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Backstroke start kinematic and kinetic changes due todifferent feet positioningKarla de Jesusa, Kelly de Jesusa, Pedro Figueiredoa, Pedro Gonçalvesab, Suzana MatheusPereirac, João Paulo Vilas-Boasab & Ricardo Jorge Fernandesab

a University of Porto, Faculty of Sport, Centre of Research, Education, Innovation andIntervention in Sport, Porto, Portugalb Porto Biomechanics Laboratory, Porto, Portugalc University of the State of Santa Catarina, Health and Sports Science Centre, Florianópolis,BrazilPublished online: 20 May 2013.

To cite this article: Karla de Jesus, Kelly de Jesus, Pedro Figueiredo, Pedro Gonçalves, Suzana Matheus Pereira, João PauloVilas-Boas & Ricardo Jorge Fernandes (2013) Backstroke start kinematic and kinetic changes due to different feet positioning,Journal of Sports Sciences, 31:15, 1665-1675, DOI: 10.1080/02640414.2013.794298

To link to this article: http://dx.doi.org/10.1080/02640414.2013.794298

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Backstroke start kinematic and kinetic changes due to different feetpositioning

KARLA DE JESUS1, KELLY DE JESUS1, PEDRO FIGUEIREDO1, PEDRO GONÇALVES1,2,SUZANA MATHEUS PEREIRA3, JOÃO PAULO VILAS-BOAS1,2, & RICARDO JORGEFERNANDES1,2

1University of Porto, Faculty of Sport, Centre of Research, Education, Innovation and Intervention in Sport, Porto, Portugal,2Porto Biomechanics Laboratory, Porto, Portugal, and 3University of the State of Santa Catarina, Health and Sports ScienceCentre, Florianópolis, Brazil

(Accepted 5 April 2013)

AbstractThe backstroke swimming start international rules changed in 2005. This study compared two backstroke start variants,both with feet parallel to each other but in complete immersion and emersion. Six elite swimmers performed two sets of 4maximal 15 m bouts, each set using one of the variants. The starts were videotaped in the sagittal plane with two cameras,providing bi-dimensional dual-media kinematic evaluation, and an underwater force plate and a handgrip instrumented witha load cell collected kinetic data. Backstroke start with feet immerged displayed greater centre-of-mass horizontal startingposition, centre-of-mass horizontal velocity at hands-off and take-off angle. Backstroke start with feet emerged showedgreater wall contact time, centre-of-mass horizontal and downward vertical velocity at take-off, lower limbs horizontalimpulse, and centre-of-mass downward vertical velocity during flight phase. Backstroke start with feet immerged andemerged displayed similar centre-of-mass horizontal water reach, back arc angle and 5 m starting time. Irrespective of theswimmer’s feet positioning, coaches should emphasise each variant’s mechanical advantages during the wall contact phases.Furthermore, the maintenance of those advantages throughout the flight should be stressed for better backstroke startperformance.

Keywords: biomechanics, elite performance, swimming, dorsal start

Introduction

The final outcome of a swimming event is the sum ofthe starting, stroking and turning times and, as worldrecords continue to be broken, each part assumes acritical importance. The swimming start time (untilthe 15 m mark) is unanimously accepted as a deter-minant for success, contributing up to 30% of theoverall backstroke race time in short distance events(Lyttle & Benjanuvatra, 2005), and its importance isfurther emphasised by the small differences betweenthe individual performances of high level swimmers(Elipot, Dietrich, Hellard, & Houel, 2010). In fact,at the latest Shanghai 2011 Long Course SwimmingWorld Championship, two swimmers tied in the firstposition of the men’s 100 m backstroke, and only0.11 s and 0.22 s separated the first place from theworld record in the women’s and men’s 100 m back-stroke respectively, at the London Olympic Games

(in 2012), which highlights the importance of per-fecting the start for better performance.

Over the last 40 years, the backstroke swimmingstart has evolved from standing (feet above thewater surface fixed on the wall or on the gutter)(Stratten, 1970) to traditional (feet fully immerged)(Hohmann, Fehr, Kirsten, & Krüger, 2008; Theuth& Jensen, 2006) starting positions. Comparison ofthese two backstroke starts showed that the standingvariant was faster until the 6.09 m mark (Stratten,1970). Analysing the traditional variant, Hohmannet al. (2008) reported high correlations betweenhands-off (starting signal to hands left the handgrips)and take-off time (starting signal to feet wall release),hands-off and flight time (feet wall release to hipimmersion), and resultant peak force just before thetake-off and 7.5 m starting time. Theuth and Jensen(2006) also analysed the traditional variant and high-lighted the inexistence of kinematic advantages for

Correspondence: Ricardo Jorge Fernandes, University of Porto, Faculty of Sport, Centre of Research, Education, Innovation and Intervention in Sport andPorto Biomechanics Laboratory, Porto, Portugal. E-mail: ricfer@fade.up.pt

Journal of Sports Sciences, 2013Vol. 31, No. 15, 1665–1675, http://dx.doi.org/10.1080/02640414.2013.794298

© 2013 Taylor & Francis

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5 m backstroke starting performance between feetstaggered positioned and parallel.

Despite previous findings, the backstroke swim-ming start has not been given enough attention bythe scientific community – unlike ventral starts(Blanksby, Nicholson, & Elliot, 2002; Galbraith,Scurr, Hencken, Wood, & Graham-Smith, 2008;Seifert et al., 2009; Vantorre, Seifert, Fernandes,Vilas-Boas, & Chollet, 2010a, 2010b; Vilas-Boaset al., 2003) – and most of the studies are eitherobsolete or limited, as Federation Internationale deNatation (FINA) rules changed. In fact, nowadays itis possible to use different backstroke start technicalsolutions, particularly by placing swimmer’s feet onthe wall partially or entirely above the water surface(SW 6.1, FINA, 2012), and the use of the gutter forbetter support is not allowed. These changes datefrom 2005 and only de Jesus et al. (2011) consideredthem when trying to identify the biomechanical para-meters responsible for a fastest backstroke startingtime under different feet positioning. According tothese authors a greater take-off horizontal impulsedecreases the 5 m starting time at the variant withfeet fully immerged, and a greater centre-of-masshorizontal position at the pool-wall implies a flattertake-off angle and compromises performance at thevariant with feet fully emerged. Moreover, they con-sidered the variant performed with feet fully emergedas a more complex technical solution, requiring spe-cial attention during training sessions.

The reason for the recent FINA backstroke startrule modification may be attributed to the allegedmechanical advantages noticed when the use of thegutter was allowed, that the feet positioned above thewater surface provided greater take-off angle, archedback posture and horizontal water reach (Maglischo,2003; Stratten, 1970). In fact, the variant performedwith feet fully emerged might reduce the body watercontact at the wall set position and keep the swim-mers’ lower limbs out of the water during most ofthe aerial trajectory (Maglischo, 2003). However, thefeet fully positioned above the water surface basedon the recent FINA rules might increase feet wallcontact area and consequently increase the horizon-tal impulse. According to previous start studies theswimmers should leave the block/wall quickly gener-ating as much upper (Breed & Young, 2003; deJesus et al., 2011; Hay, Guimarães, & Grimstron,1983) and lower limbs horizontal impulse (Breed &Young, 2003; de Jesus et al., 2011; Vantorre et al.,2010a) and gaining as much horizontal take-off velo-city as possible (Hohmann et al., 2008; Takeda,Ichikawa, Takagi, & Tsubakimoto, 2009). Despitethis, upward take-off velocity and favourable take-off angle are also requirements considered to attaina maximal horizontal water reach (Galbraith et al.,2008; Seifert et al., 2009), and to minimise the

horizontal velocity decrease after water immersion(Thow, Naemi, & Sanders, 2012; Vantorre et al.,2010b); mainly when swimmers have started in thewater (Maglischo, 2003).

Having in mind the paucity of detailed biomechani-cal studies on contemporary backstroke start variants,and knowing that feet placement strategies when con-tacting the pool wall (e.g. above or below water sur-face) may determine the magnitude and direction ofthe centre-of-mass translational motion, the aim of thepresent study was to conduct a kinematic and kineticcomparison of two backstroke start variants: feet par-allel and fully immerged and emerged. It was hypothe-sised that the difference in feet positioning wouldsignificantly affect the kinematics and kinetics of thebackstroke start technique, since the variant with feetimmerged may imply greater upward take-off velocity,increasing horizontal water reach during flight andreducing the starting time, while the variant with feetemerged may lead to a greater horizontal impulse andsmaller take-off angle. The kinematic and kinetic ana-lysis can offer greater insight into a better understand-ing of the changes in the backstroke start according todifferent feet positioning.

Methods

Participants

Six high-level male swimmers (mean ± s: age22.5 ± 2.94 years, stature 1.80 ± 0.07 m, bodymass 76.6 ± 8.94 kg, training background13.3 ± 5.46 years, and a personal best of87.58 ± 2.58% of the 100 m backstroke long courseWorld Record) volunteered to participate.Swimmers were proficient in both backstroke startvariants, feet parallel and fully immerged andemerged, since they usually swim backstroke intraining practice and in competition, and they hada period of feet placement training previously to thetests. The protocol, after approval by the UniversityEthics Committee, was explained to the swimmers,who gave their written consent.

Test protocol

In a randomised order, swimmers performed twosets of 4 × 15 m maximal intensity backstroke starts,each set using one of the two variants that differedaccording to the feet position: (i) feet immerged(Figure 1a), with the hallux used as reference (posi-tioned just below water level); (ii) feet emerged(Figure 1b), performed with the whole feet placedabove the water level. In both conditions feet wereparallel and positioned against a fixed underwaterextensometric force plate. Resting periods of 120 sand 1 h were provided between repetitions and sets,

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respectively. Between the testing sessions waterdepth was lowered (or increased) by 0.10 m, allow-ing the feet to be positioned always over the fixedunderwater force plate, either immersed or emerged.The mean value of the 4 trials for each swimmer ineach variant was calculated and then, the medianvalue of the 6 swimmers in each variant was usedin subsequent statistical analysis (6 pairs tested).

Data collection

Swimmers were videotaped in the sagittal plane for bi-dimensional kinematical analysis using two video cam-eras, one underwater (DCR-HC42E, Sony®, Tokyo,Japan) and one above the water surface (SVHS-JVCGR-SX1, JVC Kenwood Corp., Tokyo, Japan).Cameras operated at a 50 Hz frequency, with 1/250 sshutter speed, and were fixed on a specially designedsupport for “dual media” video image recording (deJesus et al., 2011). This support was placed at thelateral wall of the pool, 2.5 m from the head wall,with one camera placed 0.3 m above the water surfaceand the other kept underwater in a waterproof housing(Ikelite, Ikelite Underwater Systems Corp.,Indianapolis, USA) at a depth of 0.3 m, exactlybelow the surface camera (both placed at 6.8 m fromthe movement plane). Both cameras’ images wererecorded independently and swimmers were moni-tored when passing through a specific pre-calibratedspace using a calibration frame (2.1 × 3.0 m).Participants wore a complete and standardised swimsuit, with spherical anatomical markers on the trunk,upper and lower right limbs. Images synchronisationwas obtained using a pair of lights, fixed to the calibra-tion volume, visible in the each camera field of view.

Kinetic assessment was conducted using anunderwater extensometric force plate (de Jesuset al., 2011) with a surface of 0.5 × 0.5 m, sensitivityof 2 N, error <1% and natural frequency of 60 Hz,mounted on a specially built support fixed to the

pool wall, sampling at 1000 Hz. The force platewas connected to an analogue-to-digital converter(Biopac, Biopac Systems Inc., California, USA).An iron T-bar handgrip, instrumented with a loadcell (Globus Aba Ergo Meter, Globus Corp.,Codogne, Italy), sampling at 100 Hz, was fixed tothe starting block and connected to an analogue-to-digital converter (Globus Aba Ergo Meter, GlobusCorp., Codogne, Italy) that exported the horizontalforce-time curve data to a PC. The handgrip systemwas adapted to comply with the FR 2.7 rules (FINA,2012), stating that the handgrips for the backstrokestart shall be placed within 0.3 to 0.6 m above thewater surface both horizontally and vertically. Thisallowed keeping the same handgrip elevation regard-ing water surface in both force plate conditions.

Starting signals were produced through a starterdevice (ProStart, Colorado Time SystemsCorporation, Colorado, USA), programmed andinstrumented to simultaneously produce the startingsignal and to export a light signal to the video system,and a trigger signal to the analogue-to-digital converter(Biopac, Biopac Systems Inc., California, USA) allow-ing data synchronisation. Load cell output was back-wards synchronised through video images from theinstant of “hands-off”.

Data analysis

The independent digitisation from each camera viewwas reconstructed using the calibration volume (withsix control points in total). The separate video imageswere digitised manually and frame by frame usingAPASystem (Ariel Dynamics Inc., San Diego, USA).The anthropometric biomechanical model used(Zatsiorsky & Seluyanov, 1983, with an adaptation byde Leva, 1996) had 13 anatomical markers – head’svertex, right ear lobe, acromion, lateral humeral epi-condyle, ulnar styloid process, third distal phalanx,fifth rib, iliac crest, prominence of great femoral tro-chanter, lateral femoral epicondyle, lateral malleolus,calcaneus and hallux – and the trunk was divided intothree articulated parts: (i) superior, from acromion tofifth rib; (ii) medium, from fifth rib to iliac crest, and;(iii) inferior, from iliac crest to prominence of greatfemoral trochanter. Image coordinates were trans-formed to bi-dimensional object-space coordinatesusing the Direct Linear Transformation algorithm(Abdel-Aziz & Karara, 1971). After residual analysisfor a wide range of cut-off frequencies, 5 Hz wasselected as the optimal cut-off frequency for the rawdata smoothing using a low-pass digital filter. Rootmean square reconstruction errors of six validationpoints on the calibration frame, which did not serveas control points, were (respectively for horizontal andvertical axes): (i) 3.79 and 2.39 mm, representing 0.18and 0.23% of the calibrated space for above water, and

Figure 1. The backstroke start variants in preparatory positionbefore the starting signal. The backstroke start variant with feetparallel and totally immerged (a). The backstroke start variantwith feet parallel and totally emerged (b).

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(ii) 4.78 and 6.01 mm, representing 0.22 and 0.30%of the calibrated space for underwater.

The backstroke start was divided into three phases(cf. de Jesus et al., 2011): (i) hands-off, between thestarting signal and the frame the hands left the hand-grips; (ii) take-off, from the hands-off until the framethe feet left the force plate; and (iii) flight, betweenthe take-off until the frame the fingertip contactedthe water surface.

Several linear and angular kinematic parameterswere determined: (i) the centre-of-mass horizontaland vertical position at the starting signal; (ii) thecentre-of-mass horizontal and vertical velocity at thehands-off instant; (iii) wall contact time, from start-ing signal and the time the feet left the force plate;(iv) the centre-of-mass horizontal and vertical velo-city at the take-off instant; (v) take-off angle, mea-sured between the lateral femoral epicondyle andlateral malleolus with the horizontal axis; (vi) thecentre-of-mass horizontal and vertical velocity dur-ing flight phase; (vii) the back arc angle, determinedamong the superior trunk, thigh and horizontal axisat the fingertip water contact; (viii) the centre-of-mass horizontal water reach, as the difference ofhorizontal centre-of-mass coordinates between start-ing wall and the fingertip water contact; and (ix)starting time - from the starting signal to the instantfingertips reached the 5 m mark.

To determine the accuracy of the digitising proce-dure, two repeated digitisations of a randomly selectedtrial were performed, and the coefficients of repeatabil-ity with limits of agreement (95%) were calculatedusing Bland and Altman (1986) method for each vari-able of interest and were described in Table I.

The upper limbs horizontal force-time curves (per-pendicular to the handgrip) were analysed from thestarting signal until the swimmer’s hands-off. Thelower limbs horizontal force-time curves (perpendicu-lar to the force-plate) were analysed from the firstreaction, defined as the first instant before the firstpeak value in which force corresponds to 25% of therespective first peak, until the swimmer’s feet take-off.The low-pass Butterworth was used to filter the upperand lower limbs force-time curves (fourth-order,10 Hz and 100 Hz, respectively); and all data werenormalised to the swimmer’s body weight (BW) usinga custom-designed software program (MatLabR2007a, MathWorks Inc., Natick, MA, USA). Thelower limbs normalised force from the first reactionto the swimmer’s feet take-off was calculated. Thenormalised impulse was assessed from the upper andlower limbs force-time curves as the time integral ofthe horizontal force component during the hands-offphase and from the first reaction to the swimmer’s feettake-off, respectively. Each individual upper and lowerlimbs horizontal force-time curve for each backstrokestart variant was normalised in time from the starting

signal until the hands-off instant, and from the firstreaction until the swimmer’s feet take-off, respectively,using a customised module. Critical time referenceinstants were characterised in each individual upperand lower limbs force-time curve allowing the assess-ment of the following variables: (i) first upper limbsmaximal force and time during hands-off phase; (ii)second upper limbs maximal force and time duringhands-off phase; (iii) lower limbs force at first reaction;(iv) lower limbs maximal force and time during thehands-off phase; (v) lower limbs force and time at theinstant of hands-off and; (vi) lower limbs maximalforce and time during take-off phase. The meanvalue of force and time for each critical instant wascalculated from the four upper limbs force-time curvesfor each swimmer in each variant and then, the medianvalue of the six swimmers in each variant was calcu-lated and used in subsequent statistical analysis. Thisprocedure was also performed for each critical instantof the lower limbs force-time curves. The four upperlimbs force-time curves of each swimmer in each var-iant were averaged, and then, these averaged valueswere used to calculate a force-time curve representa-tive of each variant (ensemble average of the six swim-mers). This procedure was also performed for thelower limbs force-time curves.

Table I. Coefficient of repeatability and respective limits ofagreements (95%) for all centre of mass (CM) and angularvariables of interest at hands-off (HO), take-off (TO) and flight(FL) phases.

Variables (units)Coefficient ofrepeatability

Limits of agreement(95%)

CM horizontalposition atsignal (m)

0.0013 [–0.0011 to 0.0015]

CM verticalposition atsignal (m)

0.0007 [–0.0006 to 0.0009]

CM horizontalvelocity atHO (m ∙ s–1)

0.0051 [–0.0048 to 0.0053]

CM vertical velocityat HO (m ∙ s–1)

0.0019 [–0.0009 to 0.0020]

CM horizontalvelocity atTO (m ∙ s–1)

0.0496 [–0.0368 to 0.0624]

CM vertical velocityat TO (m ∙ s–1)

0.0196 [–0.0192 to 0.0200]

CM horizontalvelocity duringFL (m ∙ s–1)

0.0098 [–0.0096 to 0.0101]

CM vertical velocityduringFL (m ∙ s–1)

0.0178 [–0.0149 to 0.0207]

Take-off angle (º) 1.5068 [–1.4872 to 1.5264]Back arc angle (º) 5.6271 [–4.5342 to 6.7106]CM horizontal

water reach (m)0.0137 [–0.0169 to 0.0105]

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Statistical procedures

The normality of distribution was checked for allvariables using the Shapiro-Wilk test before com-parative analysis and a normal distribution was notcompletely confirmed. The variables were presentedas median and interquartile range (Q1–Q3), whichare rather suitable values of central tendency anddispersion respectively, for non-parametric data.The upper and lower limbs horizontal force-timecurve was reported as mean (± s). The Wilcoxonsigned-rank test was selected to determine the effectscaused by the independent variable (backstroke startvariant with feet immerged versus emerged) on thedependent variables (i.e., kinematic and kinetic). Allstatistical procedures were conducted with IBM®SPSS® Statistics system 20 and an exact P-value ≤ 0.05 was accepted to define statistical signif-icance. The effect size (r) for each variable was cal-culated in accordance with Cohen (1988) tomeasure the magnitude of differences between thevariant with immerged and emerged. The criteria forinterpreting the absolute effect size were based onCohen’s (1988) suggestion, considering a trivialeffect size if 0 ≤ r ≤ 0.09, small if 0.1 ≤ r <0.3,medium if 0.3 ≤ r < 0.5, large if 0.5 ≤ r ≤ 0.69, andvery large if r ≥ 0.7.

Results

Table II presents the median and respective inter-quartile range (Q1–Q3) of each kinematic and upperand lower limbs kinetic parameter during the hands-off phase, with the exact P-value and effect size (r)reported for the comparisons between the backstrokestart variant with feet immerged and emerged.

Two of the four kinematic parameters analysedduring the hands-off phase differed between the stu-died backstroke start variants. For the centre-of-mass position at the starting signal, backstroke startwith feet immerged showed greater horizontal valuewith large effect size, and the variants did not differfor vertical position with moderate effect size.Regarding the centre-of-mass velocity at the hands-off instant, backstroke start with feet immergedincreased the horizontal value with large effect size,and similar upward vertical velocity was observed forboth start variants, feet immerged and emerged, withtrivial effect size. The median value for the upperlimbs horizontal impulse of both variants did notdiffer with small magnitude of effect size. For theupper limbs force-time curve critical instants, thefirst upper limbs maximal force during the hands-off phase did not differ between variant with feetimmerged and emerged with moderate effect size;although variant with feet immerged achieved thisfirst peak value in shorter time than variant withfeet emerged with very large effect size. Backstrokestart variant with feet immerged displayed greatersecond upper limbs maximal force during thehands-off phase and longer time to achieve thispeak value, both with very large effect size. InFigure 2, the mean upper limbs horizontal force-time curves and respective standard deviations, andthe critical instants analysed for variant with feetimmerged and emerged are displayed, and a dou-ble-peak profile, representative of the head liftingand maximal upper limbs extension, is observed forboth variants.

For the lower limbs force-time curve critical instants,the variant with feet emerged displayed greater force atfirst reaction, maximal force and time during hands-off

Table II. Median and respective interquartile range (Q1 and Q3) of each kinematic and upper and lower limbs kinetic parameter during thehands-off (HO) phase, with the exact P-value and effect size (r) reported for the comparisons of the backstroke start variant with feetimmerged (BSFI) and emerged (BSFE). Force data are presented as a fraction of the swimmer’s body weight (BW).

Variables BSFI BSFE P-value Effect size (r)

CM horizontal position at signal (m) 0.57 (0.45–0.61) 0.39 (0.34–0.43) 0.01* −0.63CM vertical position at signal (m) 0.11 (0.05–0.13) 0.15 (0.07–0.21) 0.15 −0.33CM horizontal velocity at HO

instant (m ∙ s–1)1.69 (1.45–2.16) 1.14 (1.04–1.59) 0.01* −0.63

CM vertical velocity at HO instant (m ∙ s–1) 0.68 (0.36–0.82) 0.60 (0.46–0.81) 0.50 −0.03Upper limbs horizontal impulse ((N/BW) · s) 0.21 (0.19–0.23) 0.20 (0.17–0.22) 0.34 −0.15First upper limbs maximal force during HO phase (N/BW) 0.47 (0.34–0.53) 0.41 (0.22–0.55) 0.18 −0.37Time of first upper limbs maximal force during HO phase (%) 38.0 (36.5–39.0) 41.0 (40.1–42.4) 0.001* −1.04Second upper limbs maximal force during HO phase (N/BW) 0.70 (0.66–0.79) 0.45 (0.35–0.62) 0.001* −0.96Time of second upper limbs maximal force during HO phase (%) 86.0 (84.0–88.5) 78.0 (73.2–79.3) 0.001* −1.03Lower limbs force at first reaction (N/BW) 0.48 (0.39–0.53) 0.72 (0.58–0.82) 0.01* −0.63Lower limbs maximal force during HO phase (N/BW) 1.53 (1.28–1.78) 1.73 (1.63–1.78) 0.03* −0.57Time of lower limbs maximal force during HO phase (%) 33.63 (30.69–37.06) 41.50 (36.50–44.87) 0.01* −0.63Lower limbs force at the instant of HO (N/BW) 1.23 (1.10–1.28) 1.17 (0.98–1.24) 0.07 −0.45Time of lower limbs force at the instant of HO (%) 61.27 (60.0–65.06) 65.85 (61.50–73.75) 0.04* −0.58

Note: Centre of mass (CM). * Significant at P ≤ 0.05.

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phase and longer time to reach the force at instant ofhands-off, all variables with a large magnitude of effectsize. Backstroke start variants with feet immerged andemerged displayed similar force at the instant of hands-off with moderate magnitude of effect size. The meanlower limbs horizontal force-time curves and respectivestandard deviations, and the critical instants analysedfor both variants are displayed in Figure 3. Backstrokestart variants with feet immerged and emerged showeda double-peak force profile.

Table III presents the median and respective inter-quartile range (Q1 and Q3) of each kinematic andlower limbs kinetic parameter during the take-off

phase and the lower limbs kinetics from the firstreaction to the swimmer’s feet take-off, with theexact P-value and effect size (r) reported for thecomparisons between the variant with feet immergedand emerged.

Backstroke start variant with feet emerged increasedwall contact time, and centre-of-mass horizontal anddownward vertical velocity at take-off, all with largeeffect size. The backstroke start variant with feetimmerged displayed greater take-off angle with largeeffect size. Backstroke start variants with feetimmerged and emerged did not differ for the lowerlimbs horizontal force with moderate effect size, and

Figure 2. Mean upper limbs horizontal force-time curves of the six swimmers for backstroke start with feet immerged (continuous line) andemerged (dashed line), expressed as a percentage of hands-off (HO) phase. The vertical dotted and continuous parallel lines denote thebackstroke start with feet immerged and emerged standard deviations, respectively. The stick figures illustrate the position at the startingsignal, first upper limbs maximal force during hands-off phase, trunk lifting and second upper limbs maximal force during hands-off phase.Force data are presented as a fraction of the swimmer’s body weight (BW).

Figure 3. Mean lower limbs horizontal force-time curves of the six swimmers for backstroke start with feet immerged (continuous line) andemerged (dashed line), expressed as a percentage of the first force reaction to the hands-off (HO) instant and the take-off (TO) phasecombined. The vertical dotted and continuous parallel lines denote the backstroke start with feet immerged and emerged standarddeviations, respectively. The time between the first force reaction and the hands-off instant, and the take-off phase are identified verticallywith continuous (backstroke start with feet immerged) and dashed lines (backstroke start with feet emerged). The four critical instants arealso represented and illustrated with stick figures. Force data are presented as a fraction of the swimmer’s body weight (BW).

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the variant with feet emerged increased the lower limbshorizontal impulse with large magnitude of effect size.For the lower limbs force-time curve critical instants,the variant with feet immerged showed greater max-imal force and shorter time to reach the respectivepeak value during the take-off phase with large andmoderate effect size, respectively. The lower limbsforce-time curve critical instants analysed for both var-iants are displayed in Figure 3.

Table IV presents the median and respective inter-quartile range (Q1 and Q3) of each kinematic para-meter during the flight phase and the 5 m startingtime, with the exact P-value and effect size (r)reported for the comparisons between the backstrokestart variant with feet immerged and emerged.

Regarding the centre-of-mass velocities during theflight phase, the two variants did not differ for hor-izontal value with moderate effect size; although thevariant with feet emerged increased downward ver-tical velocity with large effect size. The backstrokestart variant with feet immerged and emerged didnot affect the back arc angle, centre-of-mass hori-zontal water reach and 5 m starting time with small,moderate and small effect size, respectively.

Discussion

As swimmers are no longer required to position theirfeet fully under the water surface during the

backstroke start and no previous study has analysedthe kinematic and kinetic effects of different feetpositions based on the new FINA rules, comparingthe backstroke start variant with feet immerged andemerged is actual and pertinent, as it provides objec-tive and relevant indicators for coaches and swim-mers. Results did not confirm the hypotheses thatwhen starting with feet fully positioned below thewater surface a greater upward take-off velocitywould be observed, increasing centre-of-mass hori-zontal water reach and leading to a shorter startingtime. Conversely, as hypothesised, when assuming ahigher feet position, greater horizontal impulse andsmaller take-off angle were observed. The currentresults showed several kinematic and kinetic differ-ences between the studied start variants, althoughwhen considering the overall starting performance(time to 5 m) the backstroke start variant with feetimmerged and emerged were similar.

The primary effect of different feet placement wasobserved in the centre-of-mass horizontal position atthe starting signal, which was greater for the variantwith feet immerged. This finding indicates thatswimmers starting with feet immerged were able toposition their centre-of-mass further away from thepool wall, which might be due to the smaller feet wallcontact area (Maglischo, 2003). Previous ventralstart studies reported that the most forward set posi-tion from the starting block front edge leads to a

Table III. Median and respective interquartile range (Q1 and Q3) of each kinematic and lower limbs kinetic parameter during the take-off(TO) phase and lower limbs kinetics from the first reaction to the swimmer’s feet take-off, with the exact P-value and effect size (r) reportedfor the comparisons of the backstroke start variant with feet immerged (BSFI) and emerged (BSFE). Kinetic data are presented as a fractionof the swimmer’s body weight (BW).

Variables BSFI BSFE P-value Effect size (r)

Wall contact time (s) 0.76 (0.75–0.78) 0.82 (0.77–0.91) 0.03* −0.57CM horizontal velocity at TO instant (m ∙ s–1) 3.29 (3.01–3.58) 3.80 (3.42–4.10) 0.04* −0.56CM vertical velocity at TO instant (m ∙ s–1) −0.03 (–0.42–0.28) −0.40 (–1.29–0.27) 0.03* −0.57Take-off angle (º) 22.82 (12.59–30.10) 14.85 (7.97–26.15) 0.01* −0.63Lower limbs horizontal force (N/BW) 1.12 (0.97–1.24) 1.22 (1.10–1.29) 0.07 −0.45Lower limbs horizontal impulse ((N/BW) · s) 0.55 (0.51–0.57) 0.69 (0.67–0.75) 0.01* −0.63Lower limbs maximal force during TO phase (N/BW) 1.49 (1.37–1.63) 1.38 (1.26–1.54) 0.04* −0.51Time of lower limbs maximal force during TO phase (%) 85.0 (78.63–88.06) 87.0 (85.75–89.18) 0.04* −0.48

Note: Centre of mass (CM). * Significant at P ≤ 0.05.

Table IV. Median and interquartile range (Q1 and Q3) of each kinematic parameter during the flight (FL) phase, with the exact P-value andeffect size (r) reported for the comparisons of the backstroke start variant with feet immerged (BSFI) and emerged (BSFE).

Variables BSFI BSFE P-value Effect size (r)

CM horizontal velocity during FL phase (m ∙ s–1) 3.31 (2.97–3.38) 3.50 (2.86–3.87) 0.15 −0.33CM vertical velocity during FL phase (m ∙ s–1) −0.66 (–0.77––0.39) −0.89 (–1.28––0.74) 0.01* −0.63Back arc angle (º) 134.85 (128.92–142.87) 144.09 (131.82–149.46) 0.21 −0.27CM horizontal water reach (m) 1.74 (1.59–1.88) 1.58 (1.39–1.77) 0.07 −0.45Starting time (s) 1.96 (1.87–2.28) 2.11 (1.76–2.47) 0.28 −0.21

Note: Centre of mass (CM). * Significant at P ≤ 0.05.

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shorter centre-of-mass acceleration distance,decreasing block time (Blanksby et al., 2002; Vilas-Boas et al., 2003). According to de Jesus et al.(2011), due to a closer centre-of-mass position tothe wall, swimmers starting with the feet emergedspend a longer time in contact with the force plate.Analysing the centre-of-mass vertical position at thestarting signal, results indicated no significant feetpositioning effect, and both variants showed anabove water surface centre-of-mass set positioning.In accordance with Maglischo (2003) and Stratten(1970), to position the body as high out of the wateras possible is considered an elementary mechanicalcharacteristic to optimise backstroke start perfor-mance, especially when starting with feet emerged,to reduce feet wall contact area (cf. Figure 1b) andallow the increase of vertical reaction force. Ourstudy did not measure the vertical force componentand, therefore, the explanations for this topic aremerely speculative, suggesting future studies wouldbe required to draw detailed conclusions.

The centre-of-mass horizontal velocity at hands-off was affected by the different feet placement, andthe variant with feet immerged displayed a greatervalue, which occurs due to a greater centre-of-masshorizontal position at the starting signal, decreasingthe time to displace the centre-of-mass away fromthe wall during the hands-off phase. A previousbackstroke start study has shown that to reduce thetime between the starting signal and the hands-offrelates to the shortest time between the starting sig-nal and the swimmer’s feet take-off (Hohmann et al.,2008), which was also observed in the current study.Regarding the centre-of-mass vertical velocity athands-off, the backstroke start variant with feetimmerged and emerged presented similar values,indicating that independently of the feet positioningbackstrokers raise the centre-of-mass out of thewater during the hands-off phase. According toMaglischo (2003) and Stratten (1970), the upwardvertical velocity is required to determine a mostappropriate aerial trajectory, which is essential tokeep the lower limbs out of the water during mostof the flight. As previously mentioned for ventralstarts, the upward vertical velocity may directly influ-ence start performance (Galbraith et al., 2008; Hayet al., 1983), and has been pointed out as the specificcoaching feedback for the hands-off phase duringbackstroke start (Maglischo, 2003; Stratten, 1970).

The upper limbs kinetics displayed similar back-stroke start variant with feet immerged and emergedhorizontal impulse, and both force-time curves(Figure 2) showed that swimmers pull the handgripsagainst themselves during the overall hands-offphase, which confirms previous backstroke startstudy statements regarding the role played by theupper limbs in driving the centre-of-mass above the

water surface (de Jesus et al., 2011; Hohmann et al.,2008). In opposition, the ventral start studies haveshown the upper limbs contribution to the total hor-izontal impulse (Breed & Young, 2003; Galbraithet al., 2008; Hay et al., 1983). Three-dimensionalupper limbs kinetic analysis is required to bettersupport the idea that upper limbs might contributeto the resultant impulse during backstroke start, andalso to verify possible effects of different feet andhands positioning on other force components.From the double-peak upper limbs force-timecurve profiles of both variants (Figure 2), similarbackstroke start variant with feet immerged andemerged first upper limbs maximal force values dur-ing the hands-off phase were noted, and the variantwith feet immerged achieved this peak force earlierthan the variant with feet emerged, which might bedue to the greater centre-of-mass horizontal positionat the starting signal. Adding to that, the variant withfeet immerged also displayed greater second upperlimbs maximal force during the hands-off phase andachieved this peak force later than the variant withfeet emerged, which indicated that backstrokersstarting with feet parallel and fully immerged wereable to coordinate their upper limbs movements toapply a greater force just before the hands-off.

A more detailed analysis of the lower limbs hor-izontal force-time curves revealed a double-peakprofile for both variants (Figure 3), which corrobo-rates Hohmann et al.’s (2008) findings. Concerningthe lower limbs force-time curve critical instantsduring the hands-off phase, the variant with feetemerged displayed a greater value of force at firstreaction, maximal force and time during hands-offphase, and time to reach the force at the instant ofhands-off, which was expected due to the high andlarge feet wall contact area, increasing centre-of-mass wall approximation, the horizontal force perbody weight and the hands-off phase time. Thebackstroke start variant with feet immerged andemerged showed a notable force decrease from themaximal force observed during the hands-off phaseuntil the instant the swimmer’s hands leave the start-ing grips, although no differences were foundbetween variants for the force at the instant ofhands-off. This force decrease was accompanied bysimilar upward centre-of-mass vertical velocity athands-off instant in both variants, once swimmershave been technically advised to thrust their bodiesup and away from the wall (Maglisho, 2003;Stratten, 1970). From the current results, it mightbe suggested that the hands-off is the referenceinstant for the transition of the upward movementto the most propulsive lower limbs action.

As previously observed for ventral starts (Blanksbyet al., 2002), the centre-of-mass horizontal positionfurther away from the pool wall at the starting signal

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also led to the shorter wall contact time for the variantwith feet immerged. In contrast, the variant with feetemerged displayed greater centre-of-mass horizontaland downward vertical velocity at take-off, smallertake-off angle, similar lower limbs horizontal forceand greater impulse. The results supported thehypotheses that feet positioned fully above the watersurface lead to the greater horizontal impulse and tothe flattest take-off angle. The ventral starts findingsshowed that since swimmers’ centre-of-mass is posi-tioned further behind the block, greater block time,horizontal impulse and take-off velocity are provided(Blanksby et al., 2002; Lyttle & Benjanuvatra, 2005;Vilas-Boas et al., 2003). The short starting time forventral techniques has been considered dependent onthe capacity to generate great horizontal take-off velo-city (Seifert et al., 2009; Takeda et al., 2009), whicharises from the compromise between enough blocktime to maximise the impulse and the briefest blocktime to minimise the time deficit (Lyttle &Benjanuvatra, 2005); however, for a backstroke startit might be beneficial to prioritise the take-off angle tominimise downward centre-of-mass vertical velocityand overcome some of the water resistance, mainlywhen starting with feet emerged (de Jesus et al., 2011).

Regarding the lower limbs force-time curve pro-files during the take-off phase, a progressive forceincrease was noted in both variants as from thehands-off instant (Figure 3), and the variant withfeet immerged displayed the greater maximal forceand shorter time to reach the respective peak forcebefore the take-off. Hohmann et al. (2008) showedsignificant correlation between the peak force justbefore the take-off instant and the backstroke start-ing time. Likewise, the greater second maximal forcevalue for the variant with feet immerged may havemeaningfully contributed to the strong associationfound by de Jesus et al. (2011) between lowerlimbs horizontal impulse and the variant with feetimmerged starting time. Hence, backstrokers shouldgenerate greater horizontal impulse and take-offvelocity without increasing wall contact time,emphasising the peak force just before the take-offinstant.

The maximum speed during a swimming event isachieved when swimmers’ feet leave the starting wall/block (Hohmann et al., 2008; Takeda et al., 2009),and its magnitude and maintenance are affected byactions performed during out of the water startingphases (Elipot et al., 2010; Thow et al., 2012). Thesimilar centre-of-mass horizontal velocity duringflight phase for both variants indicated that the smal-ler take-off angle compromised the greater centre-of-mass horizontal velocity at take-off displayed by thevariant with feet emerged. Previous ventral start stu-dies stated that swimmers should react to the startingsignal and leave the contact surface as fast as possible

with great horizontal take-off velocity and an appro-priate take-off angle (Galbraith et al., 2008; Vantorreet al., 2010a, 2010b). In fact, the greater take-offangle was found to reduce backstroke (de Jesuset al., 2011) and ventral starting time (Seifert et al.,2009), which can be explained by the minimisedforces resisting the swimmer’s forward motion dur-ing flight phase. The backstroke start variant withfeet immerged and emerged displayed a downwardcentre-of-mass vertical velocity during the flightphase, although it was greater for the variant withfeet emerged, which denotes the earlier effects ofgravity acceleration. The centre-of-mass horizontalvelocity increasing at take-off for the variant withfeet emerged seems to be beneficial, but only if it isaccompanied by an appropriate take-off angle andcentre-of-mass vertical velocity.

According to Breed and Young (2003) andHohmann et al. (2008), the swimming starts requirechanges in body position during flight targeting anoptimal performance. The current study showed asimilar back arc angle for the variant with feetimmerged and emerged, which compromised thepossible advantages obtained by the variant withfeet immerged with a greater take-off angle. Thesmaller back arc angle indicates that swimmershave travelled through the air in an arc, which allowsa smaller hole-entry size at the surface through whichthe body passes during water immersion (Lyttle &Benjanuvatra, 2005); therefore, swimmers areadvised to organise their aerial trajectory to optimisethe transition between aerial and underwater startphases (Seifert et al., 2009; Vantorre et al., 2010b).Nevertheless, our study did not focus on the kine-matics of entry and glide start phases, and no con-clusion can be drawn regarding the importance ofthe back arc angle to the underwater parameters.The centre-of-mass horizontal water reach and 5 mstarting time were also not affected by the differentfeet placements, which does not agree with ourhypothesis about the variant with feet immergedleading to the greater horizontal water reach andshorter starting time. Stratten’s (1970) findingsshowed shorter starting time for the variant per-formed with feet fully above water surface; althoughstanding with feet on the gutter. Most of the swim-ming start literature has described the necessity toperform an appropriate take-off angle; despite, themaintenance of an arched back posture during theoverall flight phase being crucial to improve horizon-tal water reach and consequently backstroke startperformances (Maglisho, 2003; Stratten, 1970). Infact, in previous ventral (Breed & Young, 2003;Vantorre et al., 2010a, 2010b) and dorsal (Theuth& Jensen, 2006) studies, the horizontal water reachhas also been mentioned as a very important perfor-mance variable in swimming starts.

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The current findings showed that, any advantageobtained by the variant with feet immerged due tothe centre-of-mass horizontal position being furtheraway from the pool wall and the greater take-offangle, and by the variant with feet emerged generat-ing greater lower limbs horizontal impulse and cen-tre-of-mass horizontal velocity at take-off wasnegated by the back arc angle and centre-of-masshorizontal water reach. Notwithstanding the origin-ality and relevance of the current data, some limita-tions should be considered. Firstly, the authorsacknowledge that enhanced statistical power andgeneralisability of main findings are dependentupon a large number of observations. Yet, consider-ing the complexity of our methodology and datacollection, present findings should be consideredpreliminary, albeit important, and used with cautionuntil data on a larger sample can be obtained.Secondly, some errors associated with image distor-tion, as a result of videotaping above and under thewater surface, with the digitisation process and withthe reconstruction, may have affected the data acqui-sition. Knowing that swimmers’ motions, mainlyduring the flight phase, might not be perpendicularlyconfined to the camera axes, a three-dimensionalbiomechanical model is essential in the near futureto provide more accurate information about theinfluence of different backstroke start set positionsin the three planes. In addition, it is necessary toinvestigate possible combined effects of the newbackstroke start rule modification and the currentinternational starting block configuration on thebackstroke start performance, not only on the 5 mmark but also up to the 15 m mark. Finally, the useof a multi-component kinetic approach would pro-vide additional insights into triaxial upper and lowerlimbs kinetic profiles during the backstroke start andits different variants.

Conclusions

This is the first study, after the FINA’s rules actua-lisation, which analysed the effect of different feetpositions on kinematics and kinetics of the back-stroke start performance. Findings showed severaldifferences between the variant with feet immergedand emerged from the starting signal to the swim-mer’s feet take-off; although for most of the flightparameters and for the 5 m starting time the twovariants were similar. The backstroke start variantwith feet immerged displayed greater centre-of-mass horizontal velocity at hands-off and take-offangle, shorter time of first upper limbs maximalforce during the hands-off phase, greater secondupper limbs maximal force during the hands-offphase and longer time to reach the respective peakforce, greater lower limbs maximal force and shorter

time to reach the respective peak value during thetake-off phase, shorter wall contact time and lowerdownward centre-of-mass vertical velocity at take-offand during the flight phase. The backstroke startvariant with feet emerged displayed shorter centre-of-mass horizontal position at the starting signal,greater lower limbs horizontal impulse and centre-of-mass horizontal velocity at take-off, greater lowerlimbs force at first reaction and lower limbs maximalforce during the hands-off phase, longer time toreach the lower limbs maximal force during thehands-off phase and the lower limbs force at theinstant of hands-off. As both technical solutions ledto similar 5 m starting performance, coaches andswimmers should consider including both feet place-ments in daily training emphasising the mechanicaladvantages of each variant and their maintenanceduring the aerial trajectory with an appropriatearched back posture to maximise distance travelledthrough the flight phase. Future research employinga large sample size should be done to analyse thecombined effects of different upper and lower limbspositioning on starting performance, while incorpor-ating a more complex and detailed biomechanicalapproach, which may improve the understanding ofthe backstroke start and its variants.

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