Effect of expertise on butterfly stroke coordination

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1 Journal of Sports Sciences, January 15th 2007; 25(2): Effect of expertise on butterfly stroke coordination L. SEIFERT 1, D. DELIGNIERES 2, L. BOULESTEIX 1, & D. CHOLLET 1 1 CETAPS Laboratory, Faculty of Sports Sciences, University of Rouen, Mont Saint Aignan and 2 Motor Efficiency and Deficiency, Faculty of Sports Sciences, University of Montpellier, Montpellier, France (Accepted 13 January 2006) Abstract The aim of this study was to compare the arm-to-leg coordination in the butterfly stroke of three groups of male swimmers of varying skill (10 elite, 10 non-elite, and 10 young swimmers) at four race paces (400-m, 200-m, 100-m, and 50-m paces). Using qualitative video analysis and a hip velocity-video system (50 Hz), key events of the arm and leg movement cycles were defined and four-point estimates of relative phase were used to estimate the arm-to-leg coordination between the propulsive (pull and push of arms and downward movement of leg undulation) and non-propulsive phases (entry, catch, and recovery of arms and upward movement of leg undulation). With increasing race pace, the velocity, stroke rate, and synchronization between the arm and leg key points also increased, indicating that velocity and stroke rate may operate as control parameters. Finally, these changes led to greater continuity between the propulsive actions, which is favourable for improving the swim velocity, suggesting that coaches and swimmers should monitor arm-to-leg coordination. Keywords: Coordination, butterfly stroke, motor control, biomechanics, expertise Introduction In the butterfly stroke, the movements of the two arms are identical and are performed simultaneously, as are those of the two legs, and the logic of this stroke is such that the arm movements alternate with those of the legs. This alternation does not favour propulsive continuity, but leads to high resistances that are overcome by the leg undulations. This stroke pattern results in a body wave motion (Sanders, Cappaert, & Devlin, 1995) that must be coordinated to be effective. Butterfly coordination in expert swimmers is based on two undulations of the legs for each complete arm cycle that is, a 2:1 frequency ratio (Colwin, 2002; Costill, Maglischo, & Richardson, 1992; Maglischo, 2003). These authors showed that: the downward time of the first undulation of the legs should occur during the catch time of the arms; the upward time of the first undulation of the legs should occur during the pull time of the arms; the downward time of the second undulation of the legs should occur during the push time of the arms; and the upward time of the second undulation of the legs should occur during the recovery of the arms. Yet while the theoretical armto-leg coordination has been globally described, the perfect timing between these two motor limbs has been difficult to achieve. Some swimmers, for example, use only a 1:1 frequency ratio between leg and arm actions. Moreover, the coordination can be constrained by the head movements necessary for breathing, which may occur at each arm stroke (1:1 frequency ratio) or every two arm strokes (1:2 frequency ratio). As described by dynamical theory, relative coordination modes (oscillators with different eigen frequencies; e.g. a 1:2 frequency ratio) are less stable than absolute coordination (1:1 frequency ratio) (von Holst, 1937/1973). In bimanual coordination, the coupling between left and right arms is more stable with a 1:1 than with a 1:2 frequency ratio (Sternad, Turvey, & Saltzman, 1999). Moreover, in two-limb (in ipsilateral and contralateral condition) and four-limb human coordination, the coupling between non-homologous limbs (arm with leg) is less stable than between homologous limbs (right arm with left arm and right leg with left leg) (Baldissera, Cavallari, Marini, & Tassone, 1991; Kelso & Jeka, 1992). When frequency increases, the wrist-to-foot coupling switches from anti-phase (wrist flexion with foot extension and wrist extension with foot flexion) to in-phase (wrist flexion with foot flexion and wrist Correspondence: L. Seifert, CETAPS Laboratory, Faculty of Sports Sciences, University of Rouen, Boulevard Siegfried, Mont Saint Aignan Cedex, France. ludovic.seifert@univ-rouen.fr ISSN print/issn X online Ó 2007 Taylor & Francis DOI: /

2 132 L. Seifert et al. extension with foot extension), indicating that for non-homologous limbs the iso-contraction mode (flexion for the two limbs or extension for the two limbs) is less stable than the iso-direction mode (flexion of one limb with extension of the other limb) (Baldissera et al., 1991). Similarly, transitions occur between the anti-phase and in-phase modes in nonhomologous limbs, whereas homologous limbs display no transitions between the two modes (Kelso & Jeka, 1992). Indeed, when frequency increases, the coupling of the right with the left arm (or the right with the left leg) is as stable in anti-phase as it is in in-phase (Kelso & Jeka, 1992). Donker, Beek, Wagenaar and Mulder (2001) examined different walking velocities and showed that the inter-limb coupling was lower for the arm movements and for the ipsilateral and controlateral combinations of arm and leg movements than for the leg movements. Indeed, whatever the walking velocity, the right and left legs were in anti-phase (1808). Conversely, when walking velocity increased, the frequency coordination of the arms to legs switched from a 2:1 to a 1:1 ratio. Thus in the ipsilateral combination, the interlimb coupling approached anti-phase ( in 2:1 frequency coordination and 1908 in 1:1 frequency coordination); in the contralateral combination, the coupling switched from anti-phase (26908 in 2:1 frequency coordination) to in-phase (108 in 1:1 frequency coordination); and the right and left arm coordination switched from in-phase (208) at low velocity to anti-phase (1808) at high velocity. Swimming studies have shown that when velocity or stroke rate increases in relation to race pace, the coordination changes as much in the simultaneous strokes (breaststroke and butterfly), in which arm-toleg coordination has been investigated (Chollet, Seifert, Leblanc, Boulesteix, & Carter, 2004; Leblanc, Seifert, & Chollet, 2005; Seifert & Chollet, 2005), as in the alternating strokes (backstroke and front crawl), in which right-to-left arm coordination has been analysed (Chollet, Chalies, & Chatard, 2000, Seifert, Chollet, & Bardy, 2004). In the breaststroke, when velocity and/or stroke rate increase, elite swimmers decrease the temporal gaps between arm and leg movements (Chollet et al., 2004), resulting in greater continuity between the propulsive actions. Propulsive continuity also increases with expertise (Leblanc et al., 2005) and is more refined in men (Seifert & Chollet, 2005). Based on the coordination data accumulated for other modes of locomotion and other swim strokes, perfect synchronization between arm and leg key points in the butterfly stroke should be the in-phase mode. The aim of this study was to determine whether butterfly stroke swimmers approach this mode of arm and leg coordination with increases in velocity, stroke rate, and skill. We hypothesized that with increases in velocity and stroke rate, an increase in continuity between arm and leg propulsion would show a mode of coordination close to in-phase by synchronization of arm and leg key points. Moreover, we assumed that the more skilled the swimmer, the closer to in-phase the coordination would be. Material and methods Participants Thirty male swimmers (10 elite, 10 non-elite, and 10 young swimmers) volunteered to participate in the study. The protocol was explained in full to each of the swimmers and they provided written consent to participate in the study, which was approved by the university s ethics committee. The elite group included a national finalist and swimmers who had participated in international competition. The nonelite group consisted of swimmers who competed in the national second division. The young swimmers were least expert as they were still mastering swim techniques; they were of regional or national standard for their age category. A one-way analysis of variance showed that the young swimmers were younger than the non-elite and elite swimmers, but did not find any differences in height, arm span, or body mass between the three groups (Table I). For the non-elite and elite swimmers, expertise was expressed as a percentage of the absolute current world record for the 100-m butterfly; that of the young swimmers was expressed in terms of the current world record standards for the junior age group (Table I). Table I. Physical characteristics of the swimmers (mean+s). Group Age (years) Height (m) Mass (kg) Arm span (cm) 100-m time (s) % of world record Elite swimmers (n ¼ 10) Non-elite swimmers (n ¼ 10) Young swimmers (n ¼ 10) a, b a, b a, b F-ratio (n ¼ 30) F 2,27 ¼ 5.73 F 2,27 ¼ F 2,27 ¼ a, significantly different from preceding group; b, significantly different from elite swimmers (P50.05).

3 Effect of expertise on butterfly stroke coordination 133 Swim trials In a 25-m pool, the swimmers performed four butterfly trials at increasing velocity. Each trial required an individually imposed swim pace corresponding to a specific race distance or training distance, as previously detailed for the front crawl and the breaststroke (Chollet et al., 2004; Seifert & Chollet, 2005; Seifert et al., 2004): the 400-m, 200-m, 100-m, and 50-m paces. After each trial, all swimmers were informed of their performance time, which was expected to be within +2.5% of the targeted race velocity. If this was not the case, the swimmer repeated the trial. During the test, pace and stroke rate were monitored with a chronometer and a Seiko Base 3-frequency meter. These measures served only to validate each trial that is, to ensure minimal discrepancy between the swim pace expected of each swimmer and the velocity at which he actually swam. The experimental data of this study, on the other hand, were obtained by the video device. Video analysis An aerial lateral video camera was superimposed on an underwater lateral video camera (Sony compact FCB-EX10L, Paris, France) with a rapid shutter speed (1/1000 s) and a sampling rate of 50 Hz, and both were fixed on a trolley. The trolley was pulled along the side of the pool by an operator who followed the swimmers, using each swimmer s head as the mark to control parallax. The cameras were connected to a four-entry audio-visual mixer (Videonics MX-1, Campbell, USA), a video timer, a video recorder, and a monitoring screen to mix the lateral underwater and aerial views on the same screen, from which stroke rate was calculated. At each hand entry, one stroke was completed so that the stroke rate was expressed for each stroke. A third camera (fixed on the wall, 50 Hz, Sony compact FCB-EX10L, Paris, France) filmed the swimmer from a frontal underwater view and was mixed and gen-locked by the audio-visual mixer (Videonics MX-1, Campbell, CA, USA) with the underwater lateral view on another screen. The two lateral views and the frontal view completed the velocity-video system and were used to analyse the arm and leg stroke phases. A fourth camera (50 Hz, Panasonic NV-MS1 HQ S-VHS, Paris, France), mixed with the lateral underwater view for time synchronization, filmed all the trials of each swimmer with a profile view from above the pool. This camera measured the time over a distance of 12.5 m (between the 10-m and the 22.5-m marks to remove the wall constraint) to obtain the velocity. The stroke length was calculated for each stroke from the mean velocity and stroke rate values [stroke length ¼ velocity 6 (stroke rate/ 60)]. Velocity-video system The velocity-video system enabled us to determine the key points in both arm and leg movements and thus to delimit with accuracy the propulsive (pull and push of arms and downward movement of leg undulation) and non-propulsive phases (entry, catch and recovery of arms and upward movement of leg undulation) (Figure 1). The video analysis was synchronized with a swim-speedometer (Fahnemann , Bockenem, Germany) as previously described for the breaststroke (Chollet et al., 2004; Costill, Lee, & D Acquisto, 1987; Craig, Bommer, Skehan, 1988; Seifert & Chollet, 2005; Tourny, Chollet, Micallef, & Macabies, 1992) and the butterfly (Buckwitz, Bähr, & Ungerechts, 2003). The swimmers wore waist belts attached by cable to an electric generator. The voltage produced by the generator was proportional to the swim velocity, and was recorded on computer. The lateral view of the underwater video camera and the video timer were mixed and gen-locked (by the audio-visual mixer Videonics MX-1, Campbell, CA, USA) with the instantaneous velocity curve read off the computer. Four complete strokes were filmed for each swimmer. The accelerations and decelerations of the hip calculated by the swim speedometer (at 0.01 s) were synchronized with the arm and leg movements recorded by the video device (at 0.02 s). Arm and leg stroke times The arm stroke was divided into four distinct phases (Figure 1) by three operators who analysed the key points of each phase at 0.02-s intervals using a blind technique that is, without any knowledge of the analyses of the other two operators. The three analyses were compared only when each operator had completed his own analysis. When the difference between the analyses did not exceed an error of 0.04 s, the mean of the three analyses was used to validate the key point of each phase. When the error exceeded 0.04 s, the three operators together undertook a new assessment of the key point. This qualitative video analysis was similar to the analysis used for the front crawl by Chollet et al. (2000), but it needs to be used with caution. Therefore, unlike this latter study, which analysed only two strokes, and more recent studies analysing three strokes (Chollet et al., 2004; Seifert & Chollet, 2005; Seifert et al., 2004), four strokes were analysed to decrease the error inherent to a small sample of strokes.

4 134 L. Seifert et al. Figure 1. Synchronized structure of the arms and legs for butterfly swimming. The four phases were as follows: 1. Entry and catch of the hands in the water, which corresponds to the time between the entry of the hands into the water and the beginning of their backward movement. In fact, this phase is composed of four parts: entry and stretch, outsweep, glide, and catch (Maglischo, 2003). 2. Pull, which corresponds to the time between the beginning of the backward movement of the hands and their entry into the plane vertical to the shoulders. 3. Push, which corresponds to the time between the positioning of the hands below the shoulders to their exit from the water. The pull and push times correspond to the arm propulsive time (PT2 in Figure 1). 4. Recovery, which corresponds to the time between the exit of the hands from the water and their re-entry into the water. The duration of each phase was measured for each stroke to a precision of 0.02 s and was expressed as a percentage of the duration of a complete arm stroke. The leg stroke consisted of two phases (Figure 1): 1. Downward, which corresponds to the time between the high and low break-even points of the feet during the undulation. 2. Upward, which corresponds to the time between the low and high break-even points of the feet during the undulation. The duration of each phase was measured for each stroke to a precision of 0.02 s and was expressed as a percentage of the duration of a complete leg stroke. According to Jensen and McIlwain (1979), the downward time of the undulation corresponds to the leg propulsive time (PT1 in Figure 1), whereas the upward time is not propulsive.

5 Effect of expertise on butterfly stroke coordination 135 Arm-to-leg coordination The arm-to-leg coordination was assessed by the point-estimated relative phase (Diedrich & Warren, 1995; Hamill, Haddad, & McDermott, 2000). This discrete method illustrates the relative timing of key events, or key points, in a movement cycle. In the butterfly stroke, it corresponds to the latency of a key point in the arm stroke time with respect to a key point in the leg stroke time. Costill et al. (1992) and Maglischo (2003) described expert butterfly coordination as two leg undulations for one arm stroke, so the coupling between arm and leg corresponds to a 2:1 frequency coordination. Swimmers with only one leg undulation were thus not admitted to this study. Costill et al. (1992) and Maglischo (2003) also reported that four arm and leg key points, which can be assessed by four point-estimated relative phases, should be in-phase during a complete stroke. Each relative phase (RP) is calculated using the formula: [RP ¼ ((t target 7t 0 )/(t reference 7t 0 ))6360]. For the four relative phases, t 0 corresponded to the entry of the hands into the water at the first stroke. The relative phases are as follows: 1. RP1: the time difference between the high break-even point of the first undulation (t target ) and when t 0 was measured. This duration was expressed in degrees relative to the time difference between the entry of the hands into the water at the second stroke (t reference ) and t 0 (Figure 1). 2. RP2: the time difference between the low break-even point of the first undulation (t target ) and when t 0 was measured. This duration was expressed in degrees relative to the time difference between the beginning of the hands backwards movement (t reference ) and t 0 (Figure 1). During this time, neither the upper nor lower limbs were performing propulsive actions; RP2 thus measured the body glide. 3. RP3: the time difference between the high break-even point of the second undulation (t target ) and when t 0 was measured. This duration was expressed in degrees relative to the time difference between the hands arrival in a vertical plane relative to the shoulders (t reference ) and t 0 (Figure 1). 4. RP4: the time difference between the low break-even point of the second undulation (t target ) and when t 0 was measured. This duration was expressed in degrees relative to the time difference between the hands exit from the water (t reference ) and t 0 (Figure 1). In each trial, each relative phase was measured for four arm strokes and was expressed in degrees. Theoretically, two coordination modes exist: inphase (0 or 3608) and anti-phase (1808). However, as adopted by Bardy, Oulllier, Bootsma and Stoffregen (2002) and Diedrich and Warren (1995), a lag of +308 was accepted in this study for the determination of a coordination mode. Therefore, between and an in-phase mode was assumed to occur, while the anti-phase mode was taken to be between 150 and Beyond this step, a coordination mode of out-of-phase was also taken into account. The concepts of anti-phase, out-ofphase, and in-phase thus seem well adapted to characterize a coordination mode at a given moment in this stroke (by the point estimate of the relative phase), instead of an analysis of the continuous interlimb coupling over the course of the stroke (by the continuous relative phase). Statistical analysis Spatio-temporal and stroke time parameters. For the mean of the four paces and the three groups, the distribution normality (Ryan Joiner test) and the homogeneity of variance (Bartlett test) were verified for each variable and allowed parametric statistics (Minitab 13.20, Minitab Inc., 2000). Twoway repeated-measure analyses of variance were used to assess the main factor effect and the interaction between group (3 levels) and pace (4 levels) and were completed by post-hoc Tukey tests (Table II) (Minitab 13.20, Minitab Inc., 2000). For this statistical analysis, the mean of four strokes of each pace was considered: n ¼ 4 paces63 groups610 participants ¼ 120. For all tests, statistical significance was set at P Coordination parameters. Watson-Williams F-tests (group 6 pace) (Oriana 2.0, W. Kovach Computing Services, ) for circular data (Baschelet, 1981) showed coordination differences between groups (3 levels) and between paces (4 levels). For this statistical analysis, the four strokes of each pace were considered: n ¼ 4 strokes64 paces63 groups610 participants ¼ 480 (Table III). For all tests, statistical significance was set at P Results Velocity, stroke rate, and stroke length Two-way (pace6group) repeated-measures analysis of variance did not show significant interactions, but revealed that for all groups velocity and stroke rate increased throughout the paces, while stroke length decreased only for elite swimmers. Moreover, the higher the standard of skill, the higher the velocity and stroke rate (Table II).

6 136 L. Seifert et al. Table II. Values of spatio-temporal parameters, according to the imposed swim paces (mean + s). Spatio-temporal parameters Groups 400-m pace 200-m pace 100-m pace 50-m pace Mean F-ratio (pace effect) Velocity (m s 71 ) Elite swimmers c c, d d, e Non-elite swimmers a a, d a, d, e a Young swimmers a, b b, c b, d b, d, e a, b Mean c c, d c, d, e F 3,109 ¼ 37.5 Statistics F2,109 ¼ 30.2 (group effect) Stroke rate (strokes min 71 ) Elite swimmers c, d, e Non-elite swimmers c, d c, d, e Young swimmers a, b a, b c, d d, e a, b Mean c c, d c, d, e F 3,109 ¼ 49.8 Statistics F 2,109 ¼ 12.1 (group effect) Stroke length (m stroke 71 ) Elite swimmers c, d d, e Non-elite swimmers Young rising swimmers Mean c, d d, e F3,109 ¼ 6.9 F-ratio (group effect) F 2,109 ¼ 00.0 a, significantly differenct from preceding group; b, significantly different from elite swimmers (P5 0.05); c, significantly different from preceding pace; d, significant different from the 400-m pace; e, significantly different from the 200-m pace (P50.05).

7 Effect of expertise on butterfly stroke coordination 137 Table III. Values of arm-to-leg coordination parameters, according to the imposed swim paces (mean + s). Relative phases Groups 400-m pace 200-m pace 100-m pace 50-m pace Mean F-ratio (pace effect) RP1 (8) Elite swimmers d d, e Non-elite swimmers c, d c, d, e Young swimmers a, b c, d d, e a, b Mean c, d d, e F3,476 ¼ 9.1 F-ratio (group effect) F2,477 ¼ 7.6 RP2 (8) Elite swimmers d, e Non-elite swimmers c a, d d, e a Young swimmers a, b b b a, b, d a, b Mean c, d d, e F 3,476 ¼ 8.3 F-ratio (group effect) F2,477 ¼ 17.9 RP3 (8) Elite swimmers Non-elite swimmers a a a a a Young rising swimmers a, b b b b b Mean F-ratio (group effect) F 2,477 ¼ 26.3 RP4 (8) Elite swimmers Non-elite swimmers a a Young rising swimmers a Mean F-ratio (group effect) F2,477 ¼ 8.1 a, significantly differenct from preceding group; b, significantly different from elite swimmers (P ); c, significantly different from preceding pace; d, significant different from the 400-m pace; e, significantly different from the 200-m pace (P ).

8 138 L. Seifert et al. Arm and leg stroke times Two-way (pace6group) repeated-measures analysis of variance did not show significant interactions, but revealed that changes in stroke times principally occurred for the arms, because the leg stroke times only changed during the first undulation. For each group, the catch time was shorter [from % at the 400-m pace to % at the 50-m pace (F 3,109 ¼ 17.8, P )] and the pull and recovery times were longer [from % and % respectively at the 400-m pace to % (F 3,109 ¼ 7.6, P ) and % respectively at the 50-m pace (F 3,109 ¼ 9.2, P )] as the pace increased. On the other hand, based on the mean of the three groups and for the elite swimmers, the first downward undulation took longer [from % at the 400-m pace to % at the 50-m pace (F 3,109 ¼ 4.7, P )] and the first upward undulation was shorter between paces [from % at the 400-m pace to % at the 50-m pace (F 3,109 ¼ 3.7, P )]. Expertise was not related to the relative duration of the leg stroke times because no difference was noted between groups, whereas for arm stroke times, based on the mean of the four paces and for each pace, the catch time was shorter [ % for young swimmers vs % for elite swimmers (F 2,109 ¼ 22.2, P )] while the push time took longer [ % for young swimmers vs % for elite swimmers (F 2,109 ¼ 19.4, P )] with increasing expertise. Arm-to-leg coordination For the mean of the four paces and for the whole population, RP1 (6.88), RP3 (320.88), and RP4 (3488) showed an arm-to-leg coordination close to the in-phase mode (Table III). Nevertheless, based on the mean of the three groups and for each group, Watson-Williams F-tests revealed differences between paces for RP1 and RP2 (Table III). From the 400-m to the 50-m pace, the arm-to-leg coordination became closer to the in-phase mode, with a significant decrease in RP1 from to 0.58 (based on the mean of the three groups). In contrast, RP2 switched from an anti-phase mode at the 400-m pace (186.58) to an out-of-phase mode at the 50-m pace (226.78). For the mean of the four paces, Watson-Williams F-tests revealed differences between groups for the four relative phases (RP1, RP2, RP3, RP4), indicating that with increasing expertise the arm-toleg coordination approached an in-phase mode (Table III). These results on the mean of the four paces were due to closer in-phase RP2 and RP3, which were different between groups at each pace, while RP1 and RP4 were only different between groups at the 200-m pace (Table III). Discussion Effect of velocity and stroke rate on coordination For the mean of the four paces and with increases in skill, the arm-to-leg butterfly coordination approached in-phase mode at three key points of the stroke (at the entry, the point of passing under the shoulder, and the release of the hand), showing that the movements of the two pairs of motor limbs were linked. Moreover, with increases in velocity and stroke rate, coordination mostly came closer to an inphase mode. In human locomotion, when walking velocity increases, the coupling between all pairs of limb movement was found to increase (Donker et al., 2001). In quadrupedal coordination (Kelso and Jeka, 1992; Schöner, Jiang, & Kelso, 1990) and in walking/ running (Diedrich & Warren, 1995), when the frequency of the oscillator movements or the velocity increases, the coordination of the non-homologous limbs switches from anti-phase to in-phase, suggesting that frequency and velocity can be considered as control parameters. In the butterfly stroke, velocity and stroke rate can be considered control parameters of coordination because their increase led to a closer in-phase mode of arm-to-leg coordination (particularly RP1 and RP2). This was due to the increase in the propulsive time (pull time of arms and first downward undulation of legs) and the decrease in the non-propulsive time (catch time of arms and first upward undulation of legs), which leads to a better timing between arm and leg key points (Delignières & Chollet, 1999). The closer in-phase mode of RP1 with velocity may have been due to the faster entry of the head after breathing and to the lower trunk inclination when the swimmers adopted high race paces. Alves, Cunha and Gomes-Pereira (1999) noted an 88 decrease in trunk inclination (from 32 to 248) and a shorter downsweep of the arms when breathing was restrained. At a high velocity and stroke rate, swimmers must reduce the time during which the head is over the water to facilitate the aerial and lateral recovery of the arms. Regarding RP2, with an increase in velocity and stroke rate, the elite and non-elite swimmers shifted from an anti-phase to an out-of-phase relation (for the young swimmers, the shift took longer) between the first low break-even point of the first undulation and the beginning of the pull time of the arms. This change in coordination corresponded to a decrease in the lag between the propulsive time of the legs and the propulsive time of the arms, usually called the

9 Effect of expertise on butterfly stroke coordination 139 glide. Swimmers have to overcome high active drag at high velocity by reducing glide duration and increasing propulsive times (Kolmogorov, Rumyantseva, Gordon, & Cappaert, 1997). Lastly, coaches may be able to manipulate velocity and stroke rate to encourage the emergence of the most appropriate coordination mode. On the other hand, they should not automatically advocate a high stroke rate in the hope of obtaining arm-to-leg coordination in the in-phase mode. This advice should be given only if a higher stroke rate does not result in too great a decrease in velocity, stroke length, and technique. For example, a swimmer could adopt the imposed stroke rate and show an inphase coordination, but with an ineffective hand sweep which slips through the water. Effect of expertise on coordination Regarding RP1, all groups became significantly closer to an in-phase relation between the entry of the hands in the water and the high break-even point of the first undulation. Nevertheless, the non-elite swimmers increased the phase coupling between their first downward undulation and the hand entry from the 400-m to the 100-m pace (RP1 was from 9.68 to 2.28) so much that at the 50-m pace they began their first downward undulation before the hand entry (RP1 was ). This coordination was not adequate because, although the legs were propelling, the arms and hands were not streamlined in an extended position to prepare the catch time (Colwin, 2002). Because the body decelerates during the arm recovery, the timing between the arm entry and the first downward undulation is critical. Indeed, this undulation is the strongest in the stroke (Hahn & Krug, 1992) and should occur soon after the arm entry (Colwin, 2002). Costill et al. (1992) advised striving for perfect synchronization between the arm entry and the first downward undulation, because the active drag caused by the arm entry can be overcome by the leg propulsion. Unlike elite swimmers, the young swimmers did not manage to synchronize these two key points to any great extent (Figure 2) and consequently opposed more active drag. For RP2, with increasing pace the non-elite swimmers shifted more slowly to the out-of-phase mode than the elite swimmers, showing a longer glide duration with the arms extended forward and no propulsive action from the legs. An overly long glide, corresponding to an anti-phase coordination between arms and legs, decreases the velocity (Mason, Tong, & Richards, 1992; Sanders, 1996); this was the case in the young swimmers, who recorded the greatest time lag (Figure 2c). Conversely, too short a glide, which corresponds to inphase coordination, did not enable an effective catch of the arms and led to slippage throughout the water. Thus, Costill et al. (1992) advised finishing the downward undulation just before the catch time of the arms. Finally, with an out-of-phase coordination of (Table III and Figures 2a and 2b), the first downward undulation of the elite swimmers provided acceleration to project the hips forward (Jensen & McIlwain, 1979) and enabled an effective catch time to prepare the pull time. So, logically, the first upward undulation occurred during the pull time (i.e. the insweep of the arms) to bring the body into a streamlined position. RP3 was based on the difference between the beginnings of two propulsive times: the push time of the arms and the second downward undulation. The more skilled the swimmer, the earlier he began the push time after the high break-even point of the second undulation, and the more his arm-to-leg coordination approached the in-phase. Indeed, the mean RP3 showed that the less skilled swimmers adopted an out-of-phase coupling between arms and legs ( for non-elite and for young swimmers), whereas the elite coordination was close to in-phase mode ( for elite swimmers). In fact, the perfect timing of these two propulsive times is important for high propulsion (Reischle, 1979; Sanders, 1996) because it provides the greatest body acceleration in the stroke (Barthels and Adrian, 1975; Martin-Silva, Alves, & Gomes-Pereira, 1999; Mason et al., 1992), whereas a lack of coordination means large velocity fluctuations that are detrimental to propulsion (Hahn & Krug, 1992). Figure 2c shows one swimmer from the young group presenting an out-of-phase coordination mode with a marked shift between the two limbs (RP3 ¼ ). This lack of coordination could lead some of the less skilled swimmers to forget the second downward undulation and to wait with the legs smoothly flexed. Thus, the hips remain in too deep a position and oppose great active drag. Conversely, the second downward undulation counters the drop of the hips caused by the push time of the arms upward (Colwin, 2002). Like RP3, RP4, which was based on the time difference between the hand exit and the low breakeven point of the second undulation, showed an inphase coordination mode. The elite swimmers had a longer relative duration of the push time than the young swimmers, which could explain their better coordination that which facilitated higher force (Schleihauf et al., 1988). Elite swimmers (mean RP4 ¼ ) revealed a closer in-phase mode of coordination than the non-elite swimmers and the young swimmers ( and respectively), which was propitious for better arm release. In fact, according to Colwin (2002), the timing of downward undulations has three important functions: (1) to ensure continuous propulsion between arms and

10 140 L. Seifert et al. Figure 2. Comparison of armto-leg coordination between different skill levels and different race paces. legs, (2) to maintain the body in a streamlined position by keeping the hips high, and (3) to aid head mechanics, notably to favour head movement for breathing. The closer the end of the second downward undulation is to the end of the push time, the more the second downward undulation will compensate the upsweep of the hands to provide hip elevation (Colwin, 2002; Maglischo, 2003); therefore, the hand exit and the release of the arm forward will be easy. The downward undulation should not be too strong, however, because if the hips are above the water, the arm recovery will be hampered. Therefore, an important finding was not only the force applied during the propulsive times, but also the coordination between these times. Moreover, elite swimmers were closer to an in-phase mode of coordination (for RP4), which favoured head movement for breathing and could explain why, although the relative duration of their arm recovery was greater than that of the young swimmers, it did not disturb their general propulsion. In fact, their arm recovery occurred during the second upward undulation and influenced the coordination of the next beginning stroke (as assessed by RP1). Conclusion With increases in velocity, stroke rate, and/or expertise, the swimmers came closer to an in-phase mode of coordination by synchronizing the key points of the arm and leg movements, suggesting a continuum of coordination evolution through the skill levels. These differences in coordination revealed greater continuity between propulsive actions at high velocity and high expertise, which was favourable for decreasing the instantaneous velocity fluctuations and improving swim velocity. This suggests that coaches and swimmers should monitor the arm-to-leg coordination by manipulating control parameters (velocity, stroke rate) in relation to biomechanical (active drag and velocity) constraints. References Alves, F., Cunha, P., & Gomes-Pereira, J. (1999). Kinematic changes with inspiratory actions in butterfly swimming. In K. L. Keskinen, P. V. Komi, & A. P. Hollander (Eds.), Swimming science VIII (pp. 9 14). Jyväskylä, Finland: University of Jyväskylä.

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