Relationship of isokinetic strength,power and ankle flexibility on underwater dolphin kick speed in competitive collegiate swimmers

Abstract

Background

Several factors can contribute to increasing dolphin kick speed that muscle activation and simulation studies have been conducted. Although driving force can be generated by lower extremity strength and power, the relationship of isokinetic strength and power on dolphin kick speed remains unclear. Therefore, to examine a cause-and-effect relationship whether isokinetic strength and power, and ankle flexibility can contribute to increasing dolphin kick speed.

Objective

Twenty-one competitive collegiate male swimmers participated in this study.

Methods

The isokinetic strength (peak moment, Nm/kg) and power (time to peak moment, sec) of lower extremity, and ankle plantarflexion and dorsiflexion range of motion (ROM) were measured. Dolphin kick speed was measured by underwater video cameras (m/s).

Results

Underwater dolphin kick speed was affected by lower extremity strength and power of knee flexion, knee extension, and ankle dorsiflexion. Moreover, time to peak moment of knee flexion, knee extension, and ankle plantarflexion were also influencing factors.

Conclusions

This study suggests that velocity-based training could be suitable for improving strength and power for swimmers as the fast contraction speed of knee and ankle measures appeared to affect the dolphin kick speed in this study.

Keywords

swimming performance swimming training physical function ankle range of motion training

1. Introduction

Underwater dolphin kicks reduce a frictional wave resistance between the body and water, which decreases drag force during underwater swimming by 50–60%¹ and improves the efficiency of the thrust force.^2,3 As such, underwater dolphin kicks are only allowed up to 15 m after the start and after every turn, which accounts for 30% of a total distance during the race. High performance of underwater dolphin kicks plays a vital role in a faster race time.^4,5 For example, an averaged race speed was 1.12–1.85 m/s at the 2004 Athens Olympic Games, the 2013 FINA World Championships, and/or the 2016 European Swimming Championships although averaged speed of underwater dolphin kicks was 2.07–2.44 m/s during the events. This indicates that the average race speed was 32–85% faster during the underwater dolphin kicks.^6,7 Moreover, due to less frictional wave resistance, spending less time on the water surface (underwater dolphin kicks) resulted in 6.4% faster race records in the European Swimming Championships held between 1982–2010.⁵ The underwater dolphin kick requires a streamlined position consisting of raising upper arms above the head and placing fingers over fingers.⁸ The underwater dolphin kicks can be performed in prone, supine, and/or lateral positions, but the prone position is mostly used and commonly seen in freestyle, backstroke, and/or butterfly.⁹ Specifically, during the underwater dolphin kicks, there is a flexion-kick (up-kick), where the hip, knee, and ankle are flexed toward the water surface although during the extension-kick (down-kick) the hip, knee, and ankle are extended toward the bottom of the pool.³

A majority of previous studies on underwater dolphin kicks were simulation analyses^3,4,9–15 and/or electromyography (EMG) muscle activation analyses.^16–18 Lower extremity muscle activation is correlated with the underwater dolphin kick speed.^16–19 Higher muscle activation during the dolphin kick is positively correlated with the dolphin kick speed including the biceps femoris (71.6%), tibialis anterior (70.9%), rectus femoris (64.7%), and/or gastrocnemius (63.6%).^17,20 Specifically, the biceps femoris was highly activated during the flexion-kick although the quadriceps was highly activated during the extension-kick.^16,21 Moreover, a positive relationship was found between the higher co-activation (the biceps femoris and rectus femoris) and the dolphin kick speed (r = 0.42).¹⁷ From the simulation analysis studies, the role of the flexion-kick and extension-kick was analyzed^{3,4,10,13–15} where the extension-kick increased the dolphin kick speed by 7–9%.^3,11 Importantly, the extension-kick motion was 63% faster than the flexion-kick as the propulsive force can be generated by the extension-kicks’ speed and power.²² However, as the thrust force can be generated by the entire lower extremity strength and power, it remains unclear how isokinetic dynamometry strength and power of the lower extremity affects underwater dolphin kick speed.²³ A few studies have examined the effect of lower extremity strength on the dolphin kick speed.^21,24 However, limitations of these strength measures include only considering knee strength,²⁴ ankle isometric strength measured by a hand-held dynamometer.²¹ Since underwater dolphin kick performance is accomplished by the entire lower extremity kinetic chain in the sagittal-plane muscles, and thus, the entire lower extremity of ankle, knee, and hip flexor and extensor should be measured in a concentric contraction mode at different contraction speed (slow and fast).

A research gap exists in kick studies in the literature. For example, most previous studies on simulation and muscle activation used correlation analysis.^{3,4,9,10,13,15,17,18,25} Previous studies could not provide a cause-and-effect relationship to identify contributing factors for underwater dolphin kick performance. Researchers should identify key factors for enhancing underwater dolphin kick speed.²¹ Specifically, the extension-kick and flexion-kick are essential for generating a driving force through up and down amplitudes and pitching during the underwater dolphin kicks. As such, lower extremity strength and power would play a crucial role in the underwater dolphin kick.²⁶ As substantial positive correlation was found between resistance training and swimming speed (r = 0.93).²⁷ it is needed to investigate the effect of isokinetic strength and power on dolphin kick speed performance. Importantly, identifying of lower extremity muscles coordination during peak moment (strength) and time to peak moment (power) could help develop individualized training for competitive swimmers.

Isokinetic dynamometry assessment is reliable and has been used as a gold standard for strength and power measures. As a strong association was found between quadriceps strength and fin swimming speed (similar to the dolphin kick) (r = 0.82),²² profiling of isokinetic strength and power in the lower extremity muscles could be beneficial in improving the dolphin kick performance. Moreover, time to peak moment measures would be an alternative muscular power measure to demonstrate how rapidly involved muscles can reach the peak moment in a given time.²⁸ However, no studies have examined the effect of isokinetic strength and power of the entire lower extremity muscles on the underwater dolphin kick speed. Additionally, most swimming coaches believe that ankle flexibility significantly influences the driving force of the dolphin kick.²⁹ However, conflicting results exist between the dolphin kick speed and ankle plantarflexion flexibility.^{11,21,25,29,30} Thus, it remains unknown whether ankle flexibility could contribute to enhancing dolphin kick performance. Theoretically, the plantarflexed position of the ankle would make a streamlined posture⁸ and could generate a propulsive driving force during the flexion-kick (down-kick). Although some previous studies show a correlation between ankle flexibility and the dolphin kick speed, more data are needed to solidify a cause-and-effect relationship between two variables via multiple logistic regression.

The primary purpose of this study was to examine the relationship of (i) ankle flexibility (plantarflexion and dorsiflexion ROM), (ii) isokinetic concentric contraction strength (peak moment), and (iii) power (time to peak moment) of ankle, knee, and hip muscles in the sagittal-plane at slow (60°/s) and fast (180°/s) speed on dolphin kick speed using a multiple logistic regression. The secondary purpose of the study was to examine a correlation relationship between aforementioned factors and the dolphin kick speed. This study would help understand the crucial factors affecting the underwater dolphin kick speed and provide useful information to develop training for individuals who want to improve the dolphin kick speed in competitive collegiate male swimmers.

2. Methods

2.1. Study design

This study was designed as a cross-sectional study to explore a cause-and-effect relationship to determine what factors (ankle ROM and isokinetic strength and power of the lower extremity in the sagittal-plane) could contribute to the underwater dolphin kick speed. As a sub-analysis, a correlation relationship between aforementioned factors and the dolphin kick speed was conducted. Participants reported to a research center twice (2 - days apart) for main outcome measures (Day 1= ROM and isokinetic measures in biomechanics lab, Day 2 = dolphin kick speed measures in the swimming center). The independent variable was a group (collegiate male swimmers) while the dependent variables were (1) the dolphin kick speed using underwater video analysis, (2) ankle ROM (dorsiflexion and plantarflexion), and (3) isokinetic strength (peak moment) and (4) power (time to peak moment) of the lower extremity muscles in the sagittal-plane. The isokinetic were measured in a concentric contraction mode at slow (60°/s) and fast (180°/s) speed.

2.2. Participants

A total of 25 healthy, competitive, collegiate male swimmers (age = 20.9 ± 1.3 years, height = 178.5 ± 4.3 cm, mass = 78.1 ± 6.7 kg, career = 10.1 ± 1.8 years) agreed to participate in this study. Twenty five participants initially enrolled in this study, four participants were withdrawn during data collection due to personal reasons (e.g., a discomfort feeling from pre-existing injury, low physical conditioning status, and/or moderate infection symptoms), and was therefore excluded from statistical analyses. Subject demographic information is shown in Table 1. Subject exclusion criteria were those who had previous fracture, dislocation and/or surgery in their lifetime, and had recent musculoskeletal injuries in the past 3 months. Prior to their participation, subjects were informed of the purpose, risks, and experimental procedures. All subjects signed a written informed consent form prior to their participation. This study was approved by the University's Institutional Review Board Research Ethics Committee.

Table 1.
Subjects demographics.

Age (yrs) Height (cm) Mass (kg) Career (yrs) Dolphin kick Ankle ROM

Speed (m/s) Record (sec) Plantarflexion (°) Dorsiflexion (°)

Male (n = 21) 20.7 ± 1.3 180.5 ± 4.3 79.1 ± 6.7 10.1 ± 1.8 1.99 ± 0.20 7.51 ± 0.88 51.8 ± 5.7 21.2 ± 5.1

	Age (yrs)	Height (cm)	Mass (kg)	Career (yrs)	Dolphin kick	Ankle ROM
Male (n = 21)	20.7 ± 1.3	180.5 ± 4.3	79.1 ± 6.7	10.1 ± 1.8	1.99 ± 0.20	7.51 ± 0.88	51.8 ± 5.7	21.2 ± 5.1

2.3. Sample size

A feasible sample size was calculated via G*Power Ver. 3.0.10 (G*Power, Franz Faul, Universität Kiel, Germany; Test family = Exact test, Statistical test = Correlation; Bivariate normal model, Type of power analysis = α priori). Based on a previous study examining ankle plantarflexion ROM, ankle maximal strength and the dolphin kick speed in elite swimmers,²¹ a sample size of 25 subjects were selected based on α error probability = 0.05, power (1-β error probability) = 0.8, and correlation H₁= 0.532 from the previous study.

2.4 Procedures

1) Joint Range of Motion (ROM)

Ankle plantarflexion and dorsiflexion ROM (Intra-rater reliability ICC: plantarflexion = 0.85, dorsiflexion = 0.91) was measured using previous methods.³¹ Specific measure procedures were described previously.³² Ankle joint ROM was measured three times from the dominant leg. Averages of the three trials were used for data analysis.

2) Isokinetic measurement

The isokinetic measurement procedures were based on previous studies.^33,34 Subjects had a practice session prior to a testing session where the subjects performed their maximum muscle contraction at 25%, 50%, 75% and 100% of their maximum muscle contraction as a gradual warm-up suggested by a previous study.³⁵ In order to reduce measurement errors of the isokinetic measurement during the testing session, we used less than 5% coefficient variation values within testing trials in each subject. If the coefficient variation did not meet the criteria, subjects had a 3–5 min break, and repeated the procedures. As all subjects had their isokinetic measurement testing on a regular basis, at least twice per year previously, most subjects completed their testing successfully with no more than 3 repeated testing sessions. Specifically, we measured isokinetic strength of ankle (plantarflexion and dorsiflexion), knee (extension and flexion), and hip (extension & flexion) peak moment (normalized by body weight %) and time to peak moment (sec) with angular speed of 60°/s and 180°/s using the isokinetic dynamometer (CSMi, Cybex, Humac Co., NY, USA; 100 Hz). Measurements positions and procedures were presented (Figure 1). Subjects performed 3 trials and each trial had 3 repetitions of the isokinetic measurement from the dominant leg. Averaged values of each isokinetic measurement were used for data analysis.

Figure 1.

Isokinetic dynamometer strength and power measures.

3) Underwater Dolphin Kick Speed

An underwater dolphin kick speed was measured based on previous studies using video analysis.^15,17 Subjects performed 5–10 practice trials and performed 5 testing trials. The fastest 3 testing trials of 5 trials were averaged and used for data analysis. Specifically, the underwater dolphin kick speed was measured from a time when a subject reached their peak knee flexion to a time when the fingers reached the 15-m line. We used two waterproof cameras (Logitech HD Webcam C270, Lausnne, Switzerland; 30fps). The dolphin kick speed was calculated using Dart Trainer software (version 2.5.3.62; Dartfish Inc., Fribourg, Switzerland) that we can calculate the time under two decimals.¹¹ The camera set-up of the underwater video analysis is presented (Figure 2).

Figure 2.

Camera set-up for underwater dolphin kick speed analysis.

2.5. Statistical analysis

The Shapiro-Wilk test was performed to verify the normality of the collected data. Multiple regression analyses using a backward conditional selection was performed to obtain a suitable model to explain the relationship of ankle ROM, isokinetic strength (peak moment) and power (time to peak moment) of hip, knee, ankle muscles in the sagittal plane on the underwater dolphin kick speed. We also performed Pearson's r correlation analysis between variables. The SPSS/PC18.0 program for Windows was used and α was set at 0.05.

3. Results

Multiple regression analyses using backward conditional selection were performed to obtain a suitable model to explain the relationship of (i) ankle plantarflexion and dorsiflexion ROM, (ii) ankle, knee, and hip extensor and flexor peak moment at slow (60°/s) and fast (180°/s) speed, and (iii) associated time to peak moment (sec) on underwater dolphin kick speed (m/s). The multiple regression model indicates that the F statistic value was 4.991 (p < 0.000), showing 54% with R² = .548 for the regression equation of explanatory power (43% according to the modifier). A Durbin-Waston value was 1.351, indicating that the regression model is suitable since there is no correlation between residuals. Since the index of more than 10 did not appear in variance inflation factors there was no concern in multicollinearity. The results of the multiple regression are presented in Table 2 where isokinetic peak moment and time to peak moment are influencing factors for the dolphin kick speed in this study, but not ankle ROM.

Table 2.
Multiple regression analysis on the dolphin kick speed.

Variables B β SE t-value Ρ-value VIF

Constant 1.778 .231 7.703 .000

PM/BW (%) 60°/s Ankle Dorsiflexion −.004 −.290 .002 −2.357 .024* 1.100

60°/s Knee Flexion .002 .232 .001 1.368 .181 2.103

180°/s Knee Extension .003 .478 .001 2.161 .038* 3.562

180°/s Knee Flexion −.005 −.492 .002 −2.046 .049* 4.215

TPM (sec) 60°/s Ankle Plantarflexion .857 .293 .382 2.241 .032* 1.250

60° /s Knee Extension −.354 −.200 .255 −1.385 .175 1.520

180°/s Knee Extension −1.691 −.433 .628 −2.692 .011* 1.885

180°/s Knee Flexion 1.380 .621 .373 3.702 .001** 2.055

Variables	B	β	SE	t-value	Ρ-value	VIF
Constant	1.778		.231	7.703	.000
PM/BW (%)	60°/s Ankle Dorsiflexion	−.004	−.290	.002	−2.357	.024*	1.100
60°/s Knee Flexion	.002	.232	.001	1.368	.181	2.103
180°/s Knee Extension	.003	.478	.001	2.161	.038*	3.562
180°/s Knee Flexion	−.005	−.492	.002	−2.046	.049*	4.215
TPM (sec)	60°/s Ankle Plantarflexion	.857	.293	.382	2.241	.032*	1.250
60° /s Knee Extension	−.354	−.200	.255	−1.385	.175	1.520
180°/s Knee Extension	−1.691	−.433	.628	−2.692	.011*	1.885
180°/s Knee Flexion	1.380	.621	.373	3.702	.001**	2.055

R = .740; R²= .548; Revised R²= .438; F = 4.991; p = .000; Durbin-Waston = 1.351.

PM/BW: peak moment /body weight; TPM: time to peak moment, SE: standard error; VIF: variance inflation factor.

Statistically significance: *p < .05, **p < .01.

As a sub-analysis, Pearson's r correlation analyses were performed between the dolphin kick speed and (1) ankle ROM, (2) isokinetic strength, (3) isokinetic power of lower extremity muscles. No correlation was found between ankle ROM and the dolphin kick speed (Table 2). Weak positive correlations were found between the dolphin kick speed and isokinetic strength (peak moment) of ankle plantarflexion (r = 0.352, p < 0.05) and knee extension at fast speed (180°/s) (r = 0.343, p < 0.05). Moreover, weak positive correlations were found between the dolphin kick speed and isokinetic power (time to peak moment) of plantarflexion at slow (60°/s) (r = 0.340, p < 0.05) and knee flexion at fast speed (180°/s) (r = 0.406, p < 0.05) (Table 3).

Table 3.

Pearson's r correlation analysis between the dolphin kick speed and ankle ROM, dolphin kick speed, and isokinetic maximal strength (peak moment) and power (time to peak moment) at slow (60°/s) and fast speed (180°/s).

ROM (°)			Ankle		Knee		Hip
ROM (°)			Plantarflexion	Dorsiflexion	Extension	Flexion	Extension	Flexion
Dolphin kick speed			.025	.061
	60°/s	PM/BW (%)	.199	−.188	.231	.168	.152	.018
	60°/s	TPM (sec)	.340*	.032	−.250	.096	−.220	−.154
	180°/s	PM/BW (%)	.352*	−.167	.343*	.269	.085	.133
	180°/s	TPM (sec)	.041	−.093	−.047	.406**	.006	.269

r, Pearson's correlation coefficient; ROM: range of motion; PM/BW: peak moment /body weight; TPM: time to peak moment.

Statistically significance: *p < .05, **p < .01.

4. Discussion

This study has two main purposes. The primary purpose of the study was to examine whether (1) ankle flexibility (2) isokinetic strength, and (3) isokinetic power of lower extremity muscles can have effect on the dolphin kick speed. The secondary purpose of the study was to examine a correlation between the dolphin kick speed and physical function. A majority of previous studies were simulation analyses and/or muscle activation analyses of the dolphin kick motion and speed, in which driving force can be generated by the amplitude and pitching of the flexion-kick and extension-kick through the hip, knee, and ankle kinetic chain.^4,13,14,20 Although relatively few studies examined the relationship between strength and dolphin kick speed these studies have some limitations where they (1) tested ankle or knee strength only, (2) measured isometric strength via the hand-held dynamometer, and (3) used non-standardized strength measure methods like fitness machine-based strength testing. There are only two studies showing a significant correlation between lower extremity strength and dolphin kick speed (r = 0.3∼0.48, p < 0.05).^21,24 To our knowledge, this study is the first study to objectively measure isokinetic strength (peak moment) and power (time to peak moment) of the entire lower extremity in the sagittal-plane (ankle, knee, and hip flexor and extensor) and ankle flexibility (e.g., ankle plantarflexion and dorsiflexion ROM). In addition, this is the first study to examine contributing factors for the underwater dolphin kick speed via multiple regression analysis.

The results of this study partially support our hypotheses. A notable finding of this study is that isokinetic power (time to peak moment) of knee flexion at fast speed (180°/s) influences underwater dolphin kick speed followed by isokinetic strength (peak moment) of knee flexion and extension at fast speed (180°/s) (Table 2). In addition, isokinetic strength (peak toque) and power (time to peak moment) of ankle dorsiflexion and plantarflexion at slow speed (60°/s) affects the underwater dolphin kick speed. Moreover, previous studies highlighted the importance of increased ankle plantarflexion flexibility.^11,25 However, our data suggest that ankle flexibility (dorsiflexion and plantarflexion ROM) neither affects the dolphin kick speed via the multiple regression analysis, nor is it associated with the dolphin kick speed via a correlation analysis. These new findings could help us understand the crucial factors affecting the underwater dolphin kick speed and how to design training. From a lower-extremity kinetic chain perspective, the efficiency of dolphin kick speed can be improved by maintaining the same whip like joint movement and amplitude during the dolphin kick. More importantly, isokinetic strength (peak moment) and power (time to peak moment) of knee flexion and extension at fast speed (180°/s) and ankle plantarflexion and dorsiflexion at slow speed (60°/s) could play a key role in the underwater dolphin kick speed in the current study^4,14 (Table 2).

The first novel finding of this study was that isokinetic power (time to peak moment) of knee flexion at fast speed (180°/s) had a significant influence on the underwater dolphin kick speed (β = 0.621, p < 0.001) and was positively correlated with the dolphin kick speed (r = 0.406, p < 0.01). Moreover, isokinetic strength (peak moment) of knee flexion at fast speed (180°/s) also demonstrated significant influence on the underwater dolphin kick speed (β = −0.492, p < 0.049). Interestingly, isokinetic strength and power of knee flexion at slow speed (60°/s) didn’t affect the underwater dolphin kick speed. This finding suggests an important message as a fast muscle contraction speed in knee flexion (e.g., peak moment and time to peak moment) could be a key to influencing the underwater dolphin kick speed as opposed to isokinetic maximal strength at slow speed (60°/s). The ability to recruit type II fast-twitch muscle fibers appears to be important consistent with the current finding. During the underwater dolphin kick, the ability to flex the knee position quickly could result in the faster dolphin kick speed as the knee extensors can be positioned to generate a driving force by shortening the knee extensors quickly.^3,14 A high level of lower extremity muscular power measured by the isokinetic dynamometer can affect the ability to rapidly increase driving force during the 15-m dolphin kick, and thus the ability to reach one's peak moment of knee flexors at fast speed (180°/s) through the concentric contraction should be considered during training.³⁶ Previous studies^16,21 suggest that the flexion-kick through hamstrings was a preliminary motion to perform the extension-kick during the underwater dolphin kick cycle although the flexion-kick helps maintain a streamlined posture by generating a driving force.³ The vertical amplitudes and pitching generated by transverse oscillations and momentum enables the unstable body to perform an effective flexion-kick in a neutral position.¹⁴ This effective motion could increase the dolphin kick speed by reducing resisting friction force. The ability to quickly contract knee flexors concentrically during the flexion-kick and faster neuromuscular activation of the knee flexors would contribute to increasing the underwater dolphin kick speed, and thus should consider these novel findings into their training for competitive male swimmers.

The second important finding of this study was that isokinetic concentric strength (peak moment) and power (time to peak moment) of knee extension at fast speed (180°/s) had the third and fourth influential factors for the underwater dolphin kick speed (β = 0.478, p < 0.038). The current finding was supported by previous studies that the extension-kick during the dolphin kick generates a driving force^3,11,17 as the speed of the extension-kick was faster than the flexion-kick.²¹ The fin swimming extension-kick was negatively correlated with swimming records (r = −0.82).²² These previous studies and the current study emphasized the importance of the underwater dolphin extension-kick speed and knee extensor strength at fast contraction speed (180°/s) rather than slow speed (60°/s) (Figure 3). The propulsive drag is created by a vortex of water enabled by the extension-kick after performing the downward extension-kick through knee extensors from the flexion-kick.²¹ The current finding supports an idea that isokinetic strength and power of knee extensors at fast speed of the concentric contraction (180°/s) influences the underwater dolphin kick speed, but not isokinetic strength at slow contraction speed (60°/s). Coaches should emphasize speed of contraction to generate knee extensor maximal strength and power.

Figure 3.

Summary of underwater dolphin kick performance with isokinetic dynamometer strength and power measures via multiple regression analysis.

The third interesting finding of this study was that isokinetic strength (peak moment; β = .293, p < 0.024) and power (time to peak moment; β = .293, p < 0.032) of ankle plantarflexion at slow speed (60°/s) was an influential factor for the dolphin kick speed. However, isokinetic strength and power of ankle function had a relatively small relationship on the dolphin kick speed compared to isokinetic strength and power of the knee flexors and extensors as described previously. As the relationship of isokinetic strength and power of ankle muscles on the dolphin kick speed is limited in the current literature, this finding is novel, but it should be confirmed in future research.

The fourth key finding of this study was that ankle plantarflexion ROM did not influence the dolphin kick speed. It is believed that greater ankle plantarflexion ROM could be key to improving the dolphin kick speed as the greater plantarflexed position of the foot could generate a higher forward driving force by pushing water using the dorsal surface of the foot during the extension-kick. However, the findings should be interpreted with caution. Maximal static ankle plantarflexion ROM in this study was 51.4 ± 6.2 degree (max 66 deg, min 43 deg), which indicates a low variability between athletes. As such, no real mathematical relationship could not be found as they had very similar ankle plantarflexion ROM, but with different dolphin kick speed and isokinetic strength and power. A previous study investigated an association between ankle plantarflexion ROM and the dolphin kick speed, in which increased ankle plantarflexion ROM is associated with the faster dolphin kick speed.¹³ Moreover, these two variables had a high positive correlation (r = 0.86). However, recent studies show that ankle ROM alone did not affect the dolphin kick speed^21,24 with no positive correlation (r = 0.105).²¹ To sum up, conflicting results exist in the literature whether ankle ROM is associated with the dolphin kick speed or not. The importance of greater ankle plantarflexion ROM during the underwater dolphin kick should be considered in future research. A driving force pushing water backward can be generated during the flexion-kick and extension-kick of the dolphin kick cycle while the foot plantarflexion position is maintained.²¹ As such, if a swimmer can’t reach the ankle plantarflexion position during the extension-kick, it is likely that the swimmer would have limited ability to generate forward driving force.

5. Conclusions

Isokinetic concentric strength (peak moment) and power (time to peak moment) of the knee flexors and extensors at fast speed (180°/s) affects the underwater dolphin kick speed. It is essential to have the ability to reach one's peak moment of knee and ankle muscles within a short period of time through a concentric contraction via a faster muscle onset timing during the underwater flexion-kick and extension-kick. The current results suggest that coaches should assess one's isokinetic concentric strength (peak moment) and power (time to peak moment) of the lower extremity muscles in the sagittal-plane during a pre-season, then training programs should be targeted for associated muscles to improve the dolphin kick speed. As isokinetic strength of lower extremity muscles at fast speed (180°/s) can be an alternative way to measure muscular power using an isokinetic dynamometer.^27,28 The current results suggest that velocity-based training could be suitable for improving strength and power for swimmers as the fast contraction speed of knee and ankle isokinetic measures appeared to affect the dolphin kick speed in this study. As such, foot and knee focused strength and power training could be encouraged, and more importantly, coaches should understand the dolphin kick sequence associated muscle function, and could combine strength and power training into dolphin kick motions to improve the dolphin kick speed.

Footnotes

Acknowledgements

The authors are thankful to all the participants and their families for their participation in this study.

ORCID iD

S. Jun Son

Funding

The authors received no financial support for the research, authorship, and/or publication of this article.

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Ethical considerations

The Ethics Committee of the University IRB approved this study. The study was conducted in accordance with the Declaration of Helsinki. All participants were thoroughly informed about the study, and written informed consent was obtained from each participant.

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Relationship of isokinetic strength,power and ankle flexibility on underwater dolphin kick speed in competitive collegiate swimmers

Abstract

Background

Objective

Methods

Results

Conclusions

Keywords

1. Introduction

2. Methods

2.1. Study design

2.2. Participants

Table 1. Subjects demographics. Age (yrs) Height (cm) Mass (kg) Career (yrs) Dolphin kick Ankle ROM Speed (m/s) Record (sec) Plantarflexion (°) Dorsiflexion (°) Male (n = 21) 20.7 ± 1.3 180.5 ± 4.3 79.1 ± 6.7 10.1 ± 1.8 1.99 ± 0.20 7.51 ± 0.88 51.8 ± 5.7 21.2 ± 5.1

2.4 Procedures

3. Results

Footnotes

Acknowledgements

ORCID iD

Funding

Declaration of conflicting interests

Ethical considerations

References

Table 1.
Subjects demographics.

Age (yrs) Height (cm) Mass (kg) Career (yrs) Dolphin kick Ankle ROM

Speed (m/s) Record (sec) Plantarflexion (°) Dorsiflexion (°)

Male (n = 21) 20.7 ± 1.3 180.5 ± 4.3 79.1 ± 6.7 10.1 ± 1.8 1.99 ± 0.20 7.51 ± 0.88 51.8 ± 5.7 21.2 ± 5.1