Nordic Walking Performance Analysis with an Integrated Monitoring System

13 Pages·2017·1.05 MB·English

sensors
Article
Nordic Walking Performance Analysis with
an Integrated Monitoring System
FrancescoMocera* ID,GiuseppeAquilinoandAurelioSomà ID
DepartmentofMechanicalandAerospaceEngineering,PolitecnicodiTorino,10129Torino,Italy;
[email protected](G.A.);[email protected](A.S.)
* Correspondence:[email protected];Tel.:[phone]
(cid:1)(cid:2)(cid:3)(cid:1)(cid:4)(cid:5)(cid:6)(cid:7)(cid:8)(cid:1)
(cid:1)(cid:2)(cid:3)(cid:4)(cid:5)(cid:6)(cid:7)
Received:28February2018;Accepted:9May2018;Published:10May2018
Abstract: ThereisagrowinginterestinNordicwalkingbothfromthefitnessandmedicalpointof
viewsduetoitspossibletherapeuticapplications.Theproperexecutionofthetechniqueisanessential
requirementtomaximizethebenefitsofthispractice. Thisisthereasonwhyamonitoringsystemfor
outdoorNordicwalkingactivitywasdeveloped. Usingdataobtainedfromsynchronizedsensors,
itispossibletohaveacompleteoverviewoftheusers’movements. Thesystemdescribedinthis
paperisabletomeasure: thepoleangleduringthepushingphase,thearmscyclefrequencyand
synchronizationandthepushingforceappliedtotheground. Furthermore,datafromaGPSmodule
giveanimageoftheenvironmentwheretheactivitysessiontakesplace,intermsofthedistance,
slope,aswellasthegroundtypology. Aheartratesensorisusedtomonitortheeffortoftheuser
throughhis/herBeatsPerMinute(BPM).Inthiswork,thedevelopedmonitoringsystemispresented,
explaininghowtousethegathereddatatoobtainthemainfeedbackparametersforNordicwalking
performanceanalysis. Thecomparisonbetweenleftandrightarmmeasurementsallowedvalidating
thesystemasatoolfortechniqueevaluation. Finally,aproceduretoestimatethepeakpushingforce
fromaccelerationmeasurementsisproposed.
Keywords: Nordicwalking;monitoringdevice;MEMSsensors;sportengineering
1. Introduction
The regular practice of physical activity is an essential requirement for a healthy lifestyle
regardlessofageorsocialbackground. Tomaximizethebenefitsofaworkoutandavoidundesirable
injuries,itisimportanttoproperlyperformmovementsandtoadapttheintensitytoeachindividual
characteristics. Forthisreason,inthelastfewyears,severaldeviceswereproposedforthefitness
market. Withoutadoubt,thesedeviceshelpprofessionalathletesimprovetheirperformance,butcan
alsobeusedbynon-professionalstomaximizethebenefitsoftheirworkouts.
This work focused on a particular sport discipline called Nordic walking. This is a modified
versionofstandardwalking,wheretheuseofaspecificpairofpolesengagestheupperbodymuscles,
addingtheircontributiontothepropulsivephase. Inrecentyears, Nordicwalkingwaspromoted
for its health benefits. The involvement of the upper body suggests a greater energy expenditure,
thusaquickerimprovementoftheoverallphysicalconditionifcomparedtostandardwalking[1–4].
Somestudiesshowthattheuseofthesepolesaffectstheweightdistributiononjointsandonthespinal
column[5–8].However,themedicalandbioengineeringscientificcommunityarestillinvestigatingthe
realbenefitsofthistechnique,alsoforitspossibletherapeuticapplications[9–12]. Althoughseveral
studiesseemedtoconfirmtheadvantagesofpracticingthisdiscipline,othersinvestigatedifthese
arereallyattributabletotheparticulartechniqueornot. Theauthorsthinkthatoneofthepossible
reason of this wide variety of discussions may be related to how and where testing activities are
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conducted. Usually,thesetestsaredesignedforindooractivities[13–19]wherehighaccuracytesting
equipmentalsoallowexploringphysiologicalandbiomechanicalaspectsinacontrolledenvironment.
Despite the high accuracy of these testing procedures, it is realistic to assert that in real workout
sessions, the variability of the testing conditions may lead to different results. To help prove the
feasibilityofoutdoorNordicwalkingtestingsessions,anintegratedmonitoringsystemwasdeveloped
tomeasureperformanceinseveralenvironments. UsingMEMSsensors[20–23],thesystemisableto
identifythemainparametersthatcharacterizethepropermovementtechniquegivingfeedbackto
theuserabouthowwellhe/sheperformed. Accelerationandforcemeasurementsallowonetotake
intoaccountthewalkingcycleintermsoffrequency,effortoftheupperbodyandpropertechnique
execution. Aspecificprocessingalgorithmwasdevelopedtoestimatetheanglebetweenthepoleand
thegroundduringthepushingphase. DatacomingfromanintegratedGPSsystemallowforuser
trackingduringtheworkoutsession,givingaccesstoinformationabouttheencounteredslopeand
environmentalconditions. Thereareseveralstudiesintheliterature[24–26]thatshowhowuseful
theapplicationofsensorsinmovementanalysiscanbeinski-deriveddisciplines. Althoughseveral
similaritiescanbefoundinskiandNordicwalkingtechniques,totheauthors’knowledge,thereislow
attentiononthebenefitsofportablemeasurementsplatformsappliedtothestudyofthisdiscipline.
Thus, this work wants to address the potential of the designed system as tool for Nordic walking
performanceevaluation.
IncollaborationwithGabel,theItalianleadermanufacturerofNordicwalkingpoles,withthe
helpofagroupofexperts,asetofrequirementsforthenewdataacquisitionsystemwasdefinedin
termsofweightspecificationsandparameterstobeevaluated. Infact,thefinalsystemmusthave
thelowestinfluencepossibleontheusers’movementsinordertoavoidundesirablecompensations.
Theconceptwasdesignedtobethestartingpointofapossiblecommercialversion. Theloadcell
includedinthisprototypemaynotbepresentinthefinalversion,consideringthataproperanalysis
onaccelerationdatacanleadtoagoodestimationofthepeakduringthepushingphase.
The paper has two main sections. The first one is mainly focused on the characteristics of
the designed monitoring system, while the second moves the attention to the data output of the
systemandhowtoextractusefulinformationfromthemtocharacterizetheNordicwalkingsession.
Thisexperimentalsetupwillallowgatheringusefulinformationaboutcycleandsessionperformance
thatcanbeusedinfutureworksforkinematicsanddynamicmultibody-basedanalysis.
2. TheMonitoringSystem
ThemonitoringofNordicwalkingsessionsisimportantbothforthedatascientificrelevance
andfortheuserexperience. Severalstudiespointedoutthattherearedifferencesbetweentheresults
obtained in the literature, and it is widely accepted that testing equipment play a big role in this
direction. Theyaredesignedforindoorapplicationsandareusuallycumbersome,forcingtheusers
unconsciouslytoadapttheirbehaviortothetestingmachine. Themonitoringsystemhereinpresented
isthoughttoovercomesomeofthelimitationsofindoorequipment,butnottoreplacethem. Infact,
theseequipmentallowforbetteraccuracyexploringphysiologicalaspectsthatcannotbeanalyzed
withoutspecifictools.
2.1. TechnicalandFunctionalRequirements
Inthissportdiscipline, thecharacteristicsofthepolesarecrucial. Theyaredesignedtohave
a good intensity level of use of the upper body muscles without introducing a sensible fatigue
contributionduringtherecoveryphase. Theyarecharacterizedbyhighstiffness, lowweightand
aspecificgeometricdesigntokeeptheimpactontheusers’movementsaslowaspossible[27].
Nordicwalkingpoles(Figure1)areextremelylight(180g)toreducetheinertiacontributionin
therecoveryphase. FromtheexperienceofseveralNordicwalkingtrainers,asetofrequirementsin
termsofweightandpositionfromthehandlewasstatedtoreducetheimpactofthesystemonthe
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movement. Moreover,toallowlongoutdoorsessions,thesystemhadtobeoptimizedintermsof
energyconsumptionandacquisitionperformance.
Figure1.Nordicwalkingpole:(a)handle;(b)glove;(c)aluminumorcarbonfiberbody;(d)steeltip.
SeveralstudiesintheliteratureagreeonwhatarethemainparameterstodescribetheNordic
walkingtechnique. Themostwidelyacceptedare:
• theanglebetweenthegroundandthepoleduringthepushingphase;
• thetimelengthofthecyclicmovement;
• thepushingforcewhenthepoleisincontactwiththeground.
Inadditiontotheseparameters,theheartratecangiveameasureoftheworkoutintensity,and
GPS coordinates allow tracking the user path in terms of ground conditions (location) and slope.
All these data were evaluated from raw sensor measurements. To correctly capture the dynamics
of the movement, a minimum acquisition frequency of 15 Hz was fixed as the requirement of the
acquisitionboard.
2.2. TheAcquisitionSystem
To build a device capable of collecting useful information about a Nordic walking session,
atwo-stepdesignprocedurewasfollowedtodefinetheboardrequirements:
• parametersidentification;
• measurementsdefinition.
Theparametersidentifiedintheprevioussectionthankstotheliteraturereviewallowedstating
therequiredmeasurements,thusthenecessaryhardwarecomponents.InTable1,themaincomponents
usedontheboardandtheirmainfunctionalityaresummarized.
• Triaxialaccelerometer: Fromaccelerationmeasurementsofaproperly-orientedaccelerometer
(Figure2),itwaspossibletoobtainseveralusefulparameters: thecyclelength,themeanangleof
thepoleduringthepushingphaseand,insomecases,anestimationoftheimpactforce. Forthe
specificapplication,thedevicewassetupwitha10-bitresolutioninthe±8-grange,usingthe
standardsamplingrateof100Hz. Notethatthemeasuredaccelerationsweredigitallyfilteredby
thedeviceitselfwithabandwidthofhalfthesamplingrate(50Hzinthiscase).
• GPS:FromGPScoordinates,itwaspossibletotracktheuserduringtheworkoutsession.Knowing
thegeographiclocation,environmentalcharacteristicssuchastheslopeandgroundpropertiescan
beevaluated. Thedevicewassetuptoupdatepositioninformationeverysecond. Theavailability
ofanabsolutetimesourceintheGPSmessagewasconsideredasasimplewaytosynchronize
databetweentheleftandrightdevice.
• Load cell: From axial force measurements (Figure 3), it was possible to understand how the
userreliesonpolesforforwardthrustmotion. Ahighermeanvalueofthepushingforcemeans
highereffortoftheupperbody,whichdoesnotnecessarilycorrespondtothecorrectapproach.
Poles should be used as a complementary driving force to the legs, not as the main pushing
component. Theaxialloadcellusedinthisapplicationwascharacterizedbyaratedforceof500N
withasensitivityof1mV/V/FS.
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• Heart rate sensor: From a simple measurement of the heart Beats Per Minute (BPM), it was
possibletohaveanideaoftheusereffort.Thisparametercanbeusedtostudyhowthesame
session impacts users with different characteristics. From the medical point of view, using
this parameter, it is possible to compare the performance of several patients and track their
improvementsovertime.
• Memorystorage: Astoragesolutionwasnecessarytorecordalltheacquireddatacomingfrom
thesensors.
• Micro-controller: Thiswasthedevicebrain,whichmanagedallthedifferentsensorssavingall
thedataonthestorageunitwithacertainformattosimplifythepost-processingphase.
Table1.Hardwarecomponentsofthemonitoringsystem.
Component DeviceName Description
Accelerometer ADXL345 Accelerationx,y,z
GPS MTK3339 Position,data,time
Loadcell KM26z-500N Forcealongxaxis
Heartratemonitor PulsesensorAmped BeatsPerMinute(BPM)
Memory µSDCard Datastorage
Microcontroller ATMEGA328P Devicemanagement
Figure2.Accelerometerreferenceframeorientation.
Figure3.Integratedloadcell.
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In Figure 4a,b, the system functional layout and the actual data acquisition board are shown.
Asdescribedinpreviouswork[28,29],toallowlongoutdoormonitoringsessions,allofthesystem
had to be optimized in terms of acquisition performance and energy consumption. The use of
aproperly-tunedenergyharvestercanextendthemonitoringtimeusingtheavailablekineticenergy
fromthecyclicarmmovement[30–33]. Theenergyconsumptiondependsalsoontheperformance
requirementsofthemonitoringsystem. Throughanoptimizationprocess,itwaspossibletoreachthe
besttradeoffbetweenperformanceandenergyconsumption. Anacquisitionfrequencyof50Hzwas
obtainedwithatotalenergyconsumptioninacquisitionmodeof60mW.
(a) (b)
Figure4.(a)Monitoringsystemlayout;(b)dataacquisitionPrintedCircuitBoard(PCB).
2.3. Post-Processing: NordicWalkingDataAnalysis
The acquisition board was developed together with a proper post-processing tool to allow
non-technicaluserstoperformaneasyevaluationofthemainparameters. Dataarestoredbythe
microcontrollerinthephysicalmemoryinacertainformattedtextfiletohelpthepost-processing
phase. Thestoredinformationiscomprisedofthefollowing:
• latitudeandlongitude;
• dateandtime;
• accelerationmeasurements;
• userheartrateontheleftpole;
• axialforceontherightpole;
• internalmicrocontrollertimeforinternaldatasynchronization.
Asitwillbediscussedinthenextsections,therawdatacanbeprocessedbythesoftwaretoolin
ordertoidentifytheparametersofinterest,bothasdirectmeasurementsonthetimehistoriesoras
acombinationbetweendifferentdatasources.
3. MeasurementsandRawData
Thedescribedsystemwascapableofacquiringseveralmeasurementsinasynchronizedway.
Therewasnotastrictdeterministicacquisition;however,thelowdynamicsofthephenomenainvolved
allowedhavingratheraccuratesimultaneousmeasurements. Thesubjectinvolvedinthesetestswas
oneoftheworkauthors,whosuccessfullyaccomplishedaNordicwalkingtrainingcourse. Thesubject
is a young trained man (27 years old) with experience in several ski disciplines and trail running.
Thesecharacteristicswereconsideredsufficientasrequirementsforthesystemtester.
3.1. AccelerationMeasurements
Acceleration measurements allowed characterizing each phase of the arm cyclic movement.
In Figure 5, it is possible to see an usual output diagram of the acceleration measured along the
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longitudinalaxisofthepoles. Theplotshowsthatthemeasureddynamicbehaviorisinagreement
withwhatcanbefoundintheliterature[29]. Thedatameasuredalongthisaxisallowedforseveral
considerations. After the first contact between the pole tip and the ground, no other dynamic
component is superimposed on the acceleration measured along this direction. Thus, since the
chosenaccelerometermeasuredalsothestaticcomponentsofthegravityacceleration,itwaspossible
toestimatetheanglebetweenthepoleaxisandthegroundduringthepushingphase. Thiscanbe
definedas:
a
cos(φ) = x (1)
acos(90−θ)
where:
• aisthemoduleoftheaccelerationvectora;
• a istheacomponentalongthexaxis(polelongitudinalaxis);
x
• θistheanglebetweenaandthex-yplane;
• φistheanglebetweentheprojectionofaonthex-yplaneandthexaxis.
Figure5.Typicallongitudinalaccelerationoutputmeasuredbytheacquisitionsystem.
However,inthespecificapplication,thedevicewasorientedinsuchawaythatinitsnominal
position,thex-yplanewasparalleltothebodysagittalplane. Thismeantthatnominally,θwas90◦,
andthus:
φ = arcosa (2)
x
Duringtheexercise,thementionedplanedidnotalwaysremainstrictlyparalleltothebody’s
sagittal plane. Nevertheless, also without a perfect execution, the deviation from the nominal
orientationofthisplanewasconsideredsmallenoughtoaccepttheapproximationofthepoleangle
with the mentioned φ angle. It should be pointed out that also ground inclination may slightly
affectthevalueofthisangle. Inthiswork,thiseffectwasnotconsideredsincethemaintestswere
conducted on plane fields. A more sophisticated version of the device may include in the future
a9DOF(DegreesOfFreedom)sensorinordertohaveadditionalinformationtoevaluatethecorrect
relativepositionbetweenpolesandground.
Looking at the diagram shown in Figure 5, three main phases can be identified from the
measuredacceleration:
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1. The recovery phase was where the poles essentially did not touch the ground and were in
agenericroto-translationalmotioncarriedbytheuser. Thepolelostcontactwiththegroundand
thenwaspulledforwardsothatapositivedynamicaccelerationcomponentwassuperimposed
onthestaticgravitycomponent.Inthelastpartofthisphase,thearmstoppeditsmotion,pushing
thepoletowardstheground(negativeaccelerationvalues).
2. Theimpactphasewaswhenthepolestouchedtheground. Theimpulsivenatureofthecontact
translatedintoahighaccelerationvaluereachedinasmallamountoftime. Boththeamplitude
andthegrowthrateofthiseventstronglydependonsoilcharacteristicsandontheusertechnique.
3. Thepushingphasewaswhentheuserpushedonthepoles,alreadyincontactwiththeground,
to move forward. The measured acceleration could not exceed 1 g in this condition since no
relevantdynamiccomponentwaspresentinthisphase.
Thetimedurationofeachcycleisaperfectmeasureoftherepetitivenessofthemovement. Inthe
particularcaseshownabove,thecyclelastsforaboutonesecond. Thedurationofeachphasecan
alsobeconsideredtoanalyzethemovement. Inthespecificcase,therecoveryphaselastedforabout
50%ofallofthecycle,whiletheimpactandthepushingphaselasted15%and35%oftheremaining
cycletime.
More interesting is the superimposition of the diagram coming from the left and right arm
(Figure6). EachpolehaditsowndataacquisitionsystemwithaproperGPSsensor. Thetwodevices
werenotabletocommunicatetogether,buttheabsolutetimegivenbytheGPSsensorsandaproper
synchronizationalgorithmallowedforapost-processingsuperimposition.
Figure6.Accelerationdatasuperimpositionbetweeneacharm.
Merging information coming from each pole, it was possible to establish how each arm was
performingintermsoftimingandmostimportantlyofpushingangle. Thiscomparisonallowsto
characterizedifferentNordicwalkingstylesastheonewiththealternatepushingphase(shownabove)
ortheparallelpushingvariantshowninFigure7. Thelattertechniqueischaracterizedbyalonger
cycletimelength,leadingtoamorediscontinuousmovementthathastobeproperlycompensatedby
theuser’scoretomaximizetheeffectivenessofthepushingphase.
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Figure7.Parallelpushingtechnique.
3.2. ForceMeasurements
Theintegratedaxialloadcellallowedmeasuringtheforcealongthepolelongitudinalaxis. Inthis
analysis,thecomplianceeffectofthepolewasneglectedduetothehighstiffnessofthecarbonbodyof
theequippedpole. Figure8showsresultsinagreementwithindoorpolingforcemeasurements[13,14],
especiallyasconcernsthepushingforcetemporaldistributionwithinthearmcycleanditspeakvalues.
Inparticular,itispossibletoidentifythethreemainphasesofthecyclicmovement:
1. Therecoveryphasewhentheforceisnegative(tension)andbelowacertainthreshold;
2. Theimpactphasecharacterizedbytheincreaseofthepushingforce;
3. Thepushingphasewhentheforceslowlydecreasesuntilthemomentwhenthepoletiploses
contactwiththeground,startinganewcycle.
Figure8.Forcemeasurementsalongthelongitudinalpoleaxis.
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3.3. GPSData
ThroughGPScoordinates,itwaspossibletotrackthegeo-locationoftheNordicwalkingsession,
ascanbeseeninFigure9a,b. Freetoolsavailableontheweballowedcreatinganicemapwiththe
followedpathandreturnedthealtimetrylevelencounteredduringthesession.
TheuniversaldateandtimeavailableontheGPSsignalplayedakeyroleinthepost-processing
synchronizationalgorithmsincethetwodevicesonthepoleswerenotabletocommunicatewirelessly
toeachotherduringthetests.
(a) (b)
Figure9.(a)GPStracking;(b)altitude.
3.4. HeartRateData
InFigure10,itispossibletoseeatypicaloutputsignaloftheheartratesensor. Valuesonthe
y-axisarethedigitalrepresentationofthesensoranalogueoutput. Ahighervoltageisregisteredwhen
thebloodpressurewavepassesbelowthelightsensor. ThenumberofBeatsPerMinute(BPM)canbe
easilycalculatedbymeansofapeakcountingaboveacertainsignalthreshold.
Figure10.HeartratesensorsignalforBPMevaluation.
Due to the intrinsic behavior of the sensor, the signal had a very high sensitivity to noise
componentscomingfromthemovement. Measurementsweretakenonlywhentheuserwasstill,
forexampleinshortpausesduringthesession. Afutureversionofthedataacquisitionsystemshould
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consider also different technologies to measure this parameter, leading to a better understanding
duringalloftheworkoutsession.
4. AccelerationandForceMeasurementsduringtheWalkingCycle
Theadoptionofanaxialloadcellwouldbetheperfectsolutionforacompleteviewofeacharm’s
performance.Althoughseverallessexpensivesolutionstomeasurethepushingforcemaybeexploited
fromacommercialperspective,heretheauthorswanttohighlighttherelationbetweenthepeakforce
andtheaccelerationmeasuredjustbeforetheimpactasapossiblesolution. Comparingacceleration
andforcedata(Figure11),thereisalwaysacorrespondencebetweenthetimewhentheforcestarts
torapidlyincreaseandtheaccelerationmeasurementoftheimpact. AccordingtoNewton’ssecond
law,thereisarelationbetweenaccelerationandforce. However,itisnotsoimmediatetoevaluate
theinertialmassinvolved. Infact,thepolemassisjustafractionoftheinertiacontributionthatwill
determinethetotalappliedimpactforce. Theequivalentcontributionofthearmandhandshouldbe
alsotakenintoaccount. Sinceitisnotreasonabletomeasurethesequantitiesforeachuser,themost
suitablewayistoestablisharelationwiththemainhumanbodyparameters(height,weightandage)
withthespecifictraininglevel,bodyfatlevelandleanmusclemass. InTable2,theresultsofsuch
akindofanalysisareshown. Consideringdatacomingfromthesameuser,adetectionalgorithm
identified20samplesovera2-minsessionasvalidsignals. Thislownumberofsamplesisstrongly
relatedtothelimitedsamplingfrequencyoftheacquisitionboard. Forfutureworksfocusedonthis
typeofanalysis,itisadvisabletoconsiderhighersamplingfrequencies.
Figure11.Acceleration-forcecomparison.
Table2.Inertiaconstantstatisticalevaluation.
NumberofSamples MeanValue(kg) StandardDeviation(kg)
20 3.6 0.3
Itmustbenotedthatthemeasuredaccelerationisindeedthecombinationoftwocomponents:
a = a +a (3)
x s d
where:
• a isthemeasuredaccelerationalongthelongitudinalpoleaxis;
x