Undulatory Underwater Swimming Performance and Kinematics:

International- vs National-Level Female Swimmers

Jesús J. Ruiz-Navarro

, Adrián Febles-Castro

, Óscar López-Belmonte

Francisco Cuenca-Fernández

and Raúl Arellano

Aquatics Lab, Department of Physical Education and Sports, Faculty of Sport Sciences, University of Granada,

Granada, Spain

Department of Didactics of Musical, Artistic and Bodily Expression, Faculty of Education, University of Valladolid,

Soria, Spain

Department of Sports and Computer Sciences, Universidad Pablo de Olavide, Seville, Spain

Keywords: Dolphin Kick, Competitive Swimming, Biomechanics, Velocity, Assessment.

Abstract: This study aimed to compare undulatory underwater swimming (UUS) performance and kinematics between

international- and national-level female swimmers. Seven international level female swimmers (18.9 ± 3.4

years; 67.2 ± 4.4 kg of body mass; 175 ± 4.8 cm of body height; and 804 ± 35 World Aquatics points) and

seven national level female swimmers (17.6 ± 1.7 years; 57.4 ± 4.1 kg of body mass; 166 ± 4.9 cm of body

height; and 662 ± 65 World Aquatics points) performed three maximal-effort 15 m UUS trials. Seven body

landmarks were auto-digitalized during UUS by a pre-trained neural network and 21 kinematic variables were

calculated. The results showed no statistically significant differences across the variables analysed (p > 0.05);

however, mean and minimum UUS velocities showed a clear trend toward better performance in international-

level swimmers (p = 0.055–0.057; d = 0.91–0.92). In conclusion, there was a tendency for superior UUS

performance among international-level swimmers compared to national-level swimmer. Furthermore,

variations in UUS technique appear to be more strongly influenced by individual physical and anatomical

characteristics than by performance level alone.

1 INTRODUCTION

Aside from the initial dive, the highest velocities in

butterfly, backstroke, and freestyle events are

achieved during the underwater phase (Ruiz-Navarro

et al., 2022) Consequently, this segment is considered

one of the most critical for overall performance

(Mason & Cossor, 2001). During this phase,

swimmers propel themselves forward using

undulatory underwater swimming (UUS), a technique

characterized by wave-like body movements while

maintaining a streamlined position with the arms

extended and held together above the head

(Connaboy et al., 2010). During each kick cycle, a

complete downward (downbeat) and upward (upbeat)

https://orcid.org/0000-0002-0010-7233

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https://orcid.org/0000-0002-6773-2359

movement of the lower limbs is performed to

generate propulsion (Higgs et al., 2016).

Current rules and regulations limit the

underwater phase during competition to a maximum

of 15 meters. In this regard, the distance covered

underwater varies across different performance levels

(Pla et al., 2021; Veiga et al., 2016). Race analyses

have shown that higher-level performers tend to

spend more time underwater, cover longer distances,

and achieve higher velocities during this phase (Pla et

al., 2021). These factors provide greater advantages

in start and turn performance, thereby contributing

positively to overall race outcomes (Arellano et al.,

1994; Morais et al., 2019).

110

Ruiz-Navarro, J. J., Febles-Castro, A., López-Belmonte, Ó., Cuenca-Fernández, F. and Arellano, R.

Undulatory Underwater Swimming Performance and Kinematics: International- vs National-Level Female Swimmers.

DOI: 10.5220/0013821400003988

Paper published under CC license (CC BY-NC-ND 4.0)

In Proceedings of the 13th International Conference on Sport Sciences Research and Technology Support (icSPORTS 2025), pages 110-115

ISBN: 978-989-758-771-9; ISSN: 2184-3201

Given the increasing importance of UUS,

various performance-related variables have been

investigated in the literature (Atkison et al., 2014;

Higgs et al., 2017; Matsuda et al., 2021). Among the

studied variables highlights the joints’ amplitude,

range of motion, and angular velocities, as well as toe

vertical velocity, kick frequency, and kick length

(Ruiz-Navarro et al., 2022) These variables are either

linked to performance outcomes or used to

characterize swimmers’ movement patterns

(Connaboy et al., 2010; Higgs et al., 2017; Ruiz-

Navarro et al., 2022). However, there is currently no

up-to-date comparison of these variables across

swimmers of different performance levels (Arellano

et al., 2002).

Female athletes have historically been

underrepresented in sport science research (Meyer &

Cobley, 2024) highlighting the need for targeted

investigations, particularly in swimming, where

significant differences in anthropometric and

physiological characteristics may influence

performance and technique(Janssen et al., 2000;

Miller et al., 1993). Thus, this study aimed to compare

UUS performance and kinematics between

international- and national-level female swimmers. It

was hypothesized that international-level female

swimmers would exhibit superior performance

outcomes compared to their national-level

counterparts.

2 METHODS

2.1 Participants

Seven international-level female swimmers (18.9 ±

3.4 years, 67.2 ± 4.4 kg of body mass, 175 ± 4.8 cm

of body height, and 804 ± 35 World Aquatics Points

(Level 2 (Ruiz-Navarro et al., 2023)) belonging to the

same national squad and seven national-level female

swimmers (17.6 ± 1.7 years, 57.4 ± 4.1 kg of body

mass, 166 ± 4.9 cm of body height, and 662 ± 65

World Aquatics Points (Level 3 (Ruiz-Navarro et al.,

2023)members of the same swimming squad

participated in this study. The protocol was explained

to the swimmers and their parents or legal guardians

(for swimmers under 18 years old) before obtaining

their written consent.

2.2 Data Collection

Swimmers were assessed in a single testing session.

After anthropometric measurements were taken using

a stadiometer (Seca 799, Hamburg, Germany),

swimmers were marked on the right side of the body

with a 3-cm-diameter circle of black body paint at the

following anatomical landmarks: the styloid process

of the ulna, the head of the humerus, the greater

trochanter of the femur, the lateral epicondyle of the

femur, the lateral malleolus of the fibula, and the 5th

metatarsal phalangeal joint of the foot (toe) (Figure

1). These specific points represent the joint centres of

the wrist, shoulder, hip, knee, and ankle and the most

distal point of the foot, respectively (Naemi &

Sanders, 2008). Subsequently, swimmers performed

their one warm-up, with special focus on UUS.

Figure 1: Representation of the 6 anatomical landmarks.

Afterwards swimmers performed 3 × 15 m trials

with at least 3 min of total recovery between trials

(Higgs et al., 2017).Swimmers were instructed to

maintain a consistent depth of 1 m throughout the 15

m to control wave drag effects, otherwise, they would

be requested to perform an extra trial (Vennell et al.,

2006). Trials were conducted in lane three, 5.2 m

from the side wall. All the trials were recorded with

one stationary underwater camera (GoPro HERO 9,

60Hz, 2.7K, California, USA) positioned at 7.5 m

from the starting wall and 1 m below the surface with

the optical axes perpendicular to the swimming

direction, recording the area between 5 and 10 m

(Ruiz-Navarro et al., 2024). This setup ensured that

two complete kick cycles were recorded per trial, with

a total of six cycles analysed to provide a

representative and reliable assessment of UUS

kinematics (Connaboy et al., 2010).

2.3 Data Analysis

The UUS trials were analysed following Papic et al.

(2020) procedures, using a pre-trained Neural

Network in DeepLabCut

with a mean test error of

5.5 mm (Ruiz-Navarro et al., 2024). The “Cinalysis”

software (Elipot et al., 2010) was used to compute the

calibration coefficients by applying a 2D direct linear

transformation with a calibration plane (2.05 × 1.60

m) containing 37 calibration points in Matlab 2016

(MathWorks Inc., Natick, Mass., USA). The

calibration error was assessed as the reprojection

error, where root-mean-square error (RMSE) of the

Undulatory Underwater Swimming Performance and Kinematics: International- vs National-Level Female Swimmers

111

reconstructed calibration marker positions were for

the x- and y-axis coordinates 0.003 m and 0.002 m,

respectively. Two full kick cycles were digitised for

each trial, with an additional 15 frames included

before and after the start and end points to preserve

signal continuity during filtering and time derivative

calculation (Vaughan, 1982). A fourth-order low pass

Butterworth filter with a cut off frequency of 6 Hz

was employed to smooth the data. Finally, the

following kinematics variables were calculated using

the methods employed by (Connaboy et al., 2010) in

Python 3.9: mean, maximum, and minimum

undulatory underwater swimming velocity (denoted

as mean U, max U, and min U, respectively), cycle

length, kick frequency, and vertical joint center

amplitudes of the wrist, shoulder, hip, knee, ankle,

and fifth metatarsal phalangeal joint. Additionally,

joint ranges of motion and mean angular velocities of

the shoulder, hip, knee, and ankle, as well as mean

and maximum vertical toe velocity, were assessed.

2.4 Statistical Analysis

The data are expressed as mean ± standard deviation

(SD). The normality of all variables was assessed

using the Shapiro-Wilk test. An independent t-test

was conducted to compare swimmers across

performance levels. Effect sizes were calculated

using Cohen’s d to estimate the magnitude of

differences in the analysed variables. The effect size

was categorized as follows: small if 0 ≤ |d| ≤ 0.5,

medium if 0.5 < |d| ≤ 0.8, and large if |d| > 0.8 (Cohen,

1988). All statistical procedures were performed

using the Jamovi software package version 2.3.28.0

(Jamovi Project 2022, Sydney, Australia, retrieved

from https://www.jamovi.org) with the level of

statistical significance set at 0.05. Subsequently, a

post hoc power analysis was conducted for the

independent t-test using G*Power version 3.1.9.7

(Universität Düsseldorf, NRW, Germany).

3 RESULTS

Table 1 presents performance level-based mean ± SD

differences for UUS performance and kinematic

variables along with corresponding p-values, and

effect sizes (Cohen’s d). The statistical power ranged

from 0.06 to 0.63 across the observed effect size

range of 0.03 to 1.12.

Table 1: Performance level-based differences.

Variable

Level

Diff. p d

Mean U

(m/s)

1.61 ±

0.14

1.49 ±

0.13

0.12 0.057 0.91

Max U (m/s)

1.90 ±

0.12

1.88 ±

0.17

0.02 0.384 0.16

Min U

(

m/s

)

1.20 ±

0.19

1.06 ±

0.12

0.14 0.055 0.92

Cycle length

(m)

0.71 ±

0.05

0.72 ±

0.11

-0.01 0.586 0.12

Kick freque-

ncy (Hz)

2.27 ±

0.31

2.10 ±

0.35

0.17 0.177 0.52

Wrist ampli-

tude

(

)

0.06 ±

0.02

0.07 ±

0.02

-0.98 0.810 0.49

Shoulder

amplitude

(m)

0.06 ±

0.01

0.07 ±

0.02

-0.75 0.790 0.45

Hip

amplitude

(m)

0.12 ±

0.03

0.13 ±

0.02

-0.82 0.714 0.31

Knee ampli-

tude

(

)

0.23 ±

0.04

0.26 ±

0.03

-3.23 0.954 0.98

Ankle ampli-

tude

(

)

0.37 ±

0.06

0.41 ±

0.04

-3.63 0.884 0.67

Toe ampli-

tude (m)

0.51 ±

0.07

0.54 ±

0.05

-2.67 0.795 0.46

Shoulder

ROM (º)

22.0 ±

5.6

24.5 ±

5.3

-2.48 0.796 0.46

Hip ROM (º)

38.6 ±

8.3

46.7 ±

-8.07 0.971 1.12

Knee

ROM (º)

72.2 ±

9.1

79.5 ±

3.8

-7.35 0.964 1.05

Ankle

ROM (º)

46.1 ±

9.5

45.0 ±

7.8

1.06 0.412 0.12

Mean

shoulder

angular

velocit

(

º/s

)

100.8 ±

25.6

107.4 ±

20.4

-6.6 0.699 0.29

Mean hip

angular

velocity (º/s)

168.2 ±

30.5

200.1 ±

47.1

-31.8 0.920 0.80

Mean knee

angular

velocity (º/s)

372.0 ±

39.8

414.8 ±

51.4

-42.7 0.946 0.93

Mean ankle

angular

velocit

(

º/s

)

277.9 ±

67.8

275.3 ±

88.6

2.58 0.476 0.03

Mean toe

vertical

velocity

(m/s)

1.14 ±

0.04

1.11 ±

0.12

2.42 0.306 0.28

Max toe

vertical

velocity

(

m/s

)

4.09

0.28

4.04

0.31

5.14 0.375 0.17

Abbreviations: U, undulatory underwater swimming

velocity; ROM, range of motion.

icSPORTS 2025 - 13th International Conference on Sport Sciences Research and Technology Support

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4 DISCUSSION

This study aimed to compare UUS performance and

kinematics between international-level (Level 2) and

national-level (Level 3) female swimmers. While no

statistically significant differences were observed

across the variables assessed, a notable trend emerged

in UUS performance and kinematics. Specifically,

both mean and minimum underwater swimming

velocity demonstrated a large effect size (p = 0.057;

d = 0.91 and p = 0.055; d = 0.92, respectively; and

statistical power (1 – β) = 0.49, for both) that might

indicate differences between performance level.

When compared to previous studies, similar UUS

velocity values were observed in a sex-mixed sample

of international-level swimmers (Arellano et al.,

2002).However, the present study focuses

exclusively on female swimmers. In the

aforementioned study, significant differences were

found between international- and national-level

swimmers, with national-level athletes demonstrating

notably lower performance (1.25 m/s) compared to

those in the current sample (1.49 m/s). Altogether,

these findings highlight the increasing importance of

the UUS phase in modern competitive swimming and

its contribution to overall performance (Pla et al.,

2021; Veiga et al., 2016).

With regard to maximum and minimum UUS

velocities, a trend toward group differences was

observed in minimum velocity. This velocity

typically occurs during the upbeat phase of the kick

(Arellano et al., 2002), which can be technically

challenging due to human anatomical constraints

(Loebbecke et al., 2009). Notably, and in accordance

to the tendency observed in our results, this phase has

been suggested to distinguish the fastest swimmers

from their peers, and it is the most responsive to

improvement following targeted UUS training

interventions in young swimmers (Ruiz-Navarro et

al., 2021).

There is an optimal combination of kick

frequency and kick length that is specific to each

swimmer (Yamakawa et al., 2017). Consequently, the

different anthropometric characteristics between

swimmers may have led to greater intra-group

variability, potentially obscuring differences between

them. Moreover, equal velocities can be reached in

different ways by increasing the magnitude of the

propulsive impulse relative to the active drag

experienced (Connaboy et al., 2016). For instance,

while some swimmers rely on greater undulations

aiming for greater propulsion, they also produce

greater resistance, while the opposite may also occur.

Thus, the lack of differences in swimming kinematics

may be more indicative of individual differences than

to the performance level and therefore, future

research should aim to compare swimmers of

different performance level within the same cluster.

As observed in elite short-course swimming,

consistency in turn performance and control of intra-

individual variation appear more indicative of high-

level execution than isolated kinematic outputs

(Cuenca-Fernández et al., 2022). This highlights the

need to consider alternative predictors that may

underlie such consistency, such as body shape,

passive drag characteristics, and gliding efficiency.

Individual anatomical differences, like torso-to-hip

ratios or streamline profiles, can substantially

influence how force is translated into forward motion

and how effectively drag is minimized. These

biomechanical and morphological features may

explain performance differences even when

kinematic metrics appear similar across swimmers.

Thus, to enhance our understanding of elite UUS

performance, future research should incorporate

multidimensional analyses that include

anthropometrics, hydrodynamic profiling, and

gliding capabilities alongside traditional kinematics.

Another point to consider is that the

measurements were not taken in a competitive

environment, with swimmers fully rested, unlike

during actual races. These measurements are indeed

commonly used to characterize swimmers’

movements (Connaboy et al., 2010). However, during

a race, factors such as physical conditioning and

fatigue may affect UUS performance (Taladriz et al.,

2015), and differences in UUS between levels might

become more pronounced as the race progresses.

Finally, it is important to acknowledge the

limitation posed by the relatively small sample size.

Although the study successfully included high-level

swimmers, who are typically difficult to recruit, the

limited number of participants resulted in reduced

statistical power, which likely contributed to the

absence of statistically significant differences by

increasing the risk of a Type II error, thus, the results

should be interpreted with caution. Moreover, the

high degree of individual variation in technique

within each group may have influenced the kinematic

outcomes, further masking potential performance-

related differences. Additionally, overall swimming

performance does not necessarily imply superiority

across all phases of the race. As such, specific

weaknesses in some international-level swimmers

may have aligned with strengths in certain national-

level swimmers, potentially contributing to the lack

of clear group differences. To better interpret

individual performance within specific race phases,

Undulatory Underwater Swimming Performance and Kinematics: International- vs National-Level Female Swimmers

113

future research should establish benchmarks and

corresponding percentiles for key UUS kinematic

variables.

5 CONCLUSIONS

In conclusion, the findings suggest a trend toward

superior UUS performance—particularly during the

upbeat phase—in Level 2 (international) compared to

Level 3 (national) female swimmers. However, no

clear differences in kinematic variables were

observed between groups, which may indicate that

variations in UUS technique are more strongly

influenced by individual physical and anatomical

characteristics than by performance level alone. Our

findings should be interpreted as exploratory rather

than confirmatory and future studies combining

biomechanics, anthropometrics, and hydrodynamics

are needed to build on these preliminary results.

ACKNOWLEDGEMENTS

We extend our sincere gratitude to all the swimmers

and coaches who voluntarily participated and allowed

us to conduct assessments as part of this study. This

study was supported by the Grant PID2022-

142147NB-I00 (SWIM III) funded by

MICIU/AEI/10.13039/501100011033/ and by

ERDF, EU. AFC holds an FPI fellowship which is

funded through the aforementioned grant.

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