Veloso, Alves, Fernandes, Conceição, Vilas-Boas (eds.) Portuguese Journal of Sport Sciences Applied Biomechanics in Sports 11 (Suppl. 3), 2011 MEASUREMENT OF DRAG TO ASSESS THE EFFECT OF SWIM SUITS Huub M. Toussaint1,2,3 Move Institute, Human Movement Sciences, VU University, Amsterdam, The Netherlands1; Academy for Physical Education, Technical University of Applied Sciences Amsterdam 2; InnoSport.NL Fieldlab Swimming, Eindhoven3 The purpose of this study was to demonstrate the effect of different suits on drag for Olympic level swimmers. The effect on drag for some suits were with 5-7% rather large given the small differences observed in competitive time during Olympic finals. It is also found that the same suit may have a drag reducing effect for one swimmer while for the other no or a detrimental effect on drag is observed. Apparently, the simple reasoning to create a level playing field for all swimmers that one suit must be available for all is not as simple as the present FINA rules suggest. KEY WORDS: active drag, passive drag, performance effect. INTRODUCTION: Swimming is a highly competitive sport where the difference between success and failure can be very small. For example, the time difference between ‘gold’ and ‘silver’ for the 100 m freestyle finals for men at the last Olympic games is roughly 0.5%, and between the winner and the slowest finalist less than 3%. Small changes to factors determining performance will have significant effects. One of the factors determining the outcome in a 100 m freestyle race is drag. Drag is the force resisting movement through water. The total drag (Fd) swimming at a constant speed consists of frictional (Ff), pressure (Fp), and wave drag (Fw) components, namely (Toussaint & Beek, 1992): Fd = Ff + Fp + Fw (1) In different phases of the race the components of drag that dominate total drag may vary as they depend on speed and swimming depth. After the start, the swimmer dives into the water and below a certain depth, wave drag may be negligible [insert animal boil] leaving pressure and friction drag as relevant components. Frictional or viscous drag originates from fluid viscosity, and produces shear stresses in the boundary layer (a layer of water extending out from the body to the point at which it is moving at 99% of free stream speed; (Prandtl & Tietjens, 1957)). The magnitude of frictional drag will depend on the wetted surface area of the body and flow conditions within the boundary layer (Webb, 1975). The basic causes of pressure drag are vortices and this type of drag depends on the difference between the dynamic pressures at the front and rear of a moving body and the exposed area to flow (Aleyev, 1977). When swimming near the surface, the pressure field around the swimmer sets up a wave system. At race speeds exceeding about 1.6 m•s-1 wave drag will be important (Vennel, Pease, & Wilson, 2006). It was estimated that wave drag amounts to 50% of total drag (Toussaint & Truijens, 2005). Active drag versus passive drag: Drag can be measured in a passive and active condition. Passive drag is the resistance on the body of the passive swimmer, when towed through the water. The relevance of measuring passive drag is to evaluate drag as occurs during the gliding phase of swimming after start and turns. This is relevant since about 15% of the distance covered in a race occurs in a passive state (Bixler, Pease, & Fairhurst, 2007). Active drag is the resistance on the body of the swimmer when propulsion is generated. This form of drag would be more closely related to changes in body position and may thus be influenced by swimming technique (Chatard, Bourgoin, & Lacour, 1990). Another factor that may influence the magnitude of both active and passive drag is the swimming suit as was first demonstrated by Toussaint et al in 1989. In the present study the effect of different suits on both passive and active drag is studied. ISBS 2011 77 Porto, Portugal Veloso, Alves, Fernandes, Conceição, Vilas-Boas (eds.) Portuguese Journal of Sport Sciences Applied Biomechanics in Sports 11 (Suppl. 3), 2011 METHODS:METHODS:TheThe measurement measurement of ofdrag drag while while swimming swimming front front crawl crawl (i.e. (i.e.‘active’ ‘active’ drag) drag) is a is a challenge.challenge. Unlike Unlike activities activities on on land land (like (like running) running) the theswimmer swimmer is not is usingnot using a fixed a fixedpoint pointto to generategenerate propulsion. propulsion. If Ifthere there would would be bea fixeda fixed point, point, it would it would be thebe idealthe ideal spot spotto put to a put force a force transducertransducer to to measure measure the the forces forces involved involved in swimming.in swimming. The The Measuring Measuring Active Active Drag-system Drag-system (MAD-system).(MAD-system). The The MAD-system MAD-system provides provides the the swimmer swimmer with with a series a series of fixed of fixed push-off push-off points points mountedmounted below below the the water water surface, surface, such such that that a front- a front-crawlcrawl like like‘swimming’ ‘swimming’ movement movement can becan be mademade (Figure (Figure 1). 1). The The push-off push-off forces forces from from the the hands hands are aremeasured measured with witha force a force transducer. transducer. If If thethe swimmer swimmer ‘swims’ ‘swims’ at atconstant constant speed speed the the average average drag drag will willequal equal the averagethe average propulsion. propulsion. ThusThus thethe MAD-system MAD-system approach approach relies relies on ona balancea balance of resistiveof resistive and andpropulsive propulsive forces. forces. PropulsivePropulsive forces forces of of the the leg leg action action can can not not be bemeasured measured using using this thisapproach, approach, so the so legs the arelegs are tiedtied togethertogether with with a arubber rubber strap strap and and supported supported by bya pulla pullbuoy buoy to keepto keep the bodythe bodyin a in a horizontalhorizontal position position similar similar to tothat that during during actual actual swimming. swimming. The The swimmer swimmer swims swims a series a series of of lapslaps on on the the system system whereby whereby each each lap lap is swumis swum at aat constant a constant speed. speed. Each Each lap resultslap results in one in one speed-dragspeed-drag datadata point. point. For For a arange range of oflap-speeds, lap-speeds, drag drag is measuredis measured and andthe relationthe relation betweenbetween speed speed and and drag drag is iscalculated calculated using using a leasta least squares squares fitting fitting approach approach (see (seealso: also: Toussaint,Toussaint, et et al., al., 1988; 1988; Toussaint, Toussaint, de deLooze, Looze, van van Rossem, Rossem, Leijdekkers, Leijdekkers, & Dignum, & Dignum, 1990; 1990; Toussaint,Toussaint, Knops, Knops, de de Groot, Groot, & &Hollander, Hollander, 1990). 1990). Measurement Measurement of drag of drag of a ofswimmer a swimmer in a in a passive,passive, stretched stretched position position (i.e. (i.e. ‘passive’ ‘passive’ drag) drag) is reislatively relatively easy. easy. An exampleAn example of such of sucha a contraptioncontraption to to do do so so is isgiven given in inFigure Figure 1 (right 1 (right panel). panel). Figure 2: Results of passive drag testing using towing device (see Figure 1 right panel). Each swimmer was towed three times at each speed and the average recorded drag value for each lap is plotted. The relation ship between speed and drag was least square fitted to give Drag = A•velocityn. The testing speed was between 1.8 and 3 m•s-1. The high speeds were set to evaluate the effect of the suit after start and turns were the swimmer is gliding through the water. FigureFigure 1: 1: Schematic Schematic drawing drawing of ofthe the MAD-system MAD-system (left (left panel) panel) mounted mounted in a 25in am 25 pool. m pool. The MAD- The MAD- systemsystem allows allows the the swimmer swimmer to topush push off off from from fixed fixed pads pads with with each each stroke. stroke. These These push-off push-off pads pads areare attachedattached to to a a22 22 m mlong long rod. rod. The The distance distance between between the thepush-off push-off pads pads can becan adjusted be adjusted (normally(normally 1.35 1.35 m). m). The The rod rod is ismounted mounted ± 0.8 ± 0.8 m belowm below the thewater water surface. surface. The rodThe is rod connected is connected to to aa force force transducer transducer enabling enabling direct direct measurement measurementof pushof push-off -forcesoff forces for eachfor each stroke stroke (see lower(see lower panel).panel). Swimming Swimming one one lap lap on on the the system system yields yields one one data data-point-point for thefor speedthe speed-drag--dragcurve.-curve. (note: (note: thethe cordcord leading leading to to the the calibration calibration device device is detachedis detached during during drag drag-measurement)-measurement). Towing. Towing devicedevice for for passive passive drag drag measurem measurementent (right (right panel). panel). A force A force transducer transducer is mounted is mounted in the in yellow the yellow buoy.buoy. RESULTSRESULTS: :Tests Tests were were conducted conducted at atthe the end end of Mayof May 2008. 2008. For Foreach each swimmer swimmer 2 suits 2 suitswere were evaluatedevaluated to to decide decide what what to towear wear during during the the finals finals in the in theOlympic Olympic Games. Games. It should It should be noted be noted thatthat for for s ubjectsubject ‘Maarten’ ‘Maarten’ the the ‘LZR ‘LZR racer racer legskin’ legskin’ was was tested tested and andthat thatthis suitthis onlysuit onlycovered covered Figure 3: Results for active drag using the MAD-system (see Figure 1 left panel).