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Three-dimensional analysis of hip and knee joint movements during dolphin kicking and butterfly swimming

Authors:
Keisuke Kobayashi Yamakawa at University of Tsukuba
Keisuke Kobayashi Yamakawa
Hideki Takagi at University of Tsukuba
Hideki Takagi
Yasuo Sengoku at University of Tsukuba
Yasuo Sengoku
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Abstract and Figures

The aims of this study were to clarify differences in hip and knee joint movements during dolphin kick alone and during butterfly stroke swimming, and to investigate which parameter relates to swimming performance in each stroke. Eight male swimmers performed three trials using dolphin kick with a kick board (BD), underwater dolphin kick (UD), and butterfly swimming (Fly) at 80% maximal effort in a water flume. Three-dimensional coordinates of the swimmers during the trials were obtained using a motion capture system, and these coordinates were used to calculate the horizontal velocity of the hip (Vhip), the hip and knee joint angle, and the angular velocity. Butterfly kicking motion was divided into first kick (Fly-1st) and second kick (Fly-2nd) according to the stroke phase, and the kinematic parameters during the 4 different kicks were used for analysis. As the main results, it was indicated that the peak hip flexion angle during Fly-2nd was significantly smaller than that during BD and UD (BD,-27.8 9.7 deg; UD,-27.7 10.5 deg; Fly-1st,-25.6 13.9 deg; Fly-2nd,-16.2 7.0 deg, p < .05) and the peak knee extension angle during Fly-2nd was significantly smaller than that during BD, UD, and Fly-1st (BD,-7.6 5.8 deg; UD,-4.4 5.7 deg; Fly-1st,-7.4 4.9 deg; Fly-2nd, 2.0 6.6 deg, p < .05). The mean Vhip during BD was significantly correlated with the peak hip external rotation angle (r =-0.77), the peak knee extension angle (r =-0.77), the peak hip external rotation angular velocity (r =-0.88) and the peak knee flexion angular velocity (r = 0.81). The mean Vhip during UD was significantly correlated with the peak knee extension angle (r =-0.87), the peak hip external rotation angular velocity (r =-0.73) and the peak knee flexion angular velocity (r = 0.95). The mean Vhip during Fly was not correlated with any of the kinematic parameters during Fly-1st and Fly-2nd. Our results demonstrate that flexion/ extension movements of the hip and knee joint are different between dolphin kick and butterfly swimming. Furthermore, larger knee extension, greater hip external rotation velocity, and greater knee flexion velocity may be important to increase dolphin kick performance.
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Three-dimensional analysis of hip and knee joint movements during dolphin kicking and butterfly swimming 1
Three-dimensional analysis of hip and knee joint
movements during dolphin kicking and butterfly
swimming
Keisuke Kobayashi YAMAKAWA, Japan Women’s College of Physical Education, Tokyo, Japan
Hideki TAKAGI, University of Tsukuba, Ibaraki, Japan
Yasuo SENGOKU, University of Tsukuba, Ibaraki, Japan
Abstract̶The aims of this study were to clarify differences
in hip and knee joint movements during dolphin kick alone
and during butterfly stroke swimming, and to investigate which
parameter relates to swimming performance in each stroke.
Eight male swimmers performed three trials using dolphin kick
with a kick board (BD), underwater dolphin kick (UD), and but-
terfly swimming (Fly) at 80% maximal effort in a water flume.
Three-dimensional coordinates of the swimmers during the tri-
als were obtained using a motion capture system, and these
coordinates were used to calculate the horizontal velocity of
the hip (Vhip), the hip and knee joint angle, and the angular ve-
locity. Butterfly kicking motion was divided into first kick (Fly-
1st) and second kick (Fly-2nd) according to the stroke phase,
and the kinematic parameters during the 4 different kicks were
used for analysis. As the main results, it was indicated that the
peak hip flexion angle during Fly-2nd was significantly smaller
than that during BD and UD (BD, -27.8 ±9.7 deg; UD, -27.7
±10.5 deg; Fly-1st, -25.6 ±13.9 deg; Fly-2nd, -16.2 ±7.0
deg, p <.05) and the peak knee extension angle during Fly-
2nd was significantly smaller than that during BD, UD, and
Fly-1st (BD, -7.6 ±5.8 deg; UD, -4.4 ±5.7 deg; Fly-1st, -
7.4 ±4.9 deg; Fly-2nd, 2.0 ±6.6 deg, p <.05). The mean
Vhip during BD was significantly correlated with the peak hip
external rotation angle (r= -0.77), the peak knee extension
angle (r= -0.77), the peak hip external rotation angular veloc-
ity (r= -0.88) and the peak knee flexion angular velocity (r=
0.81). The mean Vhip during UD was significantly correlated
with the peak knee extension angle (r= -0.87), the peak hip
external rotation angular velocity (r= -0.73) and the peak knee
flexion angular velocity (r= 0.95). The mean Vhip during Fly
was not correlated with any of the kinematic parameters dur-
ing Fly-1st and Fly-2nd. Our results demonstrate that flexion/
extension movements of the hip and knee joint are different
between dolphin kick and butterfly swimming. Furthermore,
larger knee extension, greater hip external rotation velocity,
and greater knee flexion velocity may be important to increase
dolphin kick performance.
Key words: Swimming performance; Motion capture system;
Water flume
1. Introduction
Dolphin kicks are used in butterfly swimming, and gen-
erally swimmers perform two dolphin kicks during each
butterfly stroke cycle. According to Maglischo (2003),
The downbeat of the first kick takes place during the en-
try and catch of the arms, and the downbeat of the second
kick occurs during the upsweep if the underwater arm-
stroke. A previous study indicated differences in hip and
knee joint movements between the two dolphin kicks in
each stroke cycle (Barthels & Adrian, 1971). Therefore,
swimmers and coaches should be aware of the kine-
matic differences during the practice of dolphin kicks in
the butterfly stroke. However, that previous study inves-
tigated the sagittal movement only, and did not analyze
the coronal or cross-sectional movement. Thus to ob-
tain deeper understanding of the butterfly dolphin kick
technique, a three-dimensional analysis method should
be used.
On the other hand, there are many kinematic stud-
ies examining the underwater dolphin kick compared to
studies examining the dolphin kick used during butter-
fly swimming. While a previous study investigated the
kinematic difference in lower limb movements between
dolphin kicks when swimming with a board and butterfly
swimming (Barthels & Adrian, 1971) it remains unclear
whether there is a kinematic difference between move-
ments during the underwater dolphin kick and those
during the dolphin kick in butterfly swimming.
The purpose of this study was to clarify kinematic
differences in lower limb movements between dolphin
kicks when swimming with a board, underwater dolphin
kicks and during butterfly stroke swimming. Further-
more, to describe the characteristics related to swimming
performance, we investigated parameters that correlate
with swimming velocity in each stroke.
2. Methods
2.1. Participants
Eight male collegiate swimmers participated in this
study (age, 21.3 ±0.7 years; height, 1.73 ±0.05 m;
weight, 70.3 ±4.6 kg). All participants practiced 8
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2
times each week with a collegiate swimming team. The
participants were fully informed of the risks, benefits,
and stresses of the study, and their informed consent was
obtained.
2.2. Swim trials
In a 50 m indoor pool, the participants performed three
25 m swimming trials: dolphin kick swimming with
board (BD), underwater dolphin kick swimming (UD),
and butterfly swimming (Fly). The participants were
instructed to execute each trial at maximal effort with
a push-off start. In the underwater dolphin kick swim-
ming, the participants swam approximately 1.0 m under
the water surface to exclude the effect of wave drag (Lyt-
tle, Blanksby, Elliott, & Lloyd, 2000). The swim-times
during the trials were measured using a manual stop
watch by the same examiner, and the average swimming
velocity between 15 m to 25 m were defined as the max-
imum swimming velocity (100%V) in each trial.
For analysis of the three-dimensional motion, the
participants performed BD, UD, and Fly in a water flume
(Igarashi Industrial Works Co. Ltd.). The flow velocity
during each trial was set at 80% velocity of 100%V. The
participants executed 10 stroke cycles during each trial.
2.3. Three-dimensional motion analysis
Three-dimensional motion analysis was conducted using
a motion capture system (VENUS 3D, Nobby tech Inc.).
As Figure 1, twenty cameras filmed swimmers above
the water flume or through the underwater window. For
measurement of lower limb motion, 15 landmark points
(left styloid process of ulna, lowest ribs, anterior superior
iliac spines, hip greater trochanters, lateral condyles of
the femur, medial condyles of the femur, lateral malle-
oli of the fibula, and medial malleoli of the tibia) on
the participants were marked with LED markers, and
the landmark points coordinates were used for analysis.
The right and left center of hip joint coordinates were
estimated from the anterior superior iliac spine coordi-
nates and the hip greater trochanter coordinates. The
right and left center of the knee joint coordinates were
estimated as the mid point between the lateral condyles
of the femur coordinates and the medial condyles of the
femur coordinates.
Figure 1. Experimental setting.
2.4. Calculation of kinematic parameters and joint
angles
In the present study, we defined one kick cycle as be-
ginning at the vertical highest peak of the left lateral
malleolus position and ending at the next vertical high-
est peak. Furthermore, one kick cycle was divided into
a downward kick (DK) phase and an upward kick (UK)
phase. We defined one butterfly stroke cycle as begin-
ning at left wrist entry and ending at the next left wrist
entry, and further, the one butterfly stroke cycle was
divided into five phases (entry and stretch, out-sweep,
in-sweep, up-sweep, and release and recovery) using the
trajectory of wrist coordinates according to Maglischo
(2003). The two dolphin kicks in each butterfly stroke
were separated into the first dolphin kick (Fly-1st) and
the second dolphin kick (Fly-2nd) corresponding to the
stroke phases. We defined that the DK of the first kick
takes place during the entry and catch of the arms, and
the DK of the second kick occurs during the upsweep.
Kick frequency (KF) was defined as the reciprocal of the
duration of one kick cycle. Kick amplitude (KA) was
defined as the vertical distance between the highest verti-
cal left lateral malleolus position and lowest vertical left
lateral malleolus position during one kick cycle. Swim-
ming velocity was defined as the horizontal velocity at
the mid-point between both centers of the hip joints, and
an average swimming velocity during one kick cycle
(Vhip) was estimated as swimming performance.
Hip and knee joint angles were calculated with
Cardan angles using MATLAB software (Math works
Inc.), whereby rotations about orthogonal local axes de-
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Three-dimensional analysis of hip and knee joint movements during dolphin kicking and butterfly swimming 3
rived from swimmer’s body corresponding to flexion/
extension in the sagittal plane, adduction/abduction in
the coronal plane and internal/external rotation in the
cross-sectional plane, respectively. In the present study,
only the knee flexion/extension angle was used for anal-
ysis due to the joint movement of the knee in the coronal
and cross-sectional plane being small. From the joint
angle data, peak angle, range of motion (ROM), and
peak angular velocity were calculated for analysis. Con-
sidering the cyclic variation of the dolphin kicks, three
values were obtained for all kinematic variables, and the
mean of these values was used for statistical analysis.
2.5. Statistical analysis
All data are reported as the mean and standard devia-
tion (mean ±SD). Statistical analyses were conducted
using Bell Curve for Excel (SSRI Inc., Japan). All vari-
ables were compared between the 4 dolphin kicks using
repeated one-way ANOVA, followed by Sidak’s mul-
tiple comparison post-hoc tests. Furthermore, the re-
lationships between the swimming performance (Vhip)
and kinematic parameters was investigated using Pear-
son’s correlation coefficient. The threshold values of the
correlation coefficient that represented small, moderate,
large, very large, and nearly perfect correlations were
0.1, 0.3, 0.5, 0.7, and 0.9 according to recommenda-
tions in the literature (Hopkins, Marshall, Batterham, &
Hanin, 2009). The statistical significance level was set
at 5% in this study.
3. Results
The kinematic parameters of each dolphin kick are
shown in Table 1. There was a significant main ef-
fect for the Vhip, KF, KA, DK phase, and UK phase (all
p<.05). The time-course change of hip and knee joint
angles during one kick cycle in each kick is shown in
Figure 2. The results of peak angles, ROM, and peak
angular velocities are shown in Table 2. There was a sig-
nificant main effect for peak hip extension angle, peak
hip flexion angle, ROM of hip flexion-extension, peak
hip external rotation angle, peak knee extension angle,
ROM of knee flexion-extension, peak hip extension an-
gular velocity, peak hip flexion angular velocity, and
peak knee extension angular velocity (all p<.05).
The mean Vhip during BD correlated significantly
with the peak hip external rotation angle (r= -0.77),
the peak knee extension angle (r= -0.77), the peak hip
external rotation angular velocity (r= -0.88), and the
peak knee flexion angular velocity (r= 0.81). The mean
Vhip during UD correlated significantly with the peak
knee extension angle (r= -0.87), the peak hip external
rotation angular velocity (r= -0.73), and the peak knee
flexion angular velocity (r= 0.95). The mean Vhip dur-
ing Fly did not correlate significantly with any kinematic
parameter during Fly-1st and Fly-2nd.
Table 1. Kinematic parameters during each kick cycle.
4. Discussion
The purpose of this study was to clarify kinematic dif-
ferences in lower limb movements between dolphin kick
swimming with board, underwater dolphin kick and but-
terfly stroke swimming. Our results showed that the peak
hip flexion angle and the peak knee extension angle in
Fly-2ndwere significantly smaller than in the other 3
dolphin kicks. In contrast, there were no significant dif-
ferences in the peak hip adduction/abduction angle and
angular velocity, and in hip internal/external rotation
angle and angular velocity.
Barthels and Adrian (1971) used electrogoniometry
to analyze the hip and knee joint movements during dol-
phin kicks with a board and the butterfly stroke, and
they suggested that alternating major and minor kicks
occurred either temporally, spatially, or both temporally
and spatially in all subjects during butterfly swimming
trials. In the present study, the peak hip flexion angle
in Fly-2nd was significantly smaller than that in Fly-1st
(Table 2) while there was no significant difference in
KF between Fly-1st and Fly-2nd. Therefore, it was sug-
gested that the results in this study were different from the
previous study. Maglischo (2003) noted that the down-
beat of the second kick is generally shorter with less
hip flexion than the first kick. This may be explained
in the light of the findings of Sanders, Cappaert, and
Devlin (1995). Using Fourier analysis, they established
that the motion of elite butterfly swimmers is character-
ized by vertical undulations comprising two sinusoidal
body waves- a one beat wave (H1) and a two-beat wave
(H2)., The phase relationship between them causes the
amplitude of the vertical hip motions and the knees and
ankles to differ yielding a strong beat and a weaker beat
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Figure 2. Change in hip and knee angles during each kick cycle in a typical subject (BD: Vhip=1.30 m ·s-1, UD: Vhip=1.35
m·s-1, Fly-1st: Vhip=1.21 m ·s-1, Fly-2nd: Vhip=1.53 m ·s-1)
(Sanders, 2007). In contrast, there was no difference
between Fly-1st and Fly-2nd in the variables related to
hip adduction/abduction and hip internal/external rota-
tion (Table 2). Furthermore, it was observed that there
were no major differences in the time-course changes of
hip adduction/abduction angle and hip internal/external
rotation angle. From these results, it was considered
that the hip and knee joint movements in the coronal and
cross-sectional plane during butterfly stroke swimming
were similar with that during dolphin kick swimming.
By investigating the angular characteristics (Table
2), there was no significant difference between the vari-
ables in UD and Fly-1st, while the peak hip extension
angle in UD was slightly smaller than that in Fly-1st. No
differences were observed in the time-course changes in
Figure 2. These results suggest that the hip and knee
joint movements in Fly-1st are similar to the movements
in UD. However, the peak hip extension, the peak hip
flexion angles, the peak knee extension angle, and the
ROM of knee flexion-extension in UD were significantly
different than those in Fly-2nd. These results suggest that
the second kick during the butterfly stroke may require
hip and knee joint movement whichis different from that
of the underwater dolphin kick.
In Fly-1st and Fly-2nd, Vhip showed no significant
correlation with any kinematic variable. The previ-
ous study indicated that the kick in butterfly swimming
produced large accelerations and suggested that more
propulsion comes from the kick than from the arms
(Sanders et al., 1995). From the results of this study,
it was considered that the swimming velocity during
both situations did not relate to the kick parameters di-
rectly because the arm stroke also contributes to the
propulsion. Furthermore, very large correlations be-
tween Vhip and the peak knee flexion angular velocity,
and the peak hip external rotation angular velocity in
both BD and UD were observed. A previous study re-
ported that swimming velocity during underwater dol-
phin kicks correlates to the vertical toe velocity during
the upward kick phase (Atkison, Dickey, Dragunas, &
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Three-dimensional analysis of hip and knee joint movements during dolphin kicking and butterfly swimming 5
Table 2. The peak joint angle, range of motion, and peak angular velocity during each kick cycle.
Nolte, 2014). Therefore, the greater knee flexion angu-
lar velocity may contribute to the greater toe velocity
during the upward kick phase. As shown in Figure 4,
the hip joint rotated internally during the first half of DK
and externally in the last half of DK during each dolphin
kick. These hip internal-external rotational movements
may contribute to control the direction of the dorsal side
of the foot. However, the present study did not investi-
gate foot movement during the dolphin kick. Therefore,
future studies should examine foot movements during
the dolphin kick.
In conclusion, using three-dimensional analysis, it
was confirmed that the hip flexion/extension and knee
flexion movements in the second dolphin kick during
butterfly stroke were different from movements in the
first dolphin kick during the butterfly stroke and the un-
derwater dolphin kick, while there was no difference
in hip adduction-abduction and hip internal-external ro-
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6
tation movements. Therefore, it was considered that
understanding the kinematic difference is important for
coaching and training to improve the dolphin kick tech-
nique during butterfly stroke.
References
Atkison, R. R., Dickey, J. P., Dragunas, A., & Nolte,
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Barthels, K. M., & Adrian, M. J. (1971). Variability
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Hopkins, W., Marshall, S., Batterham, A., & Hanin, J.
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Full-text available
  • Apr 2022
This study aimed to investigate the changes in kinematics and muscle activity with increasing swimming velocity during underwater undulatory swimming (UUS). In a water flume, 8 male national-level swimmers performed three UUS trials at 70, 80, and 90% of their maximum swimming velocity (70, 80, and 90%V, respectively). A motion capture system was used for three-dimensional kinematic analysis, and surface electromyography (EMG) data were collected from eight muscles in the gluteal region and lower limbs. The results indicated that kick frequency, vertical toe velocity, and angular velocity increased with increasing UUS velocity, whereas kick length and kick amplitude decreased. Furthermore, the symmetry of the peak toe velocity improved at 90%V. The integrated EMG values of the rectus femoris, biceps femoris, gluteus maximus, gluteus medius, tibialis anterior, and gastrocnemius were higher at 90%V than at the lower flow speeds, and the sum of integrated EMGs increased with increasing UUS velocity. These results suggest that an increase in the intensity of muscle activity in the lower limbs contributed to an increase in kick frequency. Furthermore, muscle activity of the biceps femoris and gastrocnemius commenced slightly earlier with increasing UUS velocity, which may be related to improving kick symmetry. In conclusion, this study suggests the following main findings: 1) changes in not only kick frequency but also in kicking velocity are important for increasing UUS velocity, 2) the intensity of specific muscle activity increases with increasing UUS velocity, and 3) kick symmetry is related to changes in UUS velocity, and improvements in kick symmetry may be caused by changes in the muscle activity patterns.
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... However, the motions of the lower-limb joints during human movement are not only extension and flexion movements but also internal/external rotations and abduction/ adduction. Yamakawa et al. (2018) examined the relationship between the threedimensional (3D) hip and knee joint motion and forward-swimming velocity at 80% of maximal velocity in a water flume, reporting significant correlation with external rotation angle. However, the study was not conducted at maximal velocity, and the ankle joint motion was not investigated. ...
Three-dimensional lower-limb kinematics during undulatory underwater swimming
Article
Full-text available
  • Nov 2021
The three-dimensional (3D) motion of lower-limb joints is evaluated during various sports. However, few studies have reported the 3D lower-limb joint movement during undulatory underwater swimming (UUS). This study aimed to investigate the relationship between 3D lower-limb kinematics and forward-swimming velocity during UUS at maximal velocity. A total of 26 male international- and national-level swimmers were assessed during UUS using a motion-capture system. The 3D angle and angular velocity of the lower-limb joints were calculated and relationships between forward-swimming velocity, angle, and angular velocity were investigated using correlation analysis. The peak angular velocities of hip internal and external rotation were significantly correlated with forward-swimming velocity (r = .48, p = .01 and r =−.74, p < .01, respectively). Peak hip internal rotation was observed at the middle of down-kicking (25% kick cycle, 243 ∘/s), whereas peak external rotation was observed at the terminal of down-kicking (50% kick cycle, −351 ∘/s). The swimmers showed a higher peak angular velocity of hip internal/external rotation with a large active range of motion for hip rotation. The swimmers moved their lower-limb joints three-dimensionally, and aside from flexion/extension movements, and hip rotation may increase UUS proficiency.
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3D Knee and Hip Angle Estimation With Reduced Wearable IMUs via Transfer Learning During Yoga, Golf, Swimming, Badminton, and Dance
Article
Full-text available
  • Jan 2024
Wearable lower-limb joint angle estimation using a reduced inertial measurement unit (IMU) sensor set could enable quick, economical sports injury risk assessment and motion capture; however the vast majority of existing research requires a full IMU set attached to every related body segment and is implemented in only a single movement, typically walking. We thus implemented 3-dimensional knee and hip angle estimation with a reduced IMU sensor set during yoga, golf, swimming (simulated lower body swimming in a seated posture), badminton, and dance movements. Additionally, current deep-learning models undergo an accuracy drop when tested with new and unseen activities, which necessitates collecting large amounts of data for the new activity. However, collecting large datasets for every new activity is time-consuming and expensive. Thus, a transfer learning (TL) approach with long short-term memory neural networks was proposed to enhance the model’s generalization ability towards new activities while minimizing the need for a large new-activity dataset. This approach could transfer the generic knowledge acquired from training the model in the source-activity domain to the target-activity domain. The maximum improvement in estimation accuracy (RMSE) achieved by TL is 23.6 degrees for knee flexion/extension and 22.2 degrees for hip flexion/extension compared to without TL. These results extend the application of motion capture with reduced sensor configurations to a broader range of activities relevant to injury prevention and sports training. Moreover, they enhance the capacity of data-driven models in scenarios where acquiring a substantial amount of training data is challenging.
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Net forces during tethered simulation of underwater streamlined gliding and kicking technique of the freestyle turn
Article
Full-text available
  • Nov 2000
We assessed the net forces created when towing swimmers while gliding and kicking underwater to establish an appropriate speed for initiating underwater kicking, and the most effective gliding position and kicking technique to be applied after a turn. Sixteen experienced male swimmers of similar body shape were towed by a motorized winch and pulley system. A load cell measured net force (propulsive force - drag force) at speeds of 1.6, 1.9, 2.2, 2.5 and 3.1 m x s(-1). At each speed, the swimmers performed a prone streamline glide, a lateral streamline glide, a prone freestyle kick, a prone dolphin kick and a lateral dolphin kick. A two-way repeated-measures analysis of variance revealed significant differences between the gliding and kicking conditions at different speeds. The results demonstrated an optimal range of speeds (1.9 to 2.2 m x s(-1)) at which to begin underwater kicking to prevent energy loss from excessive active drag. No significant differences were found between the prone and lateral streamline glide positions or between the three underwater kicking techniques. Therefore, there appears to be no significant advantage in using one streamlining technique over another or in using one kicking style over another.
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Importance of sagittal kick symmetry for underwater dolphin kick performance
Article
  • Nov 2013
The purpose of this study was to determine how sagittal kick symmetry in the underwater dolphin kick (UDK) between the downkick and upkick phases is related to UDK performance. Fifteen adult male competitive swimmers ranging from provincial to international level were filmed performing three trials each of maximum effort UDK over 15m using an underwater video camera. Video frames were manually digitized and each subjects' single fastest trial was evaluated for between-subject comparisons. Kinematic variables were calculated for each individual and Pearson product-moment correlations between the average horizontal centre of mass velocity (Vx) and all kinematic variables were calculated. Horizontal velocity during the downkick, horizontal velocity during the upkick, relative time spent in each phase, maximum chest flexion angle, maximum knee and ankle extension angles, the ratio of flexion/extension for chest, knee and ankle angles, and maximum vertical toe velocity during the upkick phase correlated significantly with Vx (p<0.05). The ratio of downkick vertical toe velocity/upkick vertical toe velocity was significantly negatively correlated with Vx (p<0.05). These results indicate the importance of kick symmetry for UDK performance, and indicate that performing the upkick phase well appears to be most important for UDK performance.
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Progressive Statistics for Studies in Sports Medicine and Exercise Science
Article
  • Jan 2009
  • MED SCI SPORT EXER
Statistical guidelines and expert statements are now available to assist in the analysis and reporting of studies in some biomedical disciplines. We present here a more progressive resource for sample-based studies, meta-analyses, and case studies in sports medicine and exercise science. We offer forthright advice on the following controversial or novel issues: using precision of estimation for inferences about population effects in preference to null-hypothesis testing, which is inadequate for assessing clinical or practical importance; justifying sample size via acceptable precision or confidence for clinical decisions rather than via adequate power for statistical significance; showing SD rather than SEM, to better communicate the magnitude of differences in means and nonuniformity of error; avoiding purely nonparametric analyses, which cannot provide inferences about magnitude and are unnecessary; using regression statistics in validity studies, in preference to the impractical and biased limits of agreement; making greater use of qualitative methods to enrich sample-based quantitative projects; and seeking ethics approval for public access to the depersonalized raw data of a study, to address the need for more scrutiny of research and better meta-analyses. Advice on less contentious issues includes the following: using covariates in linear models to adjust for confounders, to account for individual differences, and to identify potential mechanisms of an effect; using log transformation to deal with nonuniformity of effects and error; identifying and deleting outliers; presenting descriptive, effect, and inferential statistics in appropriate formats; and contending with bias arising from problems with sampling, assignment, blinding, measurement error, and researchers' prejudices. This article should advance the field by stimulating debate, promoting innovative approaches, and serving as a useful checklist for authors, reviewers, and editors.
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Wave characteristics of butterfly swimming
Article
  • Feb 1995
  • J BIOMECH
In this study it was hypothesized that elite butterfly performance is characterized by wave motions with particular frequency, amplitude, and phase characteristics. Particular emphasis was accorded the question of whether 'waves' travel along the body during the butterfly stroke. Selected body landmarks and the center of mass (CM) of eight elite males and eight elite female swimmers were quantified. Fourier analysis was conducted to determine the frequency, amplitude, and phase characteristics of the vertical undulations of the vertex of the head, shoulders, hips, knees, and ankles. The differences in phase between these landmarks for the first (H1) and second (H2) Fourier frequencies were investigated to establish whether waves travelled along the body in a caudal direction. The absolute average velocity of H1 wave travel from vertex to ankle was found to be a mean of 0.34 ms-1 faster than the forward velocity of the CM for the male swimmers and 0.17 m s-1 faster for the female swimmers. There was a very strong relationship (p < 0.01) between velocity of H1 wave travel and CM velocity. There was no evidence to suggest that elite swimmers timed their actions to minimise vertical CM displacement to reduce mechanical work. In fact, the phase relationships among adjacent segments suggested that energy gained by raising the CM was transmitted caudally and contributed to a propulsive 'whip-like' action.
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Variability in the dolphin kick under four conditions
  • Jan 1971
  • K M Barthels
  • M J Adrian
Barthels, K. M., & Adrian, M. J. (1971). Variability in the dolphin kick under four conditions. Paper presented at the First International Symposium on Biomechanics in Swimming, Waterpolo and Diving.