ofthevariationinthedata)andthepredictivepower(alsocalled
cross-validation)ofthemodel(Q2)anOrthogonalProjectiontoLatentStructures(OPLS)was
done.Thepredictivepowerisameasureoftheabilityofthemodeltopredictsamevariation
basedoncross-validation.WeconsideredmodelshavingbothanR2andaQ2>0.60to
bevalid.
PCAandOPLSgenerateascoreandloadingplot.Thescoreplotrepresentstheprojection
subjects(observations)andtheloadingplottheprojectionofthevariables(loadings).The
scoreplotshowscorrelationsbetweenobservations,identifiesiftheyarerelatedtoeachother
andifthereareanygroupsortrends.Theloadingplotshowscorrelationsbetweenvariables.
Variableswiththelargestabsolutevaluesofprojection(p)1or/andp2intheloadingplotdom-
inatetheprojectionandarethemostinfluentialvariableforthedistributionoftheobservations
inthescoreplot.Variablesclosetoeachotherarepositivelycorrelated;variablesoppositeto
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PredictionofFirefighters'PhysicalWorkCapacity
eachotherarenegativelycorrelated.Thescoreandtheloadingplotsarelinked,andtogether
theygenerateanoverviewofthevariationinthedata,providesinformationandexplanations
regardingsampledistributions,e.g.groupseparationsanddisplayvariablescorrelations.In
OPLS,Xrepresentstheregressorvariables(laboratoryandfieldtests,andanthropometric
data)andYrepresentstheresponsevariables(worktasks).Foreachworktask,anOPLSmod-
elsandseparatesthesystematicvariationinXcorrelatedtoY(predictivevariation)andthe
systematicvariationuncorrelatedtoY(orthogonalvariation).Tomakethisreporteasytoread,
mathematicalandtechnicalinformationhavebeenomitted,andfordetailsthereaderisre-
ferredtootherpublications[49–51].Foreachworktask,onemodelincludedlaboratorytests
andanthropometricdata(X=79),andoneincludedfieldtestsandanthropometrics(X=18).
Foreachmodel,astepwisevariableselectionwasexecuted:First,variableshavingthelowest
VariableImportancefortheProjection(VIP)wereexcludeduntilanoptimalmodelwasob-
tained.TheoptimalmodelincludedasfewvariablesaspossiblewithoutadecreaseinQ2>
0.05.VIPsummarizestheimportanceofthevariablesbothtoexplainXandtocorrelateto
Y.VIP-valueslargerthan1indicatesthatthevariableisimportantfortheprojection,andval-
ueslowerthan0.5indicatesthatthevariableisunimportantfortheprojection.Second,when
physicaltestvariableswerehighlycorrelated(r(cid:4)0.8)toeachother,onlythevariablehaving
thehighestVIPtothespecificworktaskwasincluded.
Externalvalidation. Externalvalidationisarigorouswayoftestingthepredictiveperfor-
manceonanindependentsetofobservationsthathasnotbeenusedinthemodelbuilding.
Theexternalvalidationofselectedphysicaltestswasexecutedwiththeprediction-set.Inorder
toachievethemostproperexternalvalidation,allXvariablesrepresentedinthetraining-set
shouldalsoberepresentedintheprediction-set[39].
Extendedanalysis. Theselectedfield-testmodelwastestedwithallsubjectsincluded
(training-set+prediction-set),onfirefighters(n=83)andoncivilians(n=45).
Secondmodeling. Withboththetraining-setandtheprediction-setincluded(n=128),a
newmodelwascreated,startingwithinclusionoftestsperformedbyboththetraining-setand
theprediction-set(Table1).Themodelselectedwithallsubjectsincludedwastestedseparately
onfirefightersandoncivilians.
Results
Subjects
Outofthe42subjectsincludedinthetraining-set,38subjectscompletedthestudyandfour
subjectsdroppedoutduetotheirlackoftime.All90subjectsincludedintheprediction-set
completedthestudy.Subjectsincludedweremalefull-timefirefighters(training-set:n=10,
prediction-set:n=12),malepart-timefirefighters(training-set:n=8,prediction-set:n=13),
civilianmen(training-set:n=8,prediction-set:n=13),femalefull-timefirefighters(training-
set:n=0,prediction-set:n=17),femalepart-timefirefighters(training-set:n=0,prediction-
set:n=23),andcivilianwomen(training-set:n=12,prediction-set:n=12).Significantdiffer-
encesbetweenthetraining-setandtheprediction-setwerefoundinphysicaltests(Table2).
Forvariablesincludedbothinthetraining-setandtheprediction-set,missingdataispre-
sentedinTable2.Additionalmissingdatafoundinthetraining-setwereforOBLAandLT
treadmillspeed,andOBLAandLT%HR :n=4,OBLA%VO :n=6and%HR at
max 2max max
200Wcycling:n=5.
Modelingandexternalvalidation
ThefirstandsecondPCAgivesanoverviewofallincludedvariables(Figs.1&2).Thefirst
PCA(Fig.1A&B)includedlaboratorytests,anthropometricsandsimulatedfirefightingwork
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PredictionofFirefighters'PhysicalWorkCapacity
Table2.Betweengroupdifferencesindescriptivedataandphysicaltests.
Variable SM Training-setn=38 Prediction-setn=90
Age(year) P 34±9.8 35±9.8
Height(m) P 1.77±0.08 1.74±0.09
Weight(kg) P 78±11.1 76±11.2
BMI(m3) P 25±2.6 25±2.7
BPSystolic(mmHg) P 130±12.8 126±12.7
BPdiastolic(mmHg) P 71±9.9 68±10.6
3000mrunning(s) NP 810±167A 867±167C
3000mrunning(s∙kg-1) NP 10.7±2.3A 11.6±3.2C
Gripstrength(kg) NP 57±24.6 43±16.9*
Benchpress(n) NP 47±56 28±23
Rowing500m(s) NP 97±18 103±16
Barbellshoulderlift(n) NP 102±79 152±111*
Broadjump(cm) NP 237±54A 210±57*
Terrain(s) NP 690±183 779±199B
Stairs(s) NP 77±40B 99±43*
Pulling(s) NP 17±10A 35±36*
Demolition(s) NP 86±50A 60±41
Rescue(s) NP 22±10A 27±12*
Betweengroupdifferencesforsubjectsincludedinthetraining-set(n=38)andintheprediction-set(n=
90).
AOnesubjectsdidnotdothetest.
BTwosubjectsdidnotdo/completethetest.
CFoursubjectsdidnotdo/completethetestBMI:BodyMassIndex.BP:Bloodpressure.Investigatedwork
taskswereCarryinghosebasketsoverterrain(Terrain),Carryinghosebasketsupstairs(Stairs),Hose
pulling(Pulling),Demolitionatorafterafire(Demolition)andVictimrescue(Rescue).Statisticalmethods
(SM):forparametricdata(P)mean±Standarddeviationispresented.Fornon-parametricdata(NP)
median±Interquartilerangeispresented.*p<0.01.
doi:10.1371/journal.pone.0118945.t002
tasks(X=79,Y=5)andgavethreesignificantcomponentsdescribingthesystematicvariation
inthedata.ThesecondPCA(Fig.2A&B)includedfieldtests,anthropometricsandsimulated
firefightingworktasks(X=18,Y=5)andgavetwosignificantcomponentsdescribingthesys-
tematicvariationinthedata.
Accordingtothescorescatterplot(Figs.1&2A)theoverallperformanceforgroupsof
menandwomenareseparatedanddifferencesoccursalsowithinsubjectgroups.Accordingto
theloadingscatterplot(Figs.1&2B)severalworktasks(Table3)andseveralphysicaltests
arecorrelated.
Thestepwisereductionstotheoptimaltraining-setmodelsforStairs,Pulling,Demolition,
andRescueworkcapacityarepresentedinTable4,andTerrainworkcapacityispresentedin
Table5.
Carryinghosebasketsupstairs. Inthefirstmodeling,thebestpredictionwiththehighest
predictivepowerincludedfieldtestsandnoanthropometricvariables(Table4,Fig.3A&B).
Asverifiedbytheexternalvalidation(Table4),theselectedfieldtestmodelwasvalidwith
allsubjectsincluded(n=126,R2=0.80,Q2=0.79,X=3),althoughtheBarbellshoulderpress
testwasexcludedfromthemodelbecausetheprediction-setdidnotdothattest.Also,themodel
wasvalidbothforfirefighters(n=83,R2=0.77,Q2=0.76)andforcivilians(n=43,R2=0.84,
Q2=0.82).
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PredictionofFirefighters'PhysicalWorkCapacity
Fig1.Principalcomponentanalysis,laboratorytestsandanthropometrics.Scorescatterplot(A)ofthetraining-setvisualizesthemainvariationinthe
data,hereseenasbetween-groupdifferences,andwithin-groupdifferences.CM:CivilianMen,CW:CivilianWomen,MFF:MaleFulltimeFirefighters,MPF:
MaleParttimeFirefighters,n=36.Loadingscatterplot(B:X=79,Y=5)visualizescorrelationsbetweenvariables:physicaltestslocatedinthesamepartof
theloadingplotarecorrelated.Thescoreplotandtheloadingplotcommunicate:subjectslocatedinthesameareainthescoreplotasvariablesinthe
loadingplothaveahighperformancewithinthesetests.UB:Upperbody,LB:Lowerbody.
doi:10.1371/journal.pone.0118945.g001
Theselectionofphysicaltestsinthesecondmodelingdidnotchangecomparedtothefirst
modeling.Rowing500m(s),Standingbroadjump(m)andRunning3000m(s∙kg-1)remained
asthemostimportantvariablesforpredictionoffirefightersStairsworkcapacity.
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