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JournalofHumanKineticsvolume29/2011,4957 49SectionIKinesiology

1UniversityofBeiraInterior.DepartmentofSportSciences(UBI,Covilh.Portugal).2 ResearchCentreinSports,HealthandHumanDevelopment(CIDESD,Portugal).3PolytechnicInstituteofBragana.DepartmentofSportSciences(IPB,Bragana.Portugal.4 University

of

Trs

os

Montes

and

Alto

Douro.

Department

of

Mechanical

Engineering

(UTAD,

Vila

Real,

Portugal).

5UniversityofPennsylvania.DepartmentofMechanicalEngineeringandAppliedMechanics(UPENN,Pennsylvania.USA).6 UniversityofTrsosMontesandAltoDouro.DepartmentofSports,HealthandExercise(UTAD,VilaReal.Portugal).

Authorssubmittedtheircontributionofthearticletotheeditorialboard.

AcceptedforprintinginJournalofHumanKineticsvol.29/2011onSeptember2011.

TheHydrodynamicStudyoftheSwimmingGliding:aTwo

DimensionalComputational

Fluid

Dynamics

(CFD)

Analysis

DanielA.Marinho1,2,TiagoM.Barbosa2,3,AbelI.Rouboa4,5,AntnioJ.Silva2,6

Nowadays the underwatergliding after the starts and the turnsplays amajor role in the overall swimming

performance.Hence,minimizing hydrodynamic drag during the underwaterphases should be amain aim during

swimming. Indeed, there are severalpostures that swimmers can assume during the underwatergliding, although

experimental resultswere not conclusive concerning the best bodyposition to accomplish this aim. Therefore, the

purposeof

this

study

was

to

analyse

the

effect

in

hydrodynamic

drag

forces

of

using

different

body

positions

during

gliding throughcomputationalfluiddynamics (CFD)methodology.For thispurpose, twodimensionalmodelsof the

human body in steadyflow conditionswere studied.Twodimensionalvirtualmodels had been created: (i) aprone

positionwiththearmsextendedatthefrontofthebody;(ii)apronepositionwiththearmsplacedalongsidethetrunk;

(iii)alateralpositionwiththearmsextendedatthefrontand;(iv)adorsalpositionwiththearmsextendedatthefront.

The dragforceswere computed between speeds of 1.6m/s and 2m/s in a twodimensional Fluent analysis.The

positionswith thearms extendedat thefrontpresented lowerdragvalues than thepositionwith thearmsaside the

trunk.The lateralpositionwas theone inwhich thedragwas lowerandseemstobe theone thatshouldbeadopted

duringtheglidingafterstartsandturns.

Key words: tests and testing; computational fluid dynamics; technique; biomechanics; numerical simulations,

swimminggliding

Introduction

Competitive swimmers should have twoaims to improve speed and thus enhanceperformance. They should: (i) maximize thepropulsive forces produced by the propellingsegments and; (ii) minimize the hydrodynamicdrag resisting forward motion (Callaway et al.,2009; Marinho et al., 2009). Regarding the latteraim,asubstantialenergyiswastedtothewaterin

orderto

overcome

the

resistance

(Toussaint

&

Beek,1992;Kolmogorovetal.,1997).Thus,expertswimmers seem to improve techniquedue toanincrease in propulsive force, as well as,minimizing hydrodynamic drag (Seifert et al.,2007).

Effortstominimizehydrodynamicdragshouldbecarriedout during all swimming phases.However, decreasing drag during the glidingafterstartsandturnsshouldbeamainconcernforswimmers and their coaches, especiallynowadays, when the underwater gliding playsamajor role to theoverall swimmingperformance(VilasBoas et al., 2010). Thus, swimmers must

adoptthe

most

hydrodynamic

position

during

gliding. Several studies (VilasBoas et al., 2000;Cossor&Mason,2001)suggestedthatratherthanthe start technique usedby the swimmer, it ishis/herbodypositionafterimmersionthatmostlydetermines thesuccessof the start. Indeed, there

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50 TheHydrodynamicStudyoftheSwimmingGliding

EditorialCommitteeofJournalofHumanKinetics

are several postures that the swimmers canassumeduring theunderwatergliding,althoughexperimental results were not conclusiveconcerningthebestbodypositiontoperformthisphase (Jiskoot & Clarys, 1975; Maglischo, 2003).

Some

swimmers

prefer

to

glide

in

a

lateral

position and others in a prone one. In addition,during gliding swimmers can change theirbodyposture.Moreover,insometechniquesswimmersmustchangetheirlimbpositions.Forinstance,inbreaststroke, the gliding is initially performedwith the arms fully extended at the front. Butthen, swimmers perform a second gliding withthe arms aside the trunk. It canbe thought thatthesedifferentposturesmight lead todifferencesinthe intensityofthedragforcesexperiencedby

the

swimmers.

Hydrodynamic drag consists of friction,pressureandwavedrag.Friction,orviscousdrag,is originated from fluid viscosity. It producesshear stresses in theboundary layer. Thus, themagnitudeoffrictiondragdependsonthewettedsurfaceareaof thebodyand the flowconditionswithintheboundarylayer.Pressure,orformdrag,is due to distortion of flow outside of theboundary layer. The flow over the swimmersbodymay separateatacertainpoint,depending

on

the

shape,

size

and

velocity

of

the

swimmer.

Behindtheseparationpoint,theflowreversesandmay roll up into distinct eddies (vortices). As aresult, a pressure differential arisesbetween thefront and the rear of the swimmer, resulting inpressuredrag.Whenswimmingneartothewatersurface,duetotheinterfacebetweentwofluidsofdifferent density, a third component of the totaldragisduetowavedrag.Kineticenergyfromtheswimmer is lost as it is changed into potentialenergy in the formations of waves (Toussaint &

Truijens,

2005).

Thedragforcecomponentsproducedbytheswimmer gliding have been analysed applyingdifferent experimental methods (Clarys, 1979;Lyttleetal.,2000).However,asabovementioned,dataobtainedon thosestudiesvaried,whichcanrepresent some of the difficulties involved withexperimentalresearch(Bixleretal.,2007).Anotherdifferentapproachthatcanbeusedtoanalysetheeffect of different postures during the gliding istheapplicationofanumericalsimulationmethod,

such

as

computational

fluid

dynamics

(CFD).

Bixleretal. (2007)presenteddatasupporting the

accuracyandvalidityofthismethodtobeusedinswimmingresearch.Sincethen,thismethodologyhasbeenapplied on regularbasis toanalyse thewaterflowaroundthehumanbodyandtheforcesinvolved during the swimmers displacement

(Rouboa

et

al.,

2006;

Silva

et

al.,

2008;

Marinho

et

al.,2009).Therefore,thepurposeofthisstudywastoanalysetheeffectinhydrodynamicdragforcesof using differentbody positions during glidingthrough computational fluid dynamicsmethodology.

MaterialandMethods

Digitalmodel

Twodimensional virtual models werecreated in CAD software to acquire the human

bodygeometry.Thisgeometrywasbasedon theanatomicalcharacteristicsofamalenationallevelswimmersgroup (n=15,20.021.37yrs,1.870.21m of height, 78.324.56 kg of body mass).Anthropometrical measures (height, arm span,arm length, leg lengths, hand length and width,foot length and width, and sitting height) of 15maleswimmerswereassessed,assumingthatthedigital model represented average values ofnational level swimmers (participating inNational Swimming Championships). Therefore,

thedigitalmodelwascreatedinCADwith1.87mheight, a finger to toe length of 2.37 m (in thepositionwith thearmsextendedat the frontandwith shoulders fully flexed) and, a vertex to toelength of 1.92 m (in the position with the armsalongthetrunk).

Four digital models were developed tocomparetheswimmersposturegliding:(i)attheprone position, with the arms extended at thefront of thebody (Figure 1. Panel A); (ii) at thepronepositionwiththearmsplacedalongsidethe

trunk (Figure 1. Panel B); (iii) at the lateralposition, with the arms extended at the front ofthebody(Figure1.PanelC)and;(iv)atthedorsalposition, with the arms extended at the front ofthebody(Figure1.PanelD).

The models surfaces were then developedusingGambit,ageometrymodellingprogramofFluent software (Fluent, Inc. Hannover, USA).Thesesurfaceswerethenmeshed,usingthesamesoftware, creating the grid which was importedintoFluentforanalysis(Figure1).

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byMarinhoD.etal. 51

JournalofHumanKineticsvolume29/2011, http://www.johk.pl

Figure1Swimmersmodelgeometrywiththebodyinthefourdifferentpositionsanalysedinthisstudy

Figure2Computationalfluiddynamicsmodelgeometrywiththemodelinadorsalposition

withthearmsextendedatthefront

Boundaryconditions

The computational domain consisted of atwodimensional grid, or mesh of cells, thatsimulate the fluid flow around the humanbodymodel.

The fluid mechanical properties, the flowcharacteristics along the outside gridboundariesand themathematical relationship toaccount forthe turbulencewere considered.The standardkepsilon turbulence model was considered andimplemented in the Fluent software. Thisturbulencemodelwasshowntobeaccuratewithmeasuredvalues inapreviousresearch (Moreiraetal.,2006).

The computational fluid dynamics analyseswere done with the models in a horizontal

position with an attack angle of 0. The attackangleisdefinedastheanglebetweenahorizontal

planeandthefoilcord(i.e.alinedrawnfromtheheadvertextotheanklejoint).

The boundary conditions of thecomputational fluid dynamics model weredesigned to represent the geometry and flow

conditionsofapartofaswimmingpoollane.Thewaterdepthofthemodelwas1.80m.Thelengthof the computational domain was 8.0 m in thestreamlinedpositions and 7.55 m in thepositionwith the arms along the trunk, allowing in allsituationsthesameflowconditionsbehindandinfront of the model. In all positions, the distancefrom theswimmershands (orhead) to the frontsurfacewas2.0mandfromtheswimmerstoetothe back surface was 3.63 m. The swimmersmiddle line (defined as a line passing through

his/her centre of mass) was placed at a waterdepth of 0.90 m, an even distance from the top

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andthebottomsurfaces(Figure2).Thedragcoefficients(Cd)andthedragforce

(Fd) were computed for speedsbetween 1.6 m/sand 2.0 m/s (with 0.1 m/s increments) in a twodimensional Fluent steady flow analysis,

according

to

equations

1

and

2:

Fd=0.5..SCd.v2 (1)

Cd=Fd/(0.5..S.v2) (2)

where Fd is the drag force, Cd is the dragcoefficient, is the fluid density, S is theprojectionsurfaceof themodelandv is the flowspeed.

Dataanalysis

Foreachpositionandspeed,dragcoefficientsand drag forces data were described andcompared. The relationshipsbetween speed anddrag coefficient,aswellas speedanddrag forcewere studied through a nonlinear fittingmathematical model, through the application ofvarious exponential equations. Added to thecomputing equations, the R2 (coefficient ofdetermination)wasalsousedasameasureofthemodelsgoodoffit.

It was assumed that total drag force

comes only from the friction and pressure dragcomponents, as the model was placed 0.90 m

underwater. Therefore, wave drag can beconsideredasnegligent,andapproximatelyclosetonullvalue.

Results

Table 1 shows the drag coefficient valuesproducedby the fourbodypositions.Thepartialcontribution of the friction and pressure dragcomponents to total drag is also presented. Thepositions with the arms extended at the frontshoweddragvalueslowerthanthepositionwiththe arms aside the trunk. The lateral positionpresented the lowest values of total drag.Moreover, the ventral and the dorsal positionsresultedinsimilardata.

Regarding each drag force component,

pressuredragwasthemainfactorresponsiblefortotaldragforce,contributingto~92%and~85%ofthetotaldrag,atthepositionwiththearmsasidethe trunk and at the positions with the armsextendedatthefront,respectively.

Figures 3 and 4 present the relationshipbetweenthedragcoefficientandspeed,aswellas,the drag force and the speed, for the differentgliding postures, respectively. For all studiedpositions, the drag coefficient values decreasedwith speed whereas the drag force values

increasedwithspeedflow.

Table1

Dragcoefficientvaluesandcontributionofpressureandfrictiondragforthetotaldrag

toeachspeedandforthefourdifferentglidingpostures

Velocity Dragcoefficient Dragcoefficient

(m/s) (Prone:armsaside thetrunk) (Prone:armsextendedatthefront)

Totaldrag%Pressure

drag%Friction

dragTotaldrag

%Pressuredrag

%Frictiondrag

1.6 1.06 92.23% 7.77% 0.92 86.13% 13.87%1.7 1.01 92.24% 7.76% 0.87 86.15% 13.85%1.8 0.95 92.29% 7.71% 0.83 86.16% 13.84%1.9 0.89 92.30% 7.70% 0.79 86.23% 13.77%2.0 0.85 92.34% 7.66% 0.76 86.24% 13.76%

(Dorsal:armsextendedatthefront) (Lateral:armsextendedatthefront)

Totaldrag%Pressure

drag%Friction

dragTotaldrag

%Pressuredrag

%Frictiondrag

1.6 0.89 85.91% 14.09% 0.57 83.87% 16.13%1.7 0.84 86.01% 13.99% 0.56 83.88% 16.12%1.8 0.80 86.07% 13.93% 0.55 83.91% 16.09%1.9 0.76 86.12% 13.88% 0.54 83.98% 16.02%2.0 0.73 86.14% 13.86% 0.53 84.05% 15.95%

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byMarinhoD.etal. 53


y = -0.52x + 1.887

R

2

= 0.9951

y = -0.102x + 0.7336

R2= 0.9985

y = -0.3957x + 1.5176

R2= 0.9939

y = -0.408x + 1.5666

R2= 0.9956

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.5 1.6 1.7 1.8 1.9 2.0

Prone Arms In Front

Prone Arms Aside

Lateral Arms In Front

Dorsal Arms In Front

Figure3

Relationshipbetweenthedragcoefficientandthespeedforthedifferentglidingpostures.

TheregressionequationsandtheR2valuesarealsopresented

y = 44.449x0.9307

R2= 0.9944

y = 34.64x1.1211

R2= 0.9997

y = 33.601x1.1177

R2= 0.9993

y = 16.659x1.6688

R2= 0.9998

20

30

40

50

60

70

80

90

1.5 1.6 1.7 1.8 1.9 2.0

Prone Arms In Front

Prone Arms Aside

Lateral Arms In Front

Dorsal Arms In Front

Figure4

Relationshipbetweenthedragforceandthespeedforthedifferentglidingpostures.

Theequations

and

the

R2

values

are

also

presented

Discussion

The aim of this study was to analyse theeffectinhydrodynamicdragforceswhendifferentbody positions are applied during underwaterglidingafterstartsand turns inswimming.Maindata suggested that the positions with the armsfullyextendedat the frontof thebodypresentedlowerdragvaluesthanthepositionwiththearmsaside the trunk. In addition, the lateral position

(with thearmsat the front)presented the lowesthydrodynamicdragduringunderwatergliding.

Nowadays,inbiomechanicalengineeringtheCFDtechniqueisoneofthebestmethodsusedforthe analysis of fluid flow. The CFD isbased oncomputer simulations, thus presents the

advantageoftestingseveralsituationsandallowsto obtain the best or optimal result, without

Dragcoefficient

Flowspeed(m/s)

Flowspeed(m/s)

Dragforce(N

)

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physical/experimental testing. The CFD wasdeveloped to be valid and accurate in a largescope of fluid environments,bodies and tasks,includingsports,beingscientificallyassumedthatthe CFD has ecological validity even for

swimming

research

(Bixler

et

al.,

2007;

Marinho

et

al., 2009). Bixler et al. (2007) compared thehydrodynamicdragbetweenadigitalCFDmodelofamale swimmeranda realmannequinbasedon the digital model. The authors (Bixler et al.,2007) found drag forces determined from thedigital model using the CFD approach to bewithin 4% of the values experimentallydetermined for the mannequin. Hence, the CFDallows to examine changes in the swim stroketechniqueandhowthesechangesmightaffectthe

swimmers

performance.

In

the

current

work,

we

have analysed the hydrodynamic drag sufferedby the swimmer at different underwater glidingbodypositionsthatareused inhighlevelevents,attemptingtohelpswimmersandtheircoachestoimprove performance due to decreasing dragduring this phase. This study was developedthroughapassivedraganalysis,withthebodyina fixed position, without any movements of thelimbs, position assumed during the first part ofthe gliding after the start and after pushingoff

from

the

wall

during

the

turns

(Konstantaki

&

Winter,2007).The four selected positions canbe observed

on a real swimming setting, after the starts andturns. For example, the gliding phase in frontcrawl, butterfly and the first gliding inbreaststroke (at a prone position with the armsextended at the front), the second gliding inbreaststroke (at a prone position with the armsaside the trunk), the gliding inbackstroke (at adorsal position with the arms extended at the

front)

and,

in

some

techniques/phases

during

the

gliding in front crawl (at a lateral position withthearmsextendedatthefront).

Regarding drag coefficient values, one cannote that there was an inverse relationshipbetweenthisvariableandthespeedflow.TheCddecreased as speed increased. The inverserelationshipbetween thedragcoefficientand thespeed found in the current study seems tocorrespondtowhatisreportedwithexperimentalsettingsforhumanbodiesfullyimmersed(Jiskoot

&

Clarys,

1975;

Lyttle

et

al.,

1999).

Jiskoot

and

Clarys (1975) foundCdvaluesbetween0.95and

1.0,withaslopeoftheregressionbetweentheCdand the speed of 0.17 (towing speeds rangingfrom1.5to1.9m/s).ThisdataissimilartotheonereportedbyLyttleetal.(1999), 0.16,usingspeedsrangingfrom1.6 to3.1m/s. In thecurrentstudy,

the

values

of

the

slope

of

the

regression

line

variedbetween 0.52and 0.10.Itisinterestingtonotice that the most hydrodynamic positionspresented the lowest slope values, and thesevaluesaresimilartoexperimentaldata.

CFD data and experimental data presentedquitesimilarCdvalues(Clarys,1979;Mollendorfet al., 2004). Clarys (1979) found Cd valuesbetween 0.58 and 1.04 for the humanbody in apassive towing situation. More recently,Mollendorf et al. (2004) also found in a passive

towing

approach

Cd

values

between

0.83

and

0.90.Ourdatarangedfrom0.53to1.06.However,ifweonlyreportedtotheproneposition(similarto the one usedby Mollendorf et al., 2004) theCFD data are very similar to experimental data(Cdvaluesrangedfrom0.76to0.92).

Nevertheless, the current CFD data stillpresented somedifferenceswith theCFD resultsobtainedby Bixler et al. (2007), using a threedimensional model of the human body. Theseauthors reported Cdvalues of0.302,0.300,0.298

and

0.297

for

speeds

of

1.5,

1.75,

2.0

and

2.25

m/s,

respectively. Probably, differencesbetween twoand threedimensional models lead to thesedifferences. Additionally, one can add thedifferent methodology to acquire the digitalmodel.Bixler etal. (2007)carriedout laser scansofamaleswimmertoobtaintheboundariesofthehumanbody; whereas in this study the modelwasdesignedinCAD.

Concerning the values of drag force,MiyashitaandTsunoda(1978)reporteddragforce

values

of

35.3

N

for

females;

whereas

Clarys

(1979) presented values of 51.9 N for malenational level swimmers, similar to the onesfound in thecurrentresearch.Lyttleetal. (1998),at a lower velocity studied (1.6m/s), and at adeepertowingpositiontheystudied(0.6mdeep),also reported values for male swimmers withinthisrange(58.1N).

Ourresultsarealsosimilartotheonesfoundby Bixler et al. (2007) using a CFD approach.These authors found drag force values of 31.58,

42.74,

55.57

and

70.08

N

for

speeds

of

1.5,

1.75,

2.0

and2.25m/s,respectively,withthehumanmodel

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byMarinhoD.etal. 55


atapronepositionwiththearmsextendedatthefront.Inthispositionwefounddragforcevaluesfrom 58.7 to 75.4 N for speeds rangingbetween1.6and2.0m/s.

Itwasalsofoundthatthebodypositionwith

the

arms

fully

extended

at

the

front

presented

lowerCdvalues than thebodypositionwith thearms aside the trunk. Furthermore, the lateralposition presented much lower Cd values incomparison to others. The prone and the dorsalpositions (both with the arms extended at thefront)presentedsimilardata.

Theanalysisof thepassivedragwasoneofthe first applications of biomechanics inswimming.Thepositionwith thearms extendedat the front was the most studied position. This

position

is

mostly

accepted

by

the

swimming

technical and scientific communities as the mosthydrodynamic one,being called the streamlinedposition (Guimares & Hay, 1985; Goya et al.,2003). The values found in this study seemed tocorroboratetheassumption.Thepositionwiththearmsfullyextendedatthefrontseemstosmooththe anatomical shape especially at the head andshoulders. This could be explained by thecompressiveeffectovertheshouldersandchestwidth producedby the extended arms and may

be

one

of

the

main

determining

factors

associated

toareduceddraginthesebodypostures.Thus,itseems possible to stress that, afterbreaststrokestarts and turns, the first gliding position isbiomechanically preferable compared to thesecond one. For this reason swimmers andcoachesshouldallowittoprevail,andshouldalsostresstheneedofbodypositioncontrolduringitsexecution (VilasBoas, 2010). This aim shouldbeaccomplished to decrease drag and thus toimproveglidingperformance.

Regarding

the

positions

with

the

arms

extended at the front, the lateral positionpresented the lowest values of hydrodynamicdrag.This finding isnotquite simple toexplain.Firstly,thereareascarcenumberofpapersintheliteratureanalysingpassivedragatdifferentbodypositions.Secondly,theuseofatwodimensionalmodel can lead to some misinterpretation of theresults and to not truly correspond to whathappensinrealswimmingconditions.

Similar values between prone and dorsal

positions,

both

with

the

arms

extended

at

the

front, seemed to be expected, since the

hydrodynamic profiles of each position aresimilar. However, in the lateral position, themodellingofatwodimensionalmodelseemedtodetermine theobtained data.Counsilman (1955),just like Jiskoot and Clarys (1975), using

experimental

approaches,

analysed

prone

and

lateral towing positions. Nevertheless, theirfindingswerecontradictoryforspeedslowerthan1.8 m/s. For higher velocities, both studiesreportedhighervaluesofhydrodynamicdragforthelateralposition.Inthecurrentstudythelateralpositionpresentedlowervaluesofdragforallthestudied speeds (1.62.0 m/s). The analysis of thehydrodynamic profile could explain this result.Asonecannote inFigure1,thebody inalateralposition lead to a better hydrodynamic body

position,

with

the

arms

and

hands

at

the

front

assumingadropofwatershape,situation thatdoesnotoccurintheothertwopositionswiththearmsextendedat the front.Thisdropofwatershape seems tobe the one that allows abetterhydrodynamicprofile,reducingpressuregradientaround the body (Vogel, 1994). Indeed, thereferreddropofwatershapeismoresimilartoother hydrodynamic shapes contributing to asmoother water flow around the human body(Massey,1989).Dropofwatershapekeeps the

boundary

layer

more

time

attached

to

the

swimmers body surface. Thus, it delays theseparation to a rear point of thebody surface(Polidorietal.,2006).

The CFD also allowed to quantify thecomponents of hydrodynamic drag and toevaluate the contribution of each one. Thecomputed drag components showed that thepressuredragwasdominantinallbodypositions.However, friction component cannot benegligible,since itrepresented16%,14%,and8%

in

the

lateral

position,

in

the

dorsal

and

prone

positions,andinthepronepositionwiththearmsaside the trunk, respectively. The largercontribution of the friction component in thepositionswiththearmsextendedatthefrontmayberelatedwiththemosthydrodynamicprofileofthis position during the gliding. This positionallowsthebodytobemoreextendedinthewater,favouring the elongated position, which mayallow both an increase of body length andslenderness,whichreducesthepressuredragand

may

increase

friction

drag

(Vogel,

1994).

Another situation could occur if the

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swimmer were at the waters surface (Nicolas &Bideau,2009). In the current study the swimmermodel was placed underwater (0.90 m deep). Inthis sense, we did not consider the wave dragcomponent since Lyttle et al. (1999) concluded

that

there

was

no

significant

wave

drag

contribution immersed at least 0.6 m. Therefore,during an underwater gliding more close to thesurface the contribution of friction componentwouldbereducedduepossiblytothedecreaseofthewettedareaand thegenerationofwavedrag(Bixler et al., 2007). This concern seems torepresentaninterestingideatodevelopinfurtherCFDresearch.

Themainlimitationsofthisstudyinclude:(i)the analysis of a passive drag situation; the

findings

of

this

study

must

be

read

with

caution,

because we only analysed a passive dragsituation. In the future, the development of thismethodologymustconsiderthebodymovementsin the CFD domain, analysing, for instance, the

second partof theglidingwhen the swimmer iskicking, allowing to study the total underwaterphase(Konstantaki&Winter,2007);(ii)theuseofatwodimensionalmodel.Theuseofasimplifiedmodelofthehumanbodydoesnotallowadirect

transfer

of

these

data

into

a

practical

situation.

In

furtherresearch,thisaimshouldbeaccomplishedusing threedimensional models of the humanbody(Bixleretal.,2007).

As a conclusion, one can state that thepositionswiththearmsfullyextendedatthefrontseem to substantially reduce the negativehydrodynamic effects of the human bodymorphology:abodywithvariouspressurepointsdue to the large changes in its shape. Thus, thisposture performed at the lateral position should

be

the

one

adopted

after

the

starts

and

turn

phases of a competitive swimming event. Forinstance, considering the breaststroke turn, thefirstgliding,performedwiththearmsatthefront,must be emphasized in relation to the secondgliding,performedwiththearmsalongthetrunk.

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Correspondingauthor:

DanielA.Marinho

UniversidadedaBeiraInterior,DepartamentodeCinciasdoDesporto.RuaMarqusdvilaeBolama.6201001Covilh,PortugalPhone:+351275329153.

Fax:

+351275320695.

Email:[email protected]

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