Isolated Core Training Improves Sprint Performance in National-Level Junior Swimmers

Abstract

Purpose:
To quantify the effects of a 12-wk isolated core-training program on 50-m front-crawl swim time and measures of core musculature functionally relevant to swimming.

Methods:
Twenty national-level junior swimmers (10 male and 10 female, 16±1 y, 171±5 cm, 63±4 kg) participated in the study. Group allocation (intervention [n=10], control [n=10]) was based on 2 preexisting swim-training groups who were part of the same swimming club but trained in different groups. The intervention group completed the core training, incorporating exercises targeting the lumbopelvic complex and upper region extending to the scapula, 3 times/wk for 12 wk. While the training was performed in addition to the normal pool-based swimming program, the control group maintained their usual pool-based swimming program. The authors made probabilistic magnitude-based inferences about the effect of the core training on 50-m swim time and functionally relevant measures of core function.

Results:
Compared with the control group, the core-training intervention group had a possibly large beneficial effect on 50-m swim time (-2.0%; 90% confidence interval -3.8 to -0.2%). Moreover, it showed small to moderate improvements on a timed prone-bridge test (9.0%; 2.1-16.4%) and asymmetric straight-arm pull-down test (23.1%; 13.7-33.4%), and there were moderate to large increases in peak EMG activity of core musculature during isolated tests of maximal voluntary contraction.

Conclusion:
This is the first study to demonstrate a clear beneficial effect of isolated core training on 50-m front-crawl swim performance.

Full text

204
www.IJSPP-Journal.com
ORIGINAL INVESTIGATION
International Journal of Sports Physiology and Performance, 2015, 10, 204-210
http://dx.doi.org/10.1123/ijspp.2013-0488
© 2015 Human Kinetics, Inc.
Isolated Core Training Improves Sprint Performance
in National-Level Junior Swimmers
Matthew Weston, Angela E. Hibbs, Kevin G. Thompson, and Iain R. Spears
Purpose: To quantify the effects of a 12-wk isolated core-training program on 50-m front-crawl swim time and measures of core
musculature functionally relevant to swimming. Methods: Twenty national-level junior swimmers (10 male and 10 female, 16
± 1 y, 171 ± 5 cm, 63 ± 4 kg) participated in the study. Group allocation (intervention [n = 10], control [n = 10]) was based on
2 preexisting swim-training groups who were part of the same swimming club but trained in different groups. The intervention
group completed the core training, incorporating exercises targeting the lumbopelvic complex and upper region extending to the
scapula, 3 times/wk for 12 wk. While the training was performed in addition to the normal pool-based swimming program, the
control group maintained their usual pool-based swimming program. The authors made probabilistic magnitude-based inferences
about the effect of the core training on 50-m swim time and functionally relevant measures of core function. Results: Compared
with the control group, the core-training intervention group had a possibly large benecial effect on 50-m swim time (–2.0%;
90% condence interval –3.8 to –0.2%). Moreover, it showed small to moderate improvements on a timed prone-bridge test
(9.0%; 2.1–16.4%) and asymmetric straight-arm pull-down test (23.1%; 13.7–33.4%), and there were moderate to large increases
in peak EMG activity of core musculature during isolated tests of maximal voluntary contraction. Conclusion: This is the rst
study to demonstrate a clear benecial effect of isolated core training on 50-m front-crawl swim performance.
Keywords: athletic training, exercise performance, functional performance, strength training
Muscle strength and power are major determinants of success
in competitive swimming,1 and, therefore, competitive swimmers
are advised to perform specic dry-land training to improve perfor-
mance.1,2 There is, however, a recognized shortage of well-designed
studies focusing on the effects of training interventions for swim-
mers.2 Of the available studies, research has focused predominantly
on the effects of land-based strength and power interventions, with
inconsistent ndings on swim performance.3–6
Exercises devised to train the core musculature are integral to
many strength and conditioning programs,7,8 as greater core stability
may provide a foundation for greater force production in the upper
and lower extremities.9 However, while good core functioning is
commonly believed to enhance athletic performance, recent reviews
have concluded that core training provides only marginal benets to
athletic performance.8 Difculty in isolating a core-training effect
on athletic performance, as core training is rarely the sole compo-
nent of athletic development,8 and lack of sport specicity10 could
explain the absence of a greater benecial effect.
Improved core stability could be particularly benecial for
sprint swimmers, allowing efcient transfer of force between the
trunk and the upper and lower extremities to propel the body through
the water.9 Furthermore, swimming is different from ground-based
sports in that the core becomes the reference point for all move-
ment.9 A targeted training program to improve core functioning of
sprint swimmers would therefore appear logical, yet currently there
have been no controlled trials examining the isolated effect of core
training on swimming performance. The aim of our study was to
quantify the effects of a 12-week isolated core-training program
on 50-m front-crawl swim time and measures of core musculature
functionally relevant to swimming.
Methods
Subjects
Twenty national-level junior swimmers participated in this study.
Ten swimmers (5 male and 5 female, 15.7 ± 1.2 y, 172 ± 6 cm,
63 ± 5 kg) formed the core-training intervention group and 10
swimmers (5 male and 5 female, 16.7 ± 0.9 y, 170 ± 3 cm, 63 ±
3 kg) formed the control group. At baseline, both groups were
performing similar weekly distances in training (average of 30
km) and the same number and type of swimming-training ses-
sions (8 sessions/wk). These consisted of recovery, tempo, and
endurance-based swimming sessions. During the study, pool-based
training sessions continued as normal and both groups completed
the same duration and intensity of training. These pool-based
training sessions were coach-led, with the 2 groups performing
the sessions at the same time but in different parts of the pool.
All participants were familiar with, but not actively engaged in,
core-training exercises before the study. The ethics committee of
the local university approved the study, and informed consent was
provided by all study participants.
Design
The design of our exploratory study was cluster controlled before
and after study, as allocation was performed at a group level, based
on 2 preexisting swim-training groups who were part of the same
Weston and Spears are with the Sport and Exercise Subject Group, Teesside
University, Middlesbrough, UK. Hibbs is with the Dept of Sport, Exercise
and Rehabilitation, Northumbria University, Newcastle Upon Tyne, UK.
Thompson is with the Research Inst for Sport and Exercise, University of
Canberra, Canberra, Australia. Address author correspondence to Matthew
Weston at m.weston@tees.ac.uk.

Core Exercises and Swimmers 205
swimming club but trained in different groups. We applied this
design because allocation on an individual level may have resulted
in signicant crossover contamination.11 The study was performed
1 month into the swim season, so all swimmers were in full training
when the study commenced.
Intervention
The intervention group completed a 12-week core-training program
in addition to their normal pool-based swimming regimen. For the
purposes of this study, the regions of the body that are included
in the term core are the upper legs, pelvis, trunk, and shoulders.7
Specically, the regions targeted in this training program were
the lower spine, lumbopelvic complex, and upper region extend-
ing to the scapula. The core-training program consisted of 6
exercises based on the existing literature (Table 1), which were
as follows: prone bridge (Figure 1[a]), side bridge (Figure 1[b]),
bird dog (Figure 1[c]), straight-leg raise (Figure 1[d]), overhead
squat (Figure 1[e]), and medicine-ball sit twist (Figure 1[f]).12–16
In a previous study14 these exercises were found to induce EMG
activity greater than threshold levels15 required for improving core
stability (10–25% of maximal voluntary contraction [MVC]) and
core strength (>60% of MVC). Notably, the side bridge has been
reported to elicit peak EMG values of 42% ± 24% of MVC in
lumbar multidus16 and the bird-dog exercise to elicit peak EMG
of 42% ± 17% of MVC and 56% ± 22% of MVC in gluteus medius
and maximus, respectively.16 The prone bridge elicited peaks of
47% ± 21% and 43% ± 21% of MVC in the external oblique and
upper rectus abdominis, respectively.16 In addition, a seventh
exercise termed an asymmetrical horizontal shoulder press was
included (Figure 1[g]). Each exercise was performed twice for a
total of 60 seconds with 60 seconds recovery between sets. A model
for exercise progression was incorporated by gradually increasing
the number of repetitions, sets, and, where appropriate, the level
of resistance (Table 1) or period of time in a hold position. Over
the 12-week training period the core exercises were performed 3
times a week. Each core-training session lasted approximately 30
minutes. The quality of the exercises was monitored during the
sessions by a national-level Amateur Swimming Association coach
and fortnightly by a British Association of Sport and Exercise Sci-
ences–accredited sport scientist. To minimize learning effects, all
participants were given a familiarization exercise session 7 days
before the intervention.
Outcome Measures
All outcome measures were assessed preintervention and postinter-
vention. Testing sessions took place 7 days either side of the training
intervention and commenced after the swimmers’ usual session
preparation, which involved a cardiovascular warm-up followed by
static and dynamic stretching. Swim performance was quantied by
a timed 50-m front-crawl race. In groups of 4, commencing with
the usual dive start, the swimmers sprinted the length of a 50-m
pool. Using split-timing stopwatches (Fastime 5, Fastime Ltd, UK)
the swim coaches recorded the times, with each coach recording
the times of 2 swimmers per race. Hand timing by experienced
swim coaches has been reported to have acceptable precision.17
To examine the effectiveness of our core-training intervention on
shoulder extension in the sagittal plane, a straight-arm latissimus
dorsi pull-down test was used. A strong relationship exists between
upper-body strength and sprint swimming performances,6,18 and
shoulder extension in the sagittal plane is integral to the front-
crawl swim stroke. Here, the participant stood facing a stacked
cable-based weight machine (Life Fitness CMDAP C/Motion Dual
Adjust Pulley, Powerhouse Fitness, UK) and held the bar with a
pronated grip and extended elbow and with the shoulder exed to
90°. Participants pulled the cable down until the hand reached the
hip. After 30-second rest periods, weight was increased in incre-
ments of 1.25 kg until the participant was no longer able to perform
the movement without observable exion of the elbow or of the
lumbar region. To examine the effectiveness of our intervention on
core endurance, participants performed a timed prone-bridge test,
Table 1 Core-Exercise Progression Over the 12-Week Training Regimen
Exercise Progression
Weeks 1–2 Weeks 3–4 Weeks 5–6
Repetitions Sets Repetitions Sets Repetitions Sets
Prone bridge Volume 30-s hold 2 60-s hold 2 90-s hold 2
Side bridge Volume 30-s hold 2 60-s hold 2 90-s hold 2
Bird dog Volume 10 3 15 3 20 3
Leg raise Volume 10 3 15 3 20 3
Overhead squat Resistance 10 (3 kg) 3 10 (4 kg) 3 15 (5 kg) 3
Sit twist Resistance 15 (3 kg) 3 15 (4 kg) 3 15 (5 kg) 3
Shoulder press Volume 10 3 10 4 15 4
Weeks 7–8 Weeks 9–10 Weeks 11–12
Prone bridge Volume 90-s hold 3 120-s hold 2 120-s hold 3
Side bridge Volume 90-s hold 3 120-s hold 2 120-s hold 3
Bird dog Volume 25 3 25 4 30 3
Leg raise Volume 25 3 25 4 30 3
Overhead squat Resistance 20 (6 kg) 3 20 (7 kg) 4 25 (7 kg) 3
Sit twist Resistance 20 (6 kg) 3 20 (7 kg) 4 25 (7 kg) 3
Shoulder press Volume 20 3 20 4 25 3

206
Figure 1 — Core-training exercise details. (a) Prone-bridge. Hold a straight body position supported on elbows and toes. Brace the abdominal
muscles and hold the back in a neutral position. (b) Side bridge. Lie on one side, ensuring top hip is positioned above the bottom hip. Push up until
there is a straight body line through feet, hips, and head. (c) Bird dog. Position hands below shoulders and knees below hips. Place back in neutral,
slowly extend 1 leg backward, and raise forward the opposite arm until level with back. Ensure that back does not extend and shoulders and pelvis
do not tilt sideways. Bring leg and arm back to start position and swap sides. (d) Leg raise. Lie on back with knees extended on oor. Place back in
neutral position and lift 1 leg straight up, keeping knee extended and other leg held out horizontally off oor. Raise leg till hip at 75°, then return
to start position and repeat with opposite leg. (e) Overhead squat. Using weighted medicine ball, place hands on either side of ball and raise above
head with straight arms. Feet shoulder width apart, squat down as low as possible while maintaining balance, keeping ball, head, and back vertical.
Straighten legs and repeat. (f) Sit twist. Sit up with knees bent and lean back at 45°. Feet off oor, keeping back in neutral, using a 4-kg medicine
ball, twist waist and shoulders to one side with ball held out in front of you. Return to forward and repeat on other side. (g) Shoulder press. Lie
prone on the oor with both arms fully extended. With a 3-kg dumbbell in each hand, raise 1 arm upward and then return the arm back to the oor.
Repeat this movement, alternating arms.

Core Exercises and Swimmers 207
as exercise performed in the prone position appears to be specic
to the core requirements of swimming.9 During this test the par-
ticipants remained in a prone-bridge position (forearms and toes
in contact with oor and with the spine in a neutral position). The
position was held until observable movements of the pelvis signaled
the end of the test.
To provide additional information on neuromuscular adapta-
tion to the intervention, and to potentially elucidate any mecha-
nisms underpinning changes in performance, we elected to analyze
EMG activity of some of the core muscles while performing
MVCs preintervention and postintervention.19 Maximal EMG
signals are representative of net neural drive20 and changes that
are considered to represent neural adaptations and seem to play a
role in explaining strength gains.19 Surface EMG data were col-
lected using a reference electrode placed on the right iliac crest,
with sensor muscle positions as per Cram.21 The maximal tests
were resisted trunk rotation targeting the external oblique, resisted
back extension targeting the supercial lumbar multidus, and
resisted hang targeting the latissimus dorsi muscle.14 The lumbar
multidus is a difcult muscle to analyze, and the resulting EMG
signal will likely include crosstalk from the thoracis longissimus
muscle22 and not likely include activity of the deep bers, which
differ in function from the supercial bers in terms of stabilizing
the spine.23 Each MVC test was performed 3 times for 15 seconds
with 1-minute rests. Further details of the procedures involved in
recording and processing the EMG variables and also the reliability
of these measures for a similar population have been published
elsewhere.14 The peak EMG amplitude during an MVC was used
as our proxy measure of net neural drive, and it is recognized
that this variable will only have a monotonic relationship with
the force-generating capacity of that muscle, does not reect its
endurance capacity, and is prone to large variability due to inac-
curate sensor placement when recording on separate days.24 We
took photographs of the skin-mounted sensors at baseline and
used these to reposition the sensors for the postintervention tests
to reduce the variability that arises from inaccurate sensor place-
ment. As is consistent with previous training studies, the EMG data
were not normalized,19 as this would mask any potential training
effects on neural adaptation.
Statistical Analysis
Data are presented as mean ± SD. Before analysis, all outcome
measures were log-transformed and then back-transformed to obtain
the percent difference, with uncertainty of the estimates expressed
as 90% condence intervals (CI), between the posttest and pretest.
This is the appropriate method for quantifying changes in athletic
performance.25 Mixed-effects linear modeling (IBM SPSS version
21.0) was used to analyze the effect of the core-stability-training
intervention on our outcome measures. This method allows for and
quanties, as a standard deviation, individual differences in response
to an intervention, which are often highly variable. An analysis of
covariance (ANCOVA) was used to compare the 2 groups, with the
pretest score, age, and body mass as covariates to control for imbal-
ance in our measures between the control and intervention groups at
baseline.26 We made probabilistic magnitude-based inferences about
the true value of the outcomes, based on the likelihood that the true
population difference was substantially positive or substantially
negative. With a between-competitions variability of ~1% for top
junior swimmers, any strategy to improve performance needs to
be at least 0.5 of this variability.27 Therefore, our threshold values
for assessing the magnitude of small, moderate, and large effects
in 50-m swim times were 0.5,% 1.5%, and 2.7%, respectively.25
Standardized thresholds for small, moderate, and large changes
(0.2, 0.6, and 1.2, respectively)25 derived from between-subjects
standard deviations of the baseline value were used to assess the
magnitude of all other effects. Inferences were then based on the
disposition of the condence interval for the mean difference to
these standardized thresholds and calculated as per the magnitude-
based inference approach using the following scale: 25% to 75%,
possibly; 75% to 95%, likely; 95% to 99.5%, very likely; >99.5%,
most likely.25 Inference was categorized as clinical for changes in
50-m swim time, with the default probabilities for declaring an
effect clinically benecial being <0.5% (most unlikely) for harm
and >25% (possibly) for benet.25 Magnitudes of effects for all other
measures were classied as unclear if the 90% condence interval
overlapped the thresholds for the smallest worthwhile positive and
negative effects.25
Results
The core-training intervention had a possibly large benecial
effect on 50-m front-crawl swim time (Table 2). The standard
deviation of the individual responses for 50-m swim time after
core training was 1.4% (90% condence interval 1.0–1.7%).
This represents the variability in the mean effect of the core-
training intervention due to individual swimmer responses. The
core-training group showed small to moderate improvements
on the prone-bridge and straight-arm pull-down tests compared
with controls. Secondary to this, there were moderate to large
increases in peak EMG activity of the latissimus dorsi, external
oblique abdominis, and lumbar extensor muscles (lumbar mul-
tidus/thoracis longissimus) during isolated tests of MVC. The
effect of the intervention on body mass of the intervention group
(1.5%; 0.8–2.3%) and the control group (0.7%; –0.1% to 1.4%)
was likely to be trivial (0.9%; –0.2% to 1.9%).
Discussion
The change in performance time needed to enhance a top junior
swimmer’s chance of winning a medal is as little as 0.5%.27 We
are therefore condent that our clear benecial effect (–2.0%) on
sprint swimming performance after an isolated core-training pro-
gram represents a true performance enhancement. Further to this
enhanced performance effect, our intervention elicited improved
shoulder extension in the sagittal plane and performance on the
prone-bridge test. In addition, improvements in maximal EMG
activity of key core musculature were also observed. As such, these
improvements in functionally relevant measures of core function
and neuromuscular adaptations in stroke-specic musculature
provide evidence of potential mechanisms subtending the observed
improvement in 50-m swim time.
There is a recognized shortage of well-designed studies focus-
ing on the effects of training interventions for swimmers.2 For
the most part, research has focused on the effects of land-based

208 Weston et al
strength and power interventions that may include, but do not
necessarily examine in isolation, the effects of core training. The
effects of these programs on swim performance are inconsistent.
A land-based training program evaluated by Tanaka et al3 did not
lead to any improvement in swim performance. Those authors
suggested that improved strength and power do not transfer to
swimming performance. In contrast to this nding, Strass4 and
Sharp et al5 reported respective improvements of 2.1% (50 m)
after a 6-week heavy, explosive strength-training program and
3.6% (22.9 m) after an 8-week swim bench training program.
However, the effect of an intervention should be measured relative
to nonintervention (ie, control), and the aforementioned studies
either were uncontrolled trials or failed to report control-group
data. With this in mind, the ~2.0% improvement in 50-m swim
time observed in our study is consistent with the work of Girold et
al,6 who examined the effect of dry-land and resisted and assisted
sprinting on swimming sprint performances and found 1.9% and
1.4% improvements, respectively, in 50-m swim times compared
with controls.
Our results demonstrate a clear benecial effect of the core-
training intervention on measures of core functioning and, given
the improvements in swim time, the improvements appear to have
transferred to 50-m front-crawl swim performance. There have
been several attempts to examine the effects of core training on
sports performance, and generally the ndings have been unclear.
For example, the effect of a Swiss-ball core-training program on
the economy of running was found to be minimal,28 and it has
subsequently been suggested that the lack of sport specicity of
core-training programs is to blame.10 The difculty in devising
a sport-specic core-training program is potentially exacerbated
for swimmers, because not only are the general biomechanics of
the core very complex but also detailed biomechanical analyses
of swimming are difcult to perform. Specically, techniques for
simultaneous kinetic and kinematic data capture required to perform
detailed analyses are not widely available in aquatic environments.
Consequently, it is difcult to develop an objectively determined
exercise program that is optimally designed for the specic needs
of swimmers. Nonetheless, our observations of improved perfor-
mance in all of our outcome measures would lend support to the
proposition that positive training transfer has occurred for some
of these core exercises.
Along with improved 50-m swim times our core-training
intervention also improved core endurance. Our training effect
on the prone-bridge test appears to be less than that reported by
Parkhouse and Ball,29 who reported static and dynamic core exer-
cises to improve core endurance by ~23.0%, although that was an
uncontrolled study. Furthermore, our baseline values in the prone-
bridge test were substantially greater, and, as such, any training
effect is likely to be smaller. Along with improved core endurance,
we also found a moderate improvement in strength on an asym-
metric straight-arm pull-down test. Comparisons with the literature
are not possible here, though, as previous studies only considered
this action in symmetric conditions. As shoulder extension was
not specically targeted in the training intervention, our nding
is difcult to explain. Presumably, improved core functioning
contributed to the improvements in shoulder-extension strength
via stabilization of the trunk. Moderate to large improvements in
EMG activity during MVCs of key core musculature help explain
improved performance on our measures, as an increase in the MVC
EMG activity is considered to reect an increase in neural drive
and neuromuscular strength of the underlying muscle.19,30 Thus,
the improvements in MVC activity in these tests are considered
benecial and comparable with MVC improvements observed in
other populations (eg, Fimland et al30).
Our experiment was performed in a pragmatic setting in which
there were no opportunities to isolate and discriminate the effects
of the individual components of the intervention. Therefore, it is
impossible to ascertain with condence which of these exercises was
the most important. Since front-crawl swimming is performed in a
prone position—requiring the maintenance of horizontal posture via
lumbar extensors—it may have been that the side bridge, shown to
elicit high levels of activity in the lumbar multidus and/or longis-
simus thoracis muscles,16 may have contributed disproportionately
to the success of our intervention. In contrast, for each stroke in
which the hand pushes against the water to provide propulsion, the
dynamic reaction forces exerted on the hands will be directed away
from the joint centers of the spine, thus creating dynamic moments
about the 3 rotational axes of the vertebrae. The ability to maintain
Table 2 Outcome Measures at Baseline With Effect Statistics and Inferences for Within- and Between-Groups
Comparisons Mean ± SD or % Mean; 90% Confidence Interval
Core-Training Group Control Group
Group Comparison
(Core–Control)
Baseline
value
Adjusted
change score
Baseline
value
Adjusted
change score
Difference
between groups QI
Performance measure
50-m swim time (s) 29.7 ± 2.1 –2.7; –4.2 to –1.1 28.0 ± 1.9 –0.7; –1.6 to 0.2 –2.0; –3.8 to –0.1 large +*
prone-bridge test (s) 211 ± 71 14.1; 9.2–19.2 221 ± 92 4.7; 0.2–9.3 9.0; 2.1–16.4 small +*
straight-arm pull-down (kg) 8.5 ± 2.6 26.2; 19.6–33.1 8.4 ± 2.4 2.5; –2.9 to 8.1 23.1; 13.7–33.4 mod +**
Peak EMG activity during an
isolated MVC
external oblique (mV) 503 ± 29 8.4; 6.4–10.5 508 ± 19 0.7; –1.3 to 2.6 7.7; 4.6–10.8 large +**
multidus (mV) 361 ± 22 17.6; 10.2–25.5 316 ± 21 1.2; –5.2 to 8.0 16.2; 3.9–30.1 large +**
latissimus dorsi (mV) 801 ± 76 4.4; 2.7–6.2 825 ± 72 –1.4; –3.0 to 0.3 5.9; 3.4–8.5 mod +*
Abbreviations: QI, qualitative inference; +, positive effect on core-training group compared with controls; MVC, maximal voluntary contraction.
*25–75%, possibly. **75–95%, likely.

Core Exercises and Swimmers 209
stability and control of the trunk during body roll and to resist these
asymmetrical moments is likely to be enhanced by dynamic asym-
metrical exercises such as the bird dog. Specically, this exercise
elicits high levels of activity in gluteus maximus, external oblique
abdominis, and gluteus medius14,16 while stabilizing the trunk in
a prone position. Thus, taken together, it may be that the static
symmetrical exercises are secondary to the dynamic asymmetrical
exercises in terms of importance (or vice versa). Similarly, it may
be that some elements of progression (eg, increasing the number of
sets) were more effective than others (eg, increasing the hold times).
Regardless, an improved understanding of swimming technique
through aquatic-based measuring tools will in time improve this
understanding and allow further renement of the intervention.
Practical Applications
Our ndings suggest that the implementation of isolated core
exercises appears to be a worthwhile addition to the programming
of a swimmer’s dry-land training routine. There were, however,
several limitations associated with our study. First, we were unable
to provide precise information with regard to the intensity of pool-
based training sessions undertaken by both groups of swimmers.
However, the breakdown of session typology between groups was
consistent, leading to similar training volumes (km). Further to this,
all sessions were prescribed and delivered by experienced swim and
strength and conditioning coaches. Second, while we have demon-
strated a clear benecial effect of our core-training intervention on
50-m swim performance, we were unable to examine the training
effect on swim-stroke mechanics, namely, stroke depth, rate, and
length, and also dive mechanics. This is an area that warrants further
research—in particular, the effect of isolated core training on stroke
rate, given that in a 50-m sprint this variable is important, because to
be efcient the 50-m swimmer has to generate relatively moderate
to high levels of maximal strength but at a high stroke rate.6 Third,
timing error may increase when the number of swimmers measured
by the coach increases. Therefore, for future studies we recommend
video analysis and/or timing pads as a solution to this potential
problem. Fourth, when one uses clustered experimental design
observations, individuals in the same cluster tend to be correlated.31
Failing to account for dependence between individual observations
and the cluster to which they belong produces condence intervals
that are too narrow.11 In the absence of any previously reported
intracluster correlation coefcients for 50-m swim performances
of elite junior swimmers after an exercise intervention, we were
unable to determine the design effect and allow for clustering in our
analysis. Fifth, our secondary measure to monitor neuromuscular
adaptation is simple yet pragmatic. Further laboratory-based work
to derive rates of activation and torque development alongside
additional measures such as MVC torques, muscle cross-sectional
area, and cocontractions would improve our understanding of the
specic nature of the muscle response to the training program.
Finally, a major hurdle when studying young athletes is that the
effects of growth and maturation may mask or be greater than the
effects of training.32 However, we found clear improvements in our
measures of performance and tness after controlling for the effect
of age and body mass. Given the short-term nature of our core-
training intervention and the age of the swimmers, it is unlikely that
maturation affected our results, especially as young swimmers tend
to be average or slightly advanced in maturity status.32 Further to
this, the effect of the intervention on body mass was trivial, which
we believe provides further support for neuromuscular gains, not
growth or maturation, subtending the improvements we observed
in all our outcome measures.
Conclusion
Our ndings represent the rst piece of evidence for the benecial
effect of isolated core training on sprint swim performance in national-
level junior swimmers. Further to this we have evidenced adaptations
that could well subtend the improved 50-m swim times, namely,
enhanced performance on functional tests relevant to the front-crawl
swim stroke and greater MVCs of the involved musculature.
References
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