Kinematical Profiling of the Front Crawl Start

16 Training & Testing

Kinematical Profiling of the Front Crawl Start

Authors

J. Vantorre1, L. Seifert1, R. J. Fernandes2, J. P. V. Boas3, D. Chollet4

Affiliations

1

Key words ▶ biomechanics ● ▶ swimming start ● ▶ cluster analysis ● ▶ coordination ●

Abstract &

Faculty of Sport Sciences, CETAPS UPRES EA 3832, MONT SAINT AIGNAN, France University of Porto, Faculty of Sport, Swimming, Porto, Portugal 3 Faculty of Sport, Porto University, Biomechanics Lab., Porto, Portugal 4 Université de Rouen, CETAPS Laboratory EA3832, Mont Saint Aignan, France 2

This study analysed the start phases of 15 elite front crawl swimmers, all specialists of sprint events. The first aim was to determine which phases were correlated with the 15-m start time. The features common to the sample of swimmers were then established and individual profiles were clustered. The subjects performed two 25-m trials at the 50-m race-pace using their preferential start technique (grab start). The kinematical analysis assessed the durations of the block, flight, entry, glide, leg kicking and full swimming phases to the 15-m mark. Strok-

Introduction &

accepted after revision September 06, 2009 Bibliography DOI http://dx.doi.org/ 10.1055/s-0029-1241208 Int J Sports Med 2010; 31: 16–21 © Georg Thieme Verlag KG Stuttgart · New York ISSN 0172-4622 Correspondence Dr. Ludovic Seifert Faculty of Sport Sciences CETAPS UPRES EA 3832 Boulevard Siegfried 76821 MONT SAINT AIGNAN France Tel.: + 33232107784 Fax: + 33232107793 [email protected]

Several studies have quantified the start time in relation to the swim time and turn time in order to assess its contribution to overall race performance [1, 9, 24, 32]. The findings indicated that the 15-m start time represents 0.8–26.1 % of the overall race time (depending on the length of event), which suggest the interest of studying the start in swimming [9]. Most of the biomechanical studies employed kinetic and kinematical analyses to compare the two principal start techniques used in individual ventral events, i. e. grab and track starts. The kinetic analyses focused on the forces applied to the block or on the training programs designed to improve the start [6, 12, 19, 26]. For instance, Vilas-Boas et al. [32] observed a faster reaction time, longer impulse time on the force plate, greater flight distance, and shorter glide time in the grab start than in the track start, independent of performance with a frontal or rear projection of the centre of gravity. Swimmers using a track start tended to leave the block faster [2, 3] and achieve a flatter trajectory, which could be explained by higher horizontal velocity [9].

Vantorre J et al. Kinematical Profiling of the Front Crawl Start … Int J Sports Med 2010; 31: 16–21

ing parameters and the index of arm coordination (IdC) were analysed for the swimming part (10–20 m) of the 25-m. Through the swimming part IdC increased while stroke length and velocity decreased (p < 0.05). The relative durations of the aerial (block, flight), entry and underwater phases were correlated with start time. Intersubject variability was observed, which suggests that various motor solutions were used for the start. Notably, four clusters led to a short 15-m start time: the leg kicking style, mixed “leg kicking/swimming” style, long glide style and short glide style.

Swimmers using the grab start applied higher impulse [3] and spent more time on the block [18]. Although all of these swimming start studies focused on the swimmer’s position on the block, they did not take into account the fact that, for the same starting position, swimmers can manage the start phases differently. Because the start technique can influence the movements that follow, competitive swimmers would probably give more importance to the aerial phases in order to cover more distance in the air and use the aquatic phases to maintain their velocity for as long as possible after the water entry. Sanders [27] and Sanders and Byatt-Smith [2] emphasised that great consideration should be given to the underwater phase, as Cossor and Mason [8] observed a negative correlation between underwater time and the 15-m start time in the 100-m freestyle. Guimarães and Hay [13] showed that reducing hydrodynamic resistance during the glide decreased the total event time. Pereira et al. [25] suggested that this phase is influenced by several variables, like a streamlined position and the stroke technique. Indeed, Arellano et al. [1] and Guimarães and Hay [13]

Training & Testing 17

BLOCK PHASE

Fig. 1

FLIGHT PHASE

ENTRY PHASE

GLIDE PHASE

LEG KICKING PHASE

SWIMMING PHASE

Start phases to 15 m.

showed that 95 % of the variance in the start time was explained by the glide time. Moreover, Vilas-Boas et al. [32] showed that the gliding phase can compensate all the differences produced during the impulse and flight phases, thereby emphasising its importance. According to the FINA rules for freestyle events, the head must break the water surface before the 15-m mark, implying that the distance usually used to assess the start time is 15 m. However, these rules do not refer to swimmers’ movements in the water, namely the duration and number of undulations in the leg kicking phase or the number of stroke cycles and the change in arm coordination needed to reach the average velocity. For the 100-m event, Seifert et al. [19, 30] showed that the IdC continuously increased over the first 25 m while the velocity tended to be stable. The first aim of this study was to determine whether all elite swimmers would show similar start organisation, by studying the relationship between each phase of the start and the 15-m start time or whether several start styles could lead to an effective 15-m start time. The second aim was to analyze the influence of the starting actions of the following swimming part (arm + leg) not only up to 15-m but up to the end of a 25 m lap. The originality of this study is therefore not only based on a kinematic analysis of the start styles but also on the starting part – full swimming part transition by assessing the stroking parameters (v, stroke rate, stroke length and arm coordination) changes up to 25 m.

Material and Methods & Participants Fifteen elite male swimmers volunteered for this study (age 21.4 ± 3.7 years, body mass 79.5 ± 9.6 kg, and height 1.87 ± 0.07 m). Their mean performance for the 100-m freestyle long course was 50.73 ± 1.41 s, which corresponds to 94.3 ± 2.1 % of the 2007 world record for this event. This elite group included a medallist at the Olympics Games for the 4 × 100 m freestyle relay, a medallist at the European Championships, and two European junior champions. The protocol, approved by the University Ethics Committee, was explained to the swimmers, who then gave their written consent to participate in the experiments.

Protocol In an indoor 25-m swimming pool, each swimmer performed a 25-m front crawl twice at the 50-m race pace. The best trial (in terms of performance over 15 m) was selected to keep one trial per swimmer. The target time was based on the competition or race simulation times and was recorded over 25 m. The target time was expected to be within more or less 2.5 % of the race time [16]. All the trials were done using the grab start (which was their preferential start technique). Two lateral fixed cameras (50 Hz, Panasonic NV-MS1 HQ S-VHS) with rapid shutter speed (1/1 000 s) were placed at the side of

the pool: one was 5 m from the edge of the pool and videotaped the block, flight and head water-entry phases, and the other was placed in front of the 15-m mark to videotape the swimmer from the instant the head emerged at the 15-m mark. These cameras were connected to a double-entry audio-video mixer, a video timer, a video recorder and a monitoring screen to genlock and mix the two lateral fixed views on the same screen. Two lateral mobile cameras (50 Hz, Sony Compact FCB-EX 10 L) were fixed on a trolley and connected to a second double-entry visual mixer, a video timer, a video recorder and a monitoring screen to genlock and mix the two lateral aerial and underwater views on the same screen. The trolley was pulled along the side of the pool by an operator following the swimmers (the subject’s head was the mark tracked by the operator to control parallax). The fixed cameras enabled the kinematical analysis of the start phases (from the starting signal to the head reaching the 15-m mark) and the mobile cameras enabled the analysis of stroking parameters – stroke rate (SR), stroke length (SL) and velocity (v), and arm coordination – during the swimming phase (from the beginning of arm propulsion, between 10–20 m).

Kinematical analysis of the start phases ▶ Fig. 1 shows that the kinematical analysis of the start could be ● divided into six phases: (i) the block phase (the time between the signal and the instant the swimmer’s feet left the block), (ii) the flight phase (the time between the instant the toes left the block to hand entry), (iii) the entry phase (the time between hand entry and toe immersion), (iv) the glide phase (the time between toe immersion and the beginning of the aquatic propulsion of the legs), (v) the leg kicking phase (the time between the beginning of leg propulsion and arm propulsion, which was equal to 0 when kicking and stroking started at the same time, > 0 when the swimmer started kicking before stroking, and < 0 when the swimmer started stroking before kicking), and (vi) the swimming phase (the time between the beginning of the first stroke and the arrival of the head at the 15-m mark). The duration of each phase was measured for each stroke with a precision of 0.02 s and expressed in s. Relative duration is ▶ Fig. 1). The expressed in percentage of the 15-m start time (● aerial phase comprised the block and flight phases and the underwater phase comprised of the glide and leg kicking phases until the head reached the water surface. The start time is the sum of all the phases and is defined as the time from the start signal to the head reaching the 15-m mark. The number of underwater leg undulations and arm stroke movements to the 15-m mark were added to characterize the start quantitatively.

Stroking parameters and index of arm coordination assessment during the swimming phase To assess v, markers were situated on either side of the pool to establish four 2.5-m zones: 10–12.5 m, 12.5–15 m, 15–17.5 m, and 17.5–20 m. SR was calculated from the duration of each stroke, which was timed with the video timer from the entry of



the left hand at the first stroke to the entry of the left hand at the second stroke and expressed as stroke. min − 1. Each 2.5-m zone contained one or two strokes, depending on whether the swimmer’s head was in the zone. In the case of a stroke situated in two zones, the zone with the longest relative stroke duration was taken into account. When two strokes were in the same 2.5m zone, the mean SR was recorded. SL was calculated for each 2.5-m zone with the equation: SL = (v/SR) * 60 and expressed in meters per stroke. A margin of error was calculated by reproducing the v calculation in the four 2.5 m zones: v was assessed three times for all the trials. The percentage of error estimation is 2.3 % and the intra-class correlation between the three measurements of v is 0.896 that represents a good reliability. Then, the values of v, SR and SL obtained during the simulated 50-m race pace were compared to the stroking parameters values obtained in international competitions [16]. The arm coordination index (IdC) was assessed from the four phases that compose an arm stroke (Chollet et al. [6]): entry and catch of the hand in the water, pull, push and recovery. The duration of each phase was measured for each arm-stroke cycle with a precision of 0.02 s (50 Hz) and expressed as a percentage of the duration of the complete cycle. The duration of the propulsive phase was the sum of the pull and push phases, and the duration of the non-propulsive phase was the sum of the entry + catch and recovery phases (the duration of a complete arm-stroke cycle was the sum of the propulsive and non-propulsive phases). The IdC calculated the time gap between the propulsion of the two arms as a percentage of the duration of the complete armstroke cycle. The coordination mode was called catch-up when IdC was < 0 % (due to a lag time between the propulsive phases of the two arms), opposition when the propulsive phase of one arm started at the time the other arm finished its propulsive phase (IdC = 0 %), and superposition when the propulsive phases of the two arms overlapped (IdC > 0 %). The IdC was calculated for each stroke and averaged for each 2.5-m zone.

Statistical analysis Mean and SD computations for descriptive analysis were obtained for all variables from the best trial for each swimmer. (all data were checked for normality distribution with th e Shapiro-Wilk test). A two-way ANOVA (fixed factor: 2.5-m zone; random factor: subject) was used to study the change in v, SR, SL and IdC through the four zones. Pearson correlation analysis was also applied to study the relationships between the start parameters and the total 15-m start time. To classify the swimmers, a cluster analysis, using the Ward linkage method with a squared Euclidean distance, was applied for all the start parameters, as previously done to classify the impulse symmetry of the feet in the grab and track start styles [3], as well as to classify the backstroke start regarding swimmers’ body segment vectors [33]. The results of the cluster analysis were described as a dendogram and Kruskal-Wallis tests analyzed the parameters that differentiated the clusters. All tests were performed with Minitab 14.20 (Minitab Inc., 2003) and the level of significance was set at p < 0.05.

Results & The study of a sample of elite swimmers showed relationship between some phases of the start and the 15-m start time. Moreover, similar organization was found from the beginning of

the stroke to the 25 m (swimming part). However, concerning the management of the start phases, several clusters were found, indicating that there exist several strategies to achieve an effective start. Consequently, the first part of this study was a descriptive analysis of the start phases and their correlation to 15 m start time. Pearson correlation analysis revealed that the 15-m start time was negatively correlated with the relative duration of the aerial phase and positively correlated with the underwater phase ▶ Table 1). duration (● The second part of this study was based on a cluster analysis to determine if different profiles of effective starts may exist. High inter-subject variability was observed, notably for the underwater phases, suggesting that the swimmers may have used different strategies to achieve success in these phases. As can be ▶ Table 2, the cluster analysis distinguished four observed in ● start styles, significantly differentiated by six variables: block phase, glide phase, leg kicking phase, swimming phase, number of undulations and number of arm strokes. The total 15-m start time did not differ between the four clusters, indicating that these elite male specialists of the 50-m front crawl event used several start styles to perform the grab start technique. Indeed, Cluster 1 (n = 4) was termed the long leg kicking style and was characterized by medium block and glide phase durations, the longest leg kicking phase, the highest number of undulations, the shortest swimming phase and the fewest arm stroke movements. Cluster 2 (n = 7) was termed the mixed “leg kicking/ swimming” style and was characterized by medium block and glide phases, a long leg kicking phase (although shorter than in Cluster 1), a short swimming phase and the most arm stroke movements (in fact, Cluster 2 differed from Cluster 1 by more arm stroke movements). Cluster 3 (n = 2) was termed the long glide style and was characterized by the longest block and glide phases, the shortest leg kicking phase, and a long swimming phase to 15 m, which is significantly different from Clusters 1 and 2. Cluster 4 (n = 2) was termed the short glide style and was characterized by the shortest block and glide phases, a medium leg kicking phase (as opposed to the other clusters), and the longest swimming phase to 15 m. Concerning the swimming part, the stroking parameters (v, SR, SL and IdC) were not significantly different between the four clusters, showing a common influence of the start on the swimming part of the 25 m length. Therefore, for the whole sample of swimmers, v decreased in the two last zones of 2.5 m while SR and SL decreased (p < 0.05) through the four 2.5-m zones com▶ Table 3). posing the swimming part (10–20 m) of the 25 m (● IdC significantly increased (p < 0.05) through the four zones of the swimming part, showing superposition coordination ▶ Table 3). The number of undulations and arm (IdC > 0 %) (● strokes was 2.8 ± 1.0 and 7.9 ± 1.4, respectively.

Discussion & One of the most important results of this study is that the block phase was negatively correlated with start time, and corresponded to two distinct actions: a rapid reaction to the starting signal and impulse performed over the starting block. Benjanuvatra et al. [3] compared two start techniques and showed no significant difference in reaction times, although differences were observed for the block phase. The reaction time needs to be as short as possible, while the block phase needs to last long


Training & Testing 19

6.42 ± 0.12

6.36 ± 0.25 short glide style

0.85 ± 0.03 13.8 ± 0.5b 0.88 ± 0.06 12.3 ± 0.8c 0.79 ± 0.04 absolute durations (s) relative durations ( %) absolute durations (s) relative durations ( %) absolute durations (s) long glide style

mixed leg kicking/ swimming style

Significant differences between groups are shown by a (from Long Leg Kicking Style) and b (from Mixed “Leg Kicking/Swimming” Style) and c (from Long Glide Style), all for a p < 0.05

2.5 ± 0.6

2.0 ± 0.4a

7.7 ± 0.9

8.5 ± 0.6a

6.58 ± 0.49

2.03 ± 0.16 23.6 ± 2.8 1.50 ± 0.20 23.4 ± 1.8 1.50 ± 0.11 1.18 ± 0.09 19.1 ± 0.7 1.21 ± 0.01 16.5 ± 0.4 1.06 ± 0.03 3.38 ± 0.45 57.3 ± 1.2a,b 3.65 ± 0.29 60.1 ± 2.2a,b 3.86 ± 0.09 1.44 ± 0.2 12.1 ± 3.3a,b 0.77 ± 0.43 16.1 ± 0.7a,b,c 1.04 ± 0.06 0.24 ± 0.09 6.4 ± 0.9b 0.40 ± 0.14 2 ± 1.1a,c 0.13 ± 0.08 0.34 ± 0.05 5.1 ± 0.1 0.33 ± 0.04 5.2 ± 0.3 0.33 ± 0.02

36 ± 1.2 2.24 ± 0.38 30.9 ± 3.8 17.8 ± 3 1.09 ± 0.01 18 ± 1.5 46.2 ± 3.1 2.9 ± 0.50 51.1 ± 1.9 24.1 ± 2.6c 1.52 ± 0.37 21.9 ± 2.8a,c 5.7 ± 2.2 0.33 ± 0.14 3.7 ± 1.1 6 ± 0.8 0.38 ± 0.06 5.2 ± 0.9 4.9 ± 1.6 0.28 ± 0.07 5 ± 0.8 13.1 ± 0.9 0.81 ± 0.06 12.9 ± 0.9 long leg kicking style

relative durations ( %) absolute durations (s) relative durations ( %)

0.33 ± 0.07 5.3 ± 1.3 0.33 ± 0.06 4.2 ± 0.8 0.27 ± 0.06

3.7 ± 0.5

2.7 ± 1.2

6.7 ± 1.1

8.9 ± 1.1a

6.22 ± 0.96

Start Time (n)

Arm Strokes Undulations

(n) Phase

Underwater Aerial

Phase Phase

Swimming Leg Kicking

Phase Phase

Glide

Phase Phase

Entry Flight Block

Phase

6.45 ± 0.52 100

Table 2 Mean ± SD values for the relative (in % of the 15 m start time) and absolute durations of the start phases for the four clusters studied.

Underwater Phases

1.91 ± 5.52 29.3 ± 0.38 r = 0.293 p = 0.04 1.18 ± 0.12 17.7 ± 2.1 r = − 0.672 p = 0.001

Aerial Phases Swimming Phase

3.4 ± 0.52 52.6 ± 5.9 1.27 ± 0.37 19.7 ± 5.3

Leg Kicking Phase Glide Phase

0.28 ± 0.13 4.4 ± 2.1 0.34 ± 0.04 5.4 ± 0.7 r = − 0.436 p = 0.042

Entry Phase Flight Phase

0.31 ± 0.07 4.9 ± 1.2 r = − 0.504 p = 0.046 0.84 ± 0.06 13.0 ± 0.8 r = − 0.596 p = 0.001

Block Phase

Table 1 Mean ± SD values for absolute and relative durations of the start phases and their correlation with the total 15 m start time.

absolute duration (s) relative duration ( %) correlation with start time

Start Time

T

enough to maximize the impulse (I = ∫ Fdt) so that the swimtO mer leaves the block with the highest possible horizontal velocity [5]. The flight and entry phases were also negatively correlated with start time. According to previous studies [15, 28], the negative correlation between the aerial phases and the 15-m start time suggests that the swimmer needs to optimize the compromise between jump as far as possible and travel the maximum distance at the high velocity developed during the block phase. To do so, the swimmers must manage the compromise between a long time spent on the block to create more force and a short time on the block to minimize the time deficit [20]. In other words, the swimmers’ aerial trajectories resulted from a compromise between the pike and flat styles. Indeed, Ruschel et al. [26] showed a significant correlation between the 15-m start time and flight distance (r = − 0.482), angle of entry (r = 0.512), depth (r = 0.515), and the average velocity during the underwater phase (r = − 0.645). In our study, the underwater phase duration was significantly and positively correlated with the 15-m start time, indicating that this phase duration should be minimized, through an increase in both the flight reach and the gliding, kicking and swimming velocities. In a study of national level swimmers, Hubert et al. [15] showed higher speed (1.3–2.14 m . s − 1) before the beginning of the stroke than after (1.52–1.76 m . s − 1), which suggested that it might be better to adopt a streamlined position during the glide and to begin the stroke immediately upon reaching the average swimming speed, not later. Although several features were common to the whole sample of elite swimmers, great inter-individual variability was observed and four start profiles were distinguished, all of which led to similar 15-m start times. The first goal of the swimmer is to react to the starting signal and leave the block as quickly as possible at an appropriate take-off angle and with the highest possible forward velocity. The short glide style was characterized by the shortest time on the block, which could lead to low take-off and/or entry velocity and would thus explain the shortest observed glide time. The swimmers of this cluster compensated their short glide by favouring a long full swimming phase (arms + legs). Conversely, the long glide style showed the longest glide time consecutive to the longest block phase, which often leads to high take-off and/or entry velocities [5, 15]. Having a longer glide phase seems more economical because the swimmers do not need to act in order to move forward and remain in a hydrodynamic position. However it cannot be expected from swimmers with a short glide style to change to the long glide style. Indeed, knowing that all styles enabled to attain a non significant 15-m start time, it was suggested that the swimmers using a long glide style have a sufficiently effective hydrodynamic position to avoid an important loss of velocity during the glide. In fact, the authors subjective impression of these swimmers was that they were slimmer than the swimmers using the short glide style, supporting the theory that the coach should assess the swimmer’s passive drag to determine their hydrodynamic body characteristic before advising to use a long glide style. The swimmer’s second goal is to determine when to end the glide and begin leg kicking. Gliding instead of kicking is a recurrent compromise that explains 95 % of the start variance related to the underwater phase [1, 20, 24, 28]. Sanders and Byatt Smith [28] observed that most swimmers start their aquatic propulsion too early, causing higher energy cost. Indeed, Lyttle and Benjanuvatra [18] stated that when swimmers kick immediately after the dive entry, the drag created by deviating from



Zones IdC ( %) SR (stroke.min − 1) SL (m.stroke − 1) v (m.s − 1)

10–12.5 m 0.9 ± 3.3 62.1 ± 5.3 2.1 ± 0.2 2.17 ± 0.17

12.5–15 m 1.3 ± 1.2 58.4 ± 5 2.09 ± 0.1 2.03 ± 0.12

15–17.5 m a

1.8 ± 2.3 56.9 ± 4.4 2.05 ± 0.2 1.94 ± 0.11a

17.5–20 m 2.1 ± 1.6 56.4 ± 4.3 1.94 ± 0.1a,b 1.82 ± 0.11a,b

Table 3 Mean ± SD values of the stroking parameters for the 4 zones of 2.5 m.

Significant differences between groups are shown by a (with zone 10–12.5 m) and b (with zone 12.5–15 m), all for a p < 0.05 IdC: index of coordination, SR: stroke rate, SL: stroke length and v: velocity

the streamlined position is likely to offset the propulsive force created by kicking. Conversely, by waiting too long before initiating underwater kicking, the full benefits of the underwater kick will not be achieved. This was confirmed by Lyttle et al. [21] in a study in which swimmers were towed as they performed underwater kicks at velocities representative of those reached during the glide phase of a start. Therefore, experienced swimmers should glide for approximately 1 s before initiating any underwater kicking [21, 28]. In fact, the glide duration relates to the initial velocity of the swimmer. The swimmer needs to begin the aquatic propulsion below 2 m . s − 1 and remain in a streamlined position above this velocity [19–21]. In our study, the swimmers with a long glide style spent 0.40 s gliding (vs. 0.13 s for the short glide style), suggesting that all the swimmers began kicking prematurely and could have delayed the beginning of leg kicking. In fact, in order to be effective, swimmers must begin the stroke with a velocity at least equal to its stroking velocity [20]. This avoids having to accelerate during the first stroke cycles and thus to use stroke cycles to maintain velocity. Elipot et al. [12] showed that expert swimmers need 6 ± 0.52 m to decrease their speed to 2 m . s − 1, which suggests that leg kicking or undulations should start about 6 m from the pool edge. For national swimmers (6.97 s for 15-m start time), Pereira et al. [25] reported an underwater phase distance (from head entry to beginning of first arm stroke) of 5.75 ± 0.87 m and an underwater phase velocity of 2.70 ± 0.36 m . s − 1. In our study, the whole population reached a swim velocity of 2.17 m . s − 1 in the 10–12.5-m zone; they started full swimming before 10 m and gradually increased this velocity through the 25 m. Other researchers also showed that, for velocities between 1.9 and 3.1 m . s − 1, swimmers should glide at approximately 0.5 m underwater to take advantage of the reduced wave drag forces and that greater glide depth produces no substantial reductions in drag forces [20, 21]. Indeed, Pereira et al. [25] observed a mean depth of 1.10 ± 0.18 m, which was positively correlated with the 15-m start time (r = 0.515). The swimmer’s third goal is to determine when the transition from leg undulation to full swimming should occur. The swimmers with the long leg kicking style spent the longest time in the leg kicking phase (24.1 % of the 15-m start time) before starting full swimming (arms + legs) and also showed the highest number of undulations (3.7) during the 15-m. Conversely, the swimmers with the mixed “leg kicking/swimming” style had a shorter leg kicking phase and favoured the full swimming phase by using the greatest number of arm strokes. It could be suggested that the non-significant shorter 15-m start time of the long leg kicking style could result from their better leg kicking/full swimming transition, notably they started full swimming later and favoured the greater number of undulations. Moreover, one swimmer of the mixed “leg kicking/swimming” style used leg flutter kicking and not dolphin undulations. Clothier et al. [7] demonstrated less deceleration during underwater dolphin kicking than flutter kicking and found that the velocity above that of free swimming

was maintained longer when using the dolphin kick. Conversely, Lyttle et al. [20] found no difference in the net forces between underwater dolphin and leg flutter kicking for experienced swimmers in steady-state towing tests, although dolphin kicking showed a tendency to produce better results. In our study, the swimmer who used flutter kicking was a bronze medalist in the 100-m freestyle of the European Championships and had a 15-m start time similar to that of the other swimmers, which suggests that his leg kicking was not detrimental to performance. Finally, although these two clusters spent similar times in the underwater phase, the challenge was to determine when the transition from leg undulation to full swimming should occur. Further investigation of the instantaneous velocity and glide depth would provide more information on how to help swimmers manage the leg undulation phase. The swimming part seemed not to be influenced by the start part because the stroking parameters and the arm coordination were not significantly different between the four clusters. Indeed, even if some clusters used one or two leg undulations and 2 arm strokes more than other clusters, these components were not correlated with the 15-m start time. According to 50-m and 100-m race analyses [16, 29], v, SR and SL decreased for the whole sample of swimmers while IdC increased through the swimming part of the 25 m. This change of inter-arm coordination indicated that the swimmers increased the propulsive continuity of the two arms actions in order to boost their stroke after the start. The fact that only IdC increased through the 25 m resulted from a change in constraints encountered by the swimmer. The diving start provided high speed during which the swimmers first glided then swam with high stroke rate until the aquatic resistance became too high to maintain this technique. Thus whatever the start style used, all the swimmers have to boost their stroke and to create swimming velocity by achieving an effective arm coordination (i. e. an increase of IdC) to minimize the stroke length decrease [29, 30].

Conclusion & This analysis of the start phase using the grab start technique demonstrated that several motor profiles lead to similar elite performances. Four profiles were distinguished in accordance with the three main goals that should be considered for an effective start: (i) reach the best compromise between enough time spent on the block (creating a higher impulse) and the briefest time on the block to minimize the time deficit, (ii) determine when to stop the glide and begin leg kicking, and (iii) determine when the transition from leg undulation to full swimming should occur. This study also showed that the swimmers continuously increased their propulsion coordination, i. e. an increase in the IdC, through the 25-m lap.


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