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スポーツパフォーマンス研究, 12, 396-407, 2020
396
エリート競泳選手を対象とした水中ドルフィンキックのパフォーマンス向上
を目指したトレーニング介入: キック頻度と推進効率に着目して
仙石泰雄 1), 山川啓介 2), 角川隆明 1), 成田健造 3)
1) 筑波大学
2) 日本女子体育大学
3) 鹿屋体育大学
キーワード: 水中ドルフィンキック,パフォーマンス,トレーニング
【抄 録】
本研究は, 1 ーズンのトレーニング前後における水中ドルフィンキック動作を分析し, 水中ドルフィ
ンキックの泳速度の向上に関与するキネマティク変数を明らかにすることを目的とした. 本研究には,
くトレーニングされた大学競泳選手 32 名(男子 19 , 女子 13 名)が参加した. トレーニング期間前
Pre)と後(Post)の水中ドルフィンキックを 2次元動作解析し, 平均泳速度, キック頻度, キック幅,
トローハル数, アップキック所要時間およびダウンキック所要時間を分析した. 水中ドルフィンキックの
泳速度を向上するため, キック頻度の上昇と推進効率の改善を目指すトレーニングを継続的に実施
した. その結果, トレーニング期間前後において男子と女子ともに泳速度が有意に向上した. また,
レーニング前後においてキック頻度が有意に上昇したものの, キック幅は変化しなかったことが明らかと
なった. 本研究結果より, 水中ドルフィンキックの泳速度を向上させるには, キック幅を変化させること
なくキック頻度を上昇させることが必要であることが明らかとなり, 本研究で用いたトレーニング方法が
有効である可能性が示唆された.
スポーツパフォーマンス研究, 12, 396-407,2020 年,受付日: 2020 年 3 月 23 日,受理日: 2020 年 7 月 27
責任著者:仙石泰雄 筑波大学 つくば市天王台 1-1-1 sengoku.yasuo.ge@u.tsukuba.ac.jp
* * * *
Training to enhance underwater dolphin kick speed in elite
swimmers: focusing on kick frequency and propelling efficiency
Yasuo Sengoku1), Keisuke Kobayashi Yamakawa2),
Takaaki Tsunokawa1), Kenzo Narita3)
1) University of Tsukuba
2) Japan Women’s College of Physical Education
3) National Institute of Fitness and Sports in Kanoya
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Key words: underwater dolphin kick, performance, training
[Abstract]
The purpose of the present study was to analyze well-trained university swimmers’
underwater dolphin kick before and after one season’s training, and to examine
kinematic parameters related to any change in their underwater dolphin kick speed.
The participants, 32 elite university swimmers (19 male, 13 female), received
underwater dolphin kick training during the 2014 winter season. To analyze the
participants’ underwater dolphin kick performance and to evaluate the effectiveness of
that season’s training, motion analysis was conducted at the beginning (Pre) and the
end (Post) of the training season. The average swimming speed, kick frequency, kick
amplitude, Strouhal number, and up-kick and down-kick durations were measured.
The average swimming speed increased significantly in both the male (Pre, 1.73 ±0.11
m/sec; Post, 1.81 ±0.11 m/sec; p<.01) and the female swimmers (Pre, 1.49 ±0.08
m/sec; Post, 1.58 ±0.08 m/sec; p <.01). Kick frequency also increased significantly in
the male (Pre, 2.42 ± 0.29 m/sec; Post, 2.67 ±0.34 m/sec; p<.01) and the female
swimmers (Pre, 2.06 ± 0.26 m/sec; Post, 2.38 ± 0.23 m/sec; p<0.01), however, none of
the swimmers’ kick amplitudes changed significantly. These data suggest that the
improvement found in the participants’ average swimming speed resulted from an
increase in the swimmers’ kick frequency, as their kick amplitudes did not change
significantly.
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1. Introduction
In addition to the four major swimming styles, butterfly, backstroke, breaststroke,
and front crawl, the underwater dolphin kick is performed as a propelling swimming
technique during competitive swimming races. The underwater dolphin kick has been
described as the ‘fifth stroke’ (Collard et al., 2013), and has been investigated by several
studies (Arellano et al., 2002, Atkison et al., 2014, Connaboy et al, 2009, Connaboy et
al., 2014, Shimojo et al., 2014). Using the underwater dolphin kick, the high velocity
acquired after starts and turns can be maintained longer compared to swimming at
the surface (Arellano et al., 2002). Therefore, race time can be improved using this
underwater technique.
To clarify factors that create a high underwater dolphin kick velocity, many cross
sectional kinematical studies have been conducted (reviewed by Connaboy et al., 2009).
These studies conclude that kick frequency, but not kick amplitude, is related to the
high underwater dolphin kick velocity. In other words, faster underwater dolphin kick
swimmers demonstrated higher kicking frequency compared to slower participants,
and no statistical difference in kick amplitude was observed between the fast and slow
swimmers. Furthermore, Zamparo et al. (2012) reported that higher propelling
efficiency was significantly related to underwater dolphin kick velocity after a turn.
These results suggest that swimmers should increase the kick frequency and/or
improve propelling efficiency to achieve higher underwater dolphin kick performance.
Previous studies investigated dolphin kick velocity in the same individual by
changing the kick frequency in sync with a metronome sound (Shimojo et al., 2014;
Yamakawa et al., 2017). Both studies reported that the swimmer's dolphin kick velocity
did not increase even when the swimmer attempted higher kick frequency than that of
the maximum effort trial. This was because the kick amplitude and propelling efficiency
decreased when the swimmers increased the kick frequency above the maximum effort.
Furthermore, Connaboy et al. (2014) conducted a four week training intervention to
clarify the training effect on maximal underwater dolphin kick speed. They could not
achieve an improvement in underwater dolphin kick velocity after a four-week training
period, and were unable to investigate factors relating to an underwater dolphin kick
velocity increase in the same individual. Therefore, the kinematic parameters related
to the enhancement in underwater dolphin kick velocity remain unclear.
As described above, there is scientific evidence reporting that higher kick frequency
is related to faster underwater kick velocity and that propelling efficiency during
underwater dolphin kick is important. However, there is a paucity in information about
the actual training methodology that can enhance underwater dolphin kick
performance.
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The primary objectives of this study were to analyze underwater dolphin kick motion
before and after one training season in well-trained university swimmers, and to clarify
the kinematical parameters related to the enhancement of underwater dolphin kick
velocity. A second objective was to develop training instruction that can improve
underwater dolphin kick performance. We hypothesized that underwater dolphin kick
velocity can be increased by increased kick frequency training and improvement in
propelling efficiency during a one season training period.
2. Methods
2.1. Participants
Well-trained university swimmers, 19 males and 13 females, participated in this
study (Males: mean age, 20.5 ± 1.3 years; mean height, 1.76 ± 0.07 m; mean weight,
70.2 ± 5.7 kg; Females: mean age, 20.3 ± 1.3 years; mean height, 1.62 ± 0.05 m; mean
weight, 56.0 ± 4.7 kg), and all of the swimmers had competed at national level
competition. Their specialized swimming style was Freestyle (n=11), Butterfly (n=6),
Backstroke (n=5), Breaststroke (n=6), and Individual Medley (n=4). The swimmers
trained for the same university swimming team. Participants trained 9-10 swim
sessions per week with an average swimming distance of 36 km per week and
participated in four strength training sessions per week. The experimental procedures
of this study were approved by the local ethics committee and informed consent was
obtained from the participants.
2.2. Underwater dolphin kick analysis
To analyze the participants’ change in underwater dolphin kick performance,
underwater motion analysis was conducted at the starting point of the winter training
season (October 2014; Pre) and at the end (February 2015; Post) of the intensive
training period. The main competitions for the investigated swimming team was March
and beginning of April.
Analyses were conducted in an indoor swimming pool (50 m, 1.35-1.80 m depth).
Before the experiment, the swimmers performed a middle-intensity 1000 m warm-up
swim. Participants performed a 15 m maximal underwater dolphin kick twice, in
accordance with the method by Shimojo et al., (2014). The underwater dolphin kick
was performed approximately 1.0 m under the water surface to exclude the effect of
wave drag (Lyttle et al., 2000).
2.3. Kinematic variables
The epiphysis of the fifth metatarsal (toe) and the greater trochanter of the femur
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(hip) were marked with LEDs (Kirameki, Nobby Tech Inc.) on the right side of the
swimmer. The experimental setting is described in Figure 1. One video camera (High
speed 1394 Camera Bcam, DKH Inc.) was positioned lateral to the swimmer for 2-
dimensional motion analysis at an underwater window to film the swimmer’s dolphin
kick motion at a 100 Hz sampling rate.
Figure 1 Schematic of the experimental setting. Underwater dolphin kick motion was filmed
between 8 m and 14 m distance after pushing off the wall.
The coordinates of all landmarks were digitized and converted to global coordinates
using the 2-D direct linear transformation method. The calibration for 2-D motion
analysis was conducted by filming a known length object. The length of calibration
objects were 1.40 m and these were set vertically at 1 m intervals between 8 m to 14
m from the start point. The calibration was carried out at every trial, throughout the
experiment. The standard errors between the actual value and the calculated value of
the calibration points were 0.008 ± 0.002 m. One-kick cycle was defined as beginning
at the highest vertical peak for the toe position and ending at the next highest peak.
One-kick cycle was divided into down-kick phase and up-kick phase according to the
vertical movement of the toe. According to Connaboy et al. (2010), all three data
obtained from consecutive three kick cycles were used for analysis. The trial with the
highest average swimming velocity was used for analysis.
Kick frequency (Hz) and kick amplitude (m) were calculated from the toe coordinates.
Kick frequency was determined from the duration of one-kick cycle, whereas kick
amplitude was the vertical displacement of the toe coordinates between the highest
and lowest position of one-kick cycle (Shimojo et al., 2014). The average swimming
velocity (m· sec-1) was the average horizontal hip velocity during one-kick cycle.
Duration of the up and down-kick phase (sec) was investigated.
To evaluate the propelling efficiency of the underwater dolphin kick, the Strouhal
number (St) was investigated (Connaboy et al., 2009). The St was calculated as:
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St = f · Atoe · U-1
where f is kick frequency, Atoe is kick amplitude, and U is average swimming velocity.
St is a dimensionless number that describes the kick amplitude normalized to the
progression given by the swimming velocity and kick frequency ratio. Past literature
reported that maximum efficiency in undulatory movement is achieved with an St range
of 0.25 to 0.4 in fish and cetaceans (Anderson et al., 1998; Triantafyllou et al., 1993).
However, the St value of underwater dolphin kick in humans is found to be outside the
range of 0.4 (Nicolas et al., 2007). In competitive swimmers, Arellano et al. (2002)
reported that the St between international level swimmers and national level swimmers
was significantly different, 0.79 vs 0.95, respectively.
2.4. Instruction for enhancing underwater dolphin kick performance
All participants were instructed to enhance their underwater dolphin kick
performance by the following two methods. The two methods were adopted according
to previous studies reporting that the higher kick frequency is related to faster
underwater kick velocity (Connaboy et al., 2009) and that propelling efficiency during
underwater dolphin kick is important (Zamparo et al., 2012).
First, Shimojo et al. (2014) reported that an effective undulatory mode can be
achieved by decreasing kick frequency 15% compared to the frequency at maximum
effort. Furthermore, as Nakashima (2009) reported, both swimming velocity and
propulsive efficiency can be improved by undulating the trunk. Therefore, the
swimmers attempted to conduct an effective dolphin kick technique by repeating
submaximal underwater dolphin kicks in the prone position (Figure 2-A), and dolphin
kicks at the water surface in the supine position (Figure 2-B). The swimmers swam 50
m at each posture alternatively, repeated it twice. During this 200 m swim, swimmers
were instructed to move their chest forward and backward using the waist for the
dolphin kick motion.
Second, high kick frequency is reported to relate significantly to high underwater
dolphin kick velocity (Connaboy et al., 2009). Swimmers performed a maximum effort
underwater dolphin kick with an elastic cord attached to their waist (Figure 2-C;
Sengoku et al., 2014). After pushing off from the wall, swimmers were instructed to
maximize their kick frequency against the elastic cord for five kick cycles. The
swimmers repeated this exercise four times with adequate rest in between.
The underwater dolphin kick training protocols described were included in the
ordinary training program by the swimming coach two to four times every week through
the experimental period.
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Figure 2 Demonstration of the training instructions prescribed to enhance
the underwater dolphin kick performance. A) submaximal underwater
dolphin kicks in the prone position, B) dolphin kicks at the water surface
in the supine position, and C) maximum effort underwater dolphin kick
with an elastic cord attached to the swimmer’s waist.
2.5. Statistical analysis
All kinematic variables are presented as the mean and the standard deviation (Mean
± SD). Two-way repeated measures ANOVA with Tukey’s post-hoc tests were used to
assess the difference between the factor of time (Pre vs. Post) and sex (male vs. female).
A p value < .05 was considered statistically significant. Statistical analyses were
conducted using SPSS for Windows 25.0 (IBM Inc.).
3. Results
The results of the two-way ANOVA are shown in Table 1. After a one month training
season, significant main effect of time was observed with the average swimming velocity
(p < .01), indicating that the underwater dolphin kick velocity was significantly
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improved in both genders. Furthermore, a significant main effect of time was observed
with the kick frequency, down-kick duration and up-kick duration (p < .01), suggesting
that subjects increased kick frequency by decreasing the duration of both down- and
up-kick. The St showed a significant main effect of time (p < .01), indicating that
propelling efficiency decreased after training in both genders. Significant change was
not observed with the kick amplitude. No difference in the change of underwater
dolphin kick kinematics between gender was observed, as there was no significant
interaction in all variables.
Table 1 The kinematical variables before and after the four month training period.
4. Discussion
The primary purpose of this study was to analyze the underwater dolphin kick
motion in well trained university swimmers before and after a one training season, and
to clarify the kinematical parameters related to the change in underwater dolphin kick
velocity. Our findings demonstrate that enhancement in average swimming speed was
achieved by increasing kick frequency while the kick amplitude was not significantly
different.
The participants’ underwater dolphin kick velocity at Pre was greater than that of
previous reports including reports examining well-trained male swimmers (Atkison et
al., 2014; Shimojo et al., 2014) and female swimmers (von Loebbecke et al., 2009).
These results suggest that the swimmers’ underwater dolphin kick performance in the
present study was relatively higher at the Pre condition compared to the subjects in
previous studies. After the one season training period, the average dolphin kick velocity
showed a significant increase in both genders.
A significant change in kick frequency was observed in male and female swimmers,
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while the kick amplitude was not statistically different after the one season training
period. Arellano et al. (2002) investigated the underwater dolphin kick performance
difference between the international and national level swimmers and reported that the
international level swimmers showed a higher average velocity with a higher kick
frequency in men and women over that of the national level swimmers. Previous cross-
sectional studies (reviewed by Connaboy et al., 2009) demonstrated that high kick
frequency was related to faster underwater dolphin kick velocity, which suggests the
importance of training to increase kick frequency. From above, a training method to
maximize kick frequency in a short duration may lead to increased underwater dolphin
kick frequency, resulting in enhancing underwater dolphin kick performance.
To increase kick frequency, swimmers shorten the duration of the up-kick and/or
down kick-phase. Our results suggest that the subjects increased the kick frequency
by decreasing the duration of both down- and up-kick. Previous studies investigating
the underwater dolphin kick using particle image velocimetry reported that a strong
backward and downward jet is generated during the down-kick, indicating that a
higher propelling force is produced by the down-kick (Hochstein and Blickhan, 2011;
Shimojo et al., 2019). Furthermore, Atkinson et al. (2014) reported that maximal
horizontal velocity was observed during the down-kick phase, however, faster
swimmers spent less time at the up-kick than slower swimmers, and the highest
correlation was observed between average velocity during the up-kick and average
horizontal swimming velocity. It was suggested that well-trained swimmers should
decrease both down- and up-kick duration to improve underwater dolphin kick
performance regardless of gender.
Velocity during the underwater dolphin kick is affected by the combination of kick
frequency and kick amplitude. Therefore, it is important to note that average velocity
will not change if the magnitude of the increase in kick frequency is equivalent with
the decrease in kick amplitude. This is consistent with results of previous studies
reporting that even if the kick frequency was increased immediately in sync with a
metronome sound, the underwater dolphin kick velocity did not increase because the
kick amplitude decreased (Shimojo et al., 2014; Yamakawa et al., 2017). The present
study suggests that swimmers could increase the kick frequency while keeping the
kick amplitude non-statistically different during longer training durations. Instructing
the swimmers not only to increase the kick frequency, but also to be aware of propelling
efficiency, may prevent decrement in the kick amplitude during maximum effort
underwater dolphin kick.
The St showed an increase in male and female swimmers, suggesting that the
propelling efficiency in these swimmers decreased with training. This result disagreed
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with our hypothesis. The Post St in the present study was lower than that reported by
Arellano et al. (2002), indicating a high efficiency level in our swimmers. However, the
increase in St was contrary to the training purpose that aimed to improve propelling
efficiency by undulating the trunk during the submaximal velocity swim (Nakashima,
2009; Shimojo et al., 2014). Yamakawa et al. (2017) reported that the trunk muscular
activation pattern is related to propelling efficiency. Therefore, the movement of the
swimmers’ trunk should be investigated in detail to evaluate their underwater dolphin
kick technique. By this further investigation, the relationship between the change in
kick frequency and the propelling efficiency may be clarified.
The training instructions prescribed to the participants in the present study was
twofold: a submaximal frequency swim to obtain efficient trunk undulatory movement,
and a maximal frequency stimulus using the elastic cord. The previous training
intervention study (Connaboy et al., 2014) only focused on training at a preferred cycle
frequency decided by the swimmer, and could not result in improving underwater
dolphin kick velocity. The present study did not clarify which training instruction was
effective, however, the combination of the two instruction methods may result in the
enhancement of underwater dolphin kick velocity.
The data obtained in this study is an evaluation from a 15 m maximal effort swim
and may differ from the performance during actual competition conditions, which is a
limitation of the present study. Furthermore, in addition to the underwater dolphin
kick training, each swimmer engaged in different types of swim training depending on
their specific swimming style and race duration. Therefore, we could not clarify the
exact training effect from the three underwater dolphin kick instruction introduced in
this study. However, 17 of 19 male swimmers and all 13 female swimmers enhanced
their maximal average swimming speed. This is the first study to clarify the kinematical
variables associated with enhancement in underwater dolphin kick velocity in the same
individual. Further research, including a more precise motion analysis of the
underwater dolphin kick, is needed to investigate the training outcome from each
training instruction.
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Thrust-producing harmonically oscillating foils are studied through force and power measurements, as well as visualization data, to classify the principal characteristics of the flow around and in the wake of the foil. Visualization data are obtained using digital particle image velocimetry at Reynolds number 1100, and force and power data are measured at Reynolds number 40 000. The experimental results are compared with theoretical predictions of linear and nonlinear inviscid theory and it is found that agreement between theory and experiment is good over a certain parametric range, when the wake consists of an array of alternating vortices and either very weak or no leading-edge vortices form. High propulsive efficiency, as high as 87%, is measured experimentally under conditions of optimal wake formation. Visualization results elucidate the basic mechanisms involved and show that conditions of high efficiency are associated with the formation on alternating sides of the foil of a moderately strong leading-edge vortex per half-cycle, which is convected downstream and interacts with trailing-edge vorticity, resulting eventually in the formation of a reverse Kármán street. The phase angle between transverse oscillation and angular motion is the critical parameter affecting the interaction of leading-edge and trailing-edge vorticity, as well as the efficiency of propulsion.
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In this study, we investigated the effects of increased kick frequency on the propelling efficiency and the muscular co-activation during underwater dolphin kick. Participants included eight female collegiate swimmers. The participants performed seven 15-m underwater dolphin kick swimming trials at different kick frequencies, which is 85, 90, 95, 100, 105, 110, and 115% of their maximum effort. The Froude (propelling) efficiency of the dolphin kick was calculated from the kinematic analysis. The surface electromyography was measured from six muscles (rectus abdominis, erector spinae, rectus femoris, biceps femoris, tibialis anterior, and gastrocnemius). From the EMG data, the co-active phase during one cycle in the trunk, thigh, and leg was evaluated. Our results show that the Froude efficiency decreased at the supra-maximum kick frequency (e.g. 100%F: 0.72 ± 0.03 vs. 115%F: 0.70±0.03, p < .05). The co-active phase in the trunk, thigh, and leg increased with increasing the kick frequency (e.g. 85%F vs. 115%F, p < 0.05). Furthermore, it was observed that there was a negative relationship between the trunk co-active phase and the Froude efficiency (r = −0.527, p < 0.05). Therefore, both the propelling efficiency and the muscular activation pattern became inefficient when the swimmer increased their kick frequency above their maximum effort.
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Undulatory underwater swimming (UUS) is an important swimming technique after a start and after turns. It was considered that a higher swimming velocity (U) resulted from a higher kick frequency (f), and greater propelling efficiency, i.e., Strouhal number (St) and Froude efficiency (gF), resulted from a lower f. The aim of this study was to investigate whether changing f affected U and St, gF plus other kinematics of UUS. Ten national-level male swimmers participated in the study. First, the swimmers performed maximal UUS (Pre; this f was defined as 100% F). Second, the swimmers synchronized their f with the sound of a metronome and with six frequencies (85% F, 90% F, 95% F, 105% F, 110% F, and 115% F) randomly presented. During the higher f sessions, kick amplitude (A) significantly decreased from Pre (115% F: -10.8%, p < .05); however, U was unchanged. In contrast, in lower f sessions, St and gF were unchanged, but the wavelength per body length (kBL), which indicates UUS mode, significantly decreased (90% F: -1.3%, p < .05). In conclusion, these results suggest that increasing f for UUS would not affect U, but a decrease in f may be suitable for human undulation training.
Article
The objectives of this study were to investigate the effect of trunk undulation on the swimming performance in underwater dolphin kick, and to clarify the ideal trunk undulation form. The reference swimming motion of an elite swimmer was firstly acquired from the video analysis, and input into the swimming human simulation model SWUM, which had been developed by the authors. The trunk motion was next optimized by the simulation for three objective functions: maximizing swimming speed, maximizing propulsive effciency, and minimizing fluid force acting on hands. The following findings were obtained: In the case of maximizing swimming speed, the whole body forms a 'C' shape due to the in-phase trunk undulation. The swimming motion of maximizing propulsive efficiency and the reference swimming are considerably similar to each other. In both cases, the trunk moves as a seesaw with a node; whereas, the lower limbs form a traveling wave in the absolute space. The values of propulsive efficiency are around 0.2 in the cases of maximizing propulsive efficiency, minimizing fluid force on hands, and the reference swimming. The swimming motion in the case of minimizing fluid force on hands is almost the same as that of maximizing propulsive efficiency. The trunk undulation with the appropriate amplitudes and phases, especially bending at the chest, is important in realizing the swimming motion which maximizes propulsive efficiency.
Article
The purposes of this article were to establish the reliability of the kinematics of maximal undulatory underwater swimming (UUS) in skilled swimmers, to determine any requirement for familiarization trials, to establish the within-subject (WS) variability of the kinematics, and to calculate the number of cycles required to accurately represent UUS performance. Fifteen male swimmers performed 20 maximal UUS trials (two cycles per trial) during four sessions. The magnitude of any systematic bias present within the kinematic variables was calculated between session, trial, and cycle. Random error calculations were calculated to determine the WS variation. An iterative intraclass correlation coefficient (ICC) process was used to determine the number of cycles required to achieve a stable representation of each kinematic variable. Significant differences were found between session 1 and all other sessions for several variables, indicating the requirement for a familiarization session. Results indicated a wide range of WS variation (coefficient of variation [CV] = 1.21%-12.42%). Reductions in WS variation were observed for all variables when the number of cycles of data used to calculate WS variation was increased. Using six cycles of data, including additional cycles of data, provided diminishing returns regarding the reduction of WS variation. The ICC analysis indicated that an average of nine cycles (mean ± SD = 9.47 ± 5.63) was required to achieve the maximum ICC values attained, and an average of four cycles (mean ± SD = 3.57 ± 2.09) was required to achieve an ICC of 0.95. After determining the systematic bias and establishing the requirement for a familiarization session, six cycles of data were found to be sufficient to provide high levels of reliability (CV(TE) = 0.86-8.92; ICC = 0.811-0.996) for each of the UUS kinematic variables.
Article
Strouhal number (St) corresponds to a wake parameter based on the tail kinematics of swimming animals according to St=A.f/U, where f is the stroke frequency, A is the fin-beat peak-to-peak amplitude, and U is the forward speed. St number is a trade-off between amplitude and frequency that generates a forward velocity. This parameter may therefore be affected by swimming technique, which can be evaluated through active drag (AD) and Froude efficiency (eta(F)). The aim of our study was therefore to (1) investigate the range of AD and eta(F) values for high level monofin-swimmers, (2) explore the range of St numbers, and (3) examine to what extent this latter non-dimensional parameter may affect AD and eta(F). To this end, experiments have been conducted on 12 international level monofin-swimmers. St number, eta(F) (calculated according to the elongated-body theory), and AD (computed with velocity perturbation method) were calculated at the same time for an underwater fin-swimming trial, at maximal speed. Lowest values of St numbers (St=0.34) corresponded to fin-swimmers with highest velocities (R=-.77, p<.05), highest efficiencies (R=-.86, p<.001), and lowest active drag (R=.91, p<.001). On the one hand, AD was increased with vertical motion of the fin (R=.84, p<.001), which increased cross-sectional area (R=.78, p<.05). On the other hand, efficiencies showed a tendency to peak at eta(F)=0.82 in a narrow range of St numbers close to 0.4. This St range corresponded to the upper limit of the 0.25-0.4 range usually claimed for maximum efficiency of marine and flying animals. Such results suggest that increasing efficiency and reducing drag improves performance per se (regardless how these parameters are related with St number).