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EDITED BY
Emiliano Cè,
University of Milan, Italy
REVIEWED BY
Shannon Siegel,
University of San Francisco, United States
Christian Doria,
University of Milan, Italy
*CORRESPONDENCE
Jiří Baláš
balas@ftvs.cuni.cz
RECEIVED 10 July 2023
ACCEPTED 18 September 2023
PUBLISHED 29 September 2023
CITATION
Javorský T, Saeterbakken AH, Andersen V and
BalášJ (2023) Comparing low volume of blood
flow restricted to high-intensity resistance
training of the finger flexors to maintain
climbing-specific strength and endurance: a
crossover study.
Front. Sports Act. Living 5:1256136.
doi: 10.3389/fspor.2023.1256136
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Baláš. This is an open-access article distributed
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terms.
Comparing low volume of blood
flow restricted to high-intensity
resistance training of the finger
flexors to maintain
climbing-specific strength and
endurance: a crossover study
TomášJavorský1,2, Atle Hole Saeterbakken3, Vidar Andersen3
and Jiří Baláš1*
1
Faculty of Physical Education and Sport, Charles University, Prague, Czech Republic,
2
Department of
Sport Science, University of Innsbruck, Innsbruck, Austria,
3
Faculty of Education, Arts and Sports, Western
Norway University of Applied Sciences, Sogndal, Norway
Introduction: It is acknowledged that training during recovery periods after injury
involves reducing both volume and intensity, often resulting in losses of sport-
specificfitness. Therefore, this study aimed to compare the effects of high-
intensity training (HIT) and low-intensity training with blood flow restriction (LIT
+ BFR) of the finger flexors in order to preserve climbing-specific strength and
endurance.
Methods: In a crossover design, thirteen intermediate climbers completed two 5-
week periods of isometric finger flexors training on a hangboard. The trainings
consisted of ten LIT + BFR (30% of max) or HIT sessions (60% of max without
BFR) and were undertaken in a randomized order. The training session consisted
of 6 unilateral sets of 1 min intermittent hanging at a 7:3 work relief ratio for
both hands. Maximal voluntary contraction (MVC), force impulse from the 4 min
all out test (W), critical force (CF) and force impulse above the critical force (W’)
of the finger flexors were assessed before, after the first, and after the second
training period, using a climbing-specific dynamometer. Forearm muscle
oxidative capacity was estimated from an occlusion test using near-infrared
spectroscopy at the same time points.
Results: Both training methods led to maintaining strength and endurance
indicators, however, no interaction (P> 0.05) was found between the training
methods for any strength or endurance variable. A significant increase (P=
0.002) was found for W, primarily driven by the HIT group (pretest—25078 ±
7584 N.s, post-test—27327 ± 8051 N.s, P= 0.012, Cohen’sd= 0.29). There were
no significant (P> 0.05) pre- post-test changes for MVC (HIT: Cohen’sd= 0.13;
LIT + BFR: Cohen’sd=−0.10), CF (HIT: Cohen’sd= 0.36; LIT + BFR = 0.05), W`
(HIT: Cohen’sd=−0.03, LIT + BFR = 0.12), and forearm muscle oxidative
capacity (HIT: Cohen’sd=−0.23; LIT + BFR: Cohen’sd=−0.07).
Conclusions: Low volume of BFR and HIT led to similar results, maintaining
climbing-specific strength and endurance in lower grade and intermediate
climbers. It appears that using BFR training may be an alternative approach after
finger injury as low mechanical impact occurs during training.
KEYWORDS
injury, hypertrophy, hypoxia, ischemia, intermittent exercise, isometric contraction,
strength, oxidative capacity
TYPE Original Research
PUBLISHED 29 September 2023
|
DOI 10.3389/fspor.2023.1256136
Frontiers in Sports and Active Living 01 frontiersin.org
Introduction
Sport climbers heavily rely on finger flexor contractions,
making finger flexor strength and endurance crucial predictors of
climbing performance (1,2). Previous research has extensively
investigated the physiological adaptations induced by high-
intensity training (HIT) on finger strength and endurance (3,4).
For example, specific maximal strength and hypertrophy training
designed for climbers have demonstrated significant increases in
finger flexor strength and endurance after 5–10 weeks of training
(5–8). However, HIT of the finger flexors may increase the risk
of injuries in the fingers, hands, elbows, or shoulders, with
chronic injuries being the most common among sport climbers
(9,10). Moreover, when recovering from injuries such as pulley
ruptures or strains it is recommended to gradually increase
training loads (11). Consequently, recovery periods require
climbers to train with decreased intensity, often resulting in
losses of sport-specificfitness.
An alternative approach to HIT for improving or maintaining
finger strength and muscle hypertrophy is training at low
intensities (typically 20%–40% of maximum strength) with blood
flow restriction (LIT + BFR), achieved by applying external
pressure to the limb proximal to the working muscle (12). LIT +
BFR exercise creates a localized hypoxic environment and
promotes recruitment of both types I and II muscle fibres,
leading to enhanced muscle strength and power (13–15).
Furthermore, changes in key markers of protein synthesis, such
as mTOR and HIF-1, support the observed adaptations in the
muscle following LIT + BFR training (16,17). Accordingly, LIT +
BFR triggers an upregulation of protein synthesis, facilitating
muscle growth and strength gains despite the use of lower
training loads (decreased mechanical stress). This suggests that
the metabolic stress induced by LIT + BFR exercise can stimulate
muscle protein synthesis to a comparable extent as high-intensity
exercise (18,19). To date there are no studies comparing HIT
and LIT + BFR in climbing-specific hangboard resistance training.
However, based on the existing literature, it is reasonable to
hypothesize that LIT + BFR and HIT may yield comparable
effects in finger flexors training in climbers.
Previous research has shown that increasing strength can be
achieved with low volume of HIT per week (20,21). However,
it remains unknown whether the same training volume of LIT
+ BFR would yield similar effects. Most studies investigating
blood flow restriction (BFR) interventions have primarily
focused on designs maximizing their effectiveness for increasing
muscle strength and hypertrophy (22,23). However, during the
recovery period following an injury, the primary objective of
training is to maintain strength and endurance levels using
minimal load and training volume (20). Low-intensity training
(LIT) with BFR training has been proposed and utilized as a
method of recovery after various types of injuries in lower
limbs such as knee osteoarthritis (24) or arthroplasty (25),
however, to authors best knowledge, there is not any literature
available on this topic on the upper extremities related to the
climbing.
Therefore, the objective of this study was to investigate the
effects of low volume of LIT + BFR training and HIT on
maintaining climbing-specific strength and endurance. We
hypothesised that HIT and LIT + BFR will be equally effective in
preserving sport specific strength and endurance in intermediate
climbers.
Methods
Participants
Thirteen lower grade to intermediate climbers [6 male, 7 female
participants: males—age, 24.3 ± 2.0 yrs; climbing ability level 13 ± 4
IRCRA (International Rock Climbing Research Association) grade;
females—age, 32.6 ± 12.5 yrs; climbing ability 9 ± 2 IRCRA grade]
volunteered to take part in the study. Participants self-reported
their climbing ability using French/Sport grade which was
transformed to the IRCRA difficulty scale ranging from 1 to 32
(26). At the beginning, all participants completed written
informed consent forms and medical health questionnaires.
Exclusion criteria included venous thrombosis, cardiovascular
diseases (including high blood pressure and diabetes),
unexplained chest pain, heart pathologies, and fainting during
physical activities. Additionally, participants with carpal tunnel
syndrome, acute upper limb injuries, tendosynovitis, or tendon
injuries in the upper limb, pregnancy, or in the injury recovery
phase were also excluded.
Participants were instructed to abstain from engaging in any
strenuous exercise, consuming caffeine, and consuming alcohol
within 24 h before each experimental testing session.
Furthermore, participants were not allowed to maintain normal
training routine or engage in any finger flexor strength and
endurance training. This was achieved partially by the ongoing
COVID lockdown when sport facilities were closed. Additionally,
participants were asked to continue their regular dietary and
supplement habits. The study was approved by Ethics Committee
of Charles University, Faculty of Physical Education and Sport.
The participants provided their written informed consent to
participate in this study.
Experimental protocol
The 13 weeks long experimental protocol is depicted at
Figure 1. All participants completed two 5 weeks periods of
finger flexors training in a cross-over randomized order with a 1-
week long washout period. The two training interventions
consisted of either isometric HIT or LIT + BFR on a hangboard.
Testing climbing specific strength and endurance was applied
before and after each period of training (Figure 1).
To eliminate interference between individual tests, the
participants underwent two separate testing sessions during the
testing week. In the first session, the muscle oxidative capacity
and the maximal voluntary contraction (MVC) were assessed.
Javorský et al. 10.3389/fspor.2023.1256136
Frontiers in Sports and Active Living 02 frontiersin.org
The second testing session involved performing a 4-min all-out test
after the measurement of blood pressure to determine the level of
occlusion.
Upon their first visit, participants were randomly assigned into
two groups based on the training intervention. They were also
familiarized with the laboratory setup. Additionally, they
completed a questionnaire and signed the medical consent form.
In the questionnaire, participants reported their climbing ability
as proposed by Draper et al. (26).
Warm-up
All subjects completed a standardized self-directed warm-up
prior to the assessment and training protocol. The warm-up
consisted of three minutes of pulse-raising activity, such as
jogging or cycling, followed by three minutes of climbing, which
is considered a sport-specific activity. In addition, the warm-up
included a series of 5:5 s work-to-rest ratio hangs on the testing
edge in a half-crimp position at ∼50% of the perceived
maximum force (27).
Training interventions
Both training interventions consisted of 10 training sessions (2
sessions per week during each 5-week period). The LIT + BFR and
HIT participants previously scheduled a time of the day for the
individual sessions of hangboard strength exercises. The intensity
for each training type was based on the MVC tested prior to
each intervention. The training was performed on the same
wooden rung as for testing MVC and all-out test (see below) in
standing position with arms ∼180° flexed in shoulder, and
slightly flexed in elbows. Participants applied the target force on
the rung by hanging (bending the knees). The online feedback of
applied force was visible on the screen of the testing/training
device (1D-SAC, Spacelab, Sofia, Bulgaria).
Blood flow restriction training
To implement BFR, we utilized a cuff provided by Occlude
ApS (Aarhaus, Denmark). Prior to each training session, the
cuff was inflated to 60% of the complete arterial occlusion
pressure (21,28) on training arm, which caused decrease in
the blood flow in the downstream vascular system by 47%–
48% (29). In each session both arms performed 6 sets over
two blocks (one block consisted of three consecutive sets)
unilaterally for each arm, and each set comprising 6
repetitions performed at 30% of MVC, with a work-to-rest
ratio of 7 to 3 s. Following the completion of set 3 (60 s rest in
between) for one arm, the cuff was deflated and participants
immediately continued with the other arm for next three sets.
In total, 36 isometric contractions for each arm were
completed (Figure 2). The cuff pressure was monitored and
controlled during the rest periods between sets.
FIGURE 2
Position of participant during the Low-intensity training with the blood
flow restriction.
FIGURE 1
Experimental design of the study.
Javorský et al. 10.3389/fspor.2023.1256136
Frontiers in Sports and Active Living 03 frontiersin.org
High-Intensity training
Participants performed HIT sessions at 60% of their MVC. The
same volume of training as for LIT + BFR was applied. Each
training session consisted of 12 working sets (i.e., 6 sets of each
arm divided into two blocks with 5 min rest in between), with
each set comprising 6 repetitions and a work-to-rest ratio of 7 to
3 s. Following the completion of the third set, participants were
given a 5 min recovery period while the other arm was exercising.
Testing climbing specific strength and
endurance
Maximal strength
The maximal strength of the finger flexors was determined
using a custom-made dynamometer (1D-SAC, Spacelab, Sofia,
Bulgaria). The participant was instructed to maintain a 5 s long
half-crimp position while “hanging”on the wooden rung. The
rung depth was 23 mm with a 10 mm radius to maximize the
activation of the flexor digitorum profundus (FDP) and flexor
digitorum superficialis (FDS) (30). Two attempts were performed
separated by a two-minute rest in between. Participants were
instructed to progressively transfer as much of their weight as
possible onto the wooden rung with their dominant arm. The
highest peak value from the two trials was considered as the
MVC of finger flexors, and this value was used to determine
relative workloads for the following training intervention.
All-out test
To assess the critical force (CF), force impulse from all
contractions (W), and impulse above the critical force (W’), the
4-min all-out test was performed (31). This test involved 24
isometric maximal voluntary contractions on the same rung as
for maximal strength (1D-SAC, Spacelab, Sofia, Bulgaria) in a
half crimp position with a 7:3 s work to rest ratio.
During the “rest”phase, participants were instructed to
maintain the anatomical position with upper-limb over the head
level and were not allowed to shake their forearms or hands, as
shaking is known to aid recovery (32). However, participants
could dry their fingers using the chalk. Loud verbal
encouragements were given to all participants to reach their
maximum force during every contraction. Force and time data
were continuously recorded throughout the test. For the visual
representation see Figure 3.
For each contraction in all tests, the length (in seconds), peak
and mean force (in kilograms), and the impulse were
determined. The CF was defined as the mean force from the last
three contractions of the test.
Muscle oxidative capacity
To assess the muscle oxidative capacity, near-infrared
spectroscopy (NIRS) (Portamon, Artinis Medical Systems BV,
The Netherlands) was employed to monitor changes in tissue
oxygenation levels of the FDP. A chartered physiotherapist
located the FDP using the technique recommended by Schweizer
and Hudek (30), where the thumb and first finger were squeezed
together, and the middle of the muscle belly was palpated (30).
The NIRS device sampling frequency was set to 10 Hz and data
were processed using the Oxysoft software (Artinis Medical
System, BV, The Netherlands). Path length factor was set to
4. Muscle oxidative capacity was estimated by calculating half-
time to recovery of the tissue oxygen saturation (O
2
HTR) after
arterial occlusion (33).
FIGURE 3
Vizualization of data acquired by the all-out test for the finger flexors. Critical force was calculated as the average force from the last three contractions.
The duration of the all-out test was 240 s with 7:3 work to rest ratio. Force impulse from all contractions was calculated as the area under the force-time
curve and represents total isometric muscle work during the test (W). Impulse above the critical force represents energy store component (W’).
Javorský et al. 10.3389/fspor.2023.1256136
Frontiers in Sports and Active Living 04 frontiersin.org
Participants were instructed to rest in a supine position with
their arm elevated above heart level for 20 min after fitting the
artery tourniquet. Following the initial measurement of the
baseline, the tourniquet was inflated to a supramaximal pressure
of 250 mmHg for 5 min. After that, the cuff was rapidly released,
and recovery muscle tissue oxygen saturation (StO
2
)valueswere
recorded for 3 min. Half-time of StO
2
recovery was calculated,
which represents a valid estimate of oxidative capacity (33).
Statistical analysis
Statistical analyses were performed using IBM SPSS for
Windows (IBM Corp. Released 2020. IBM SPSS Statistics for
Windows, Version 27.0. Armonk, NY: IBM Corp). Descriptive
statistics (mean ± standard deviation) were used to characterize
strength and endurance indicators during pretest and post-test.
The analysis of variance (ANOVA) 2 × 2 with repeated measures
was conducted to examine the main effects of time (pretest vs.
post-test) and training method (LIT + BFR vs. HIT), as well as
their interaction effect. The significance level was set at P<0.05.
Post hoc analysis using Bonferroni correction was performed to
compare specific pairs of interventions in terms of their effects on
the pretest and post-test measures. Effect sizes of 0.3, 0.5, and 0.8
were interpreted as small, medium, and large effects, respectively
(34). Utilizing the Shapiro-Wilk test, all data were determined to
be normal and met the criteria of Mauchly’s test of sphericity.
Results
At baseline, no differences for were observed between the
training methods for any of the variables (P> 0.05).
There was a significant main effect of time for impulse (delta
W = + 1568 Ns; P= 0.002). However, there was no significant
interaction of time and training method demonstrating no
substantial differences between LIT + BFR and HIT (P=0.057–
0.855).
Pairwise comparisons showed significant increases of force
impulse only for HIT method (Table 1,Figure 4). Otherwise,
non-significant improvements with small or no effect size were
found for all strength and endurance indicators and no
significant decreases of climbing specific strength or endurance
indicators were demonstrated (Table 1,Figure 4).
Discussion
The main finding of the current study was that small volume
of LIT + BFR was equally effective as HIT to maintain finger
flexor strength and endurance in lower grade and intermediate
climbers.
To evaluate maximum finger flexor strength, we employed an
ecological setting with the arm positioned overhead without any
fixation. This method has been demonstrated to be a valid and
reliable measure of climbing-specific strength, with a standard
error of measurement (SEM) of 35 N (35). Neither the HIT, nor
LIT + BFR interventions resulted in significant changes in finger
flexor strength. The observed pretest-post-test changes fell within
the previously mentioned SEM range. It has been observed that
strength decreases occur rapidly with a training interruption,
becoming more pronounced after 8 days of inactivity (36)Itis
hypothesized that neural factors such as motor unit recruitment
and synchronization, firing frequency, and intramuscular
coordination are responsible for strength losses during the early
stages of inactivity, while morphological factors contribute to
greater strength decreases thereafter (37). Our study
demonstrates that low volume of intermittent isometric HIT
(60% MVC, with a total exercise time of 36 × 10:3 s work: relief
cycles per session, two sessions per week) and an equivalent
volume of low-intensity with BFR (30% MVC) were effective in
maintaining the initial strength level for 5 weeks. All participants
were able to sustain both training protocols without premature
localized exhaustion. Therefore, it may be speculated that 2
sessions per week, with a total of 12 min of isometric non-
exhaustive exercise per arm at low intensity and with BFR,
counteracted the deteriorating changes that neural factors may
have on maximal strength due to inactivity.
During high-intensity resistance training, a single set of 6–12
repetitions with loads ranging from approximately 70%–85% 1
repetition maximum 2–3 times per week reaching volitional or
momentary failure for 8–12 weeks can produce suboptimal, yet
significant increases in squat and bench press strength in
resistance-trained men (20). Our non-exhaustive protocol with
smaller muscle groups, slightly lower intensity, and similar
volume did not result in significant improvements. It appears
that exhaustive protocols are necessary to induce structural
changes leading to strength increases (38,39). However, a similar
volume of non-exhaustive exercise may have benefits in
maintaining the current level of strength.
TABLE 1 Mean (± standard deviation) score of pretest and post-test measurements for high intensity training (HIT) and low intensity training with blood
flow restrictions (LIT + BFR).
HIT LIT + BFR
Pretest Post-test PCohen’sdPretest Post-test PCohen’sd
MVC (N) 356 ± 134 373 ± 113 0.241 0.13 376 ± 138 362 ± 125 0.158 −0.10
Cf (N) 103 ± 26 113 ± 30 0.237 0.36 114.3 ± 31 116 ± 30 0.844 0.05
W (N.s) 25,078 ± 7,583 27,327 ± 8,051 0.012 0.29 26,661 ± 8,415 27,551 ± 6,593 0.392 0.12
W’(N.s) 10,246 ± 6,011 10,092 ± 5,979 0.845 −0.03 9,494 ± 5,278 10,152 ± 5,599 0.353 0.12
O
2
HTR (s) 14.3 ± 5.1 13.1 ± 5.1 0.569 −0.23 13.6 ± 4.9 13.2 ± 4.8 0.830 −0.07
W, impulse from the 4 min all-out test; W’, impulse above the critical force; CF, critical force; O
2
HTR, oxygen saturation ½ time to recovery after arterial occlusion.
Javorský et al. 10.3389/fspor.2023.1256136
Frontiers in Sports and Active Living 05 frontiersin.org
FIGURE 4
Boxplot visualization of pretest post-test results. Left panel represent high intensity training (HIT) while right panels represent low intensity training with
blood flow restriction (LIT + BFR) The area of box shows quartile and whiskers represent 1.5 interquartile range between the first and third quartile. The
line in the middle corresponds to the mean value. W—impulse, W’—impulse above the critical force, O
2
HTR—oxygen ½ time to recovery after occlusion. *
represents significant improvements from pretest (P< 0.05).
Javorský et al. 10.3389/fspor.2023.1256136
Frontiers in Sports and Active Living 06 frontiersin.org
LIT + BFR training does not only have impact on maximal
strength improvements but may also, due to peripheric and central
adaptations, have direct or indirect impact on endurance
performance (40,41). In our study, we estimated endurance of the
finger flexors using several indicators: W, W’,CFfrom4minall
outtestandO
2
HTR from arterial occlusion test. W is an indicator
of total working capacity and represents an overall measure of
finger strength and endurance. W’is the capacity to release energy
above the CF and is often related to strength-endurance capacity
while the level of CF represents the amount of energy
predominantly released by aerobic metabolism (42). O
2
HTR is a
standardized NIRS derived functional index estimating muscle
aerobic capacity. Faster recovery of FDP has been associated with
increased climbing ability (43). Similar to maximal strength, no
decreases in any endurance indicators were observed. On the
contrary, after HIT, W was statistically higher, suggesting that low
volume of HIT may lead to overall improvement in finger flexor
working capacity in intermediate climbers as W represents both
strength and endurance components. However, the effect size for
improvement changes was low, and no differences between the
two methods were found. The maintenance of all endurance
indicators during 5-weeks LIT + BFR training is very promising as
submaximal resistance to fatigue appears to be deteriorated to a
greater extent from training interruption in comparison with
maximal force and maximal power (37).
Endurance adaptations following LIT + BFR training have been
associated with improvements in macro- and microvascular
functions, muscle redox and ionic buffering, and mitochondrial
respiratory capacity (40,41). In our study, the aerobic capacity of
the finger flexor muscles was estimated from the NIRS signal. It
is important to note that the sensitivity of StO
2
recovery as a
training indicator in climbers is still unknown, and further
experimental studies are needed to validate its use. Subsequent
studies should also aim to investigate the pathways explaining
forearm oxidative capacity and consider using NIRS technology
to independently assess skeletal muscle oxygen diffusion capacity
and mitochondrial respiratory capacity (44).
There are other strength and limitations to be stated. A
strength of the study is that all participants refrained from
engaging in any climbing-specific or upper-body strength
activities during the 13-week experimental period, ensuring that
any observed changes could be attributed to our experimental
conditions. The intervention may be regarded as a simulation of
a rehabilitation period. Participants were fit enough to train
under controlled environment but could not train/climb in an
uncontrolled environment due to lock-down restrictions. The
crossover design allowed for a direct comparison between the
two training modalities within the same group of participants,
minimizing inter-individual variability (45). However, due to
time requirements, a relatively short one-week washout period
between the training interventions was applied. Of note, a
control group was not included which might be useful of
quantifying no strength training or the short washout period.
Nevertheless, this does not seem to influence our results as no
changes in any indicator were observed after the HIT or LIT +
BFR intervention. The small group size in this study may limit
the generalizability of the findings and the ability to detect small
differences between the training modalities. Moreover, using BFR
with more advanced climbers may have provided different
results. MVC was assessed only once before each training
intervention to set the training load. In other words, the climbers
trained at the same relative intensity throughout the whole
period. This may also explain the lack of changes during the
different periods. If MVC was tested every week, there may had
been a progression in the training which ultimately may have led
to an increase in (some of) the variables. On the other hand,
during recovery periods from an injury, regular testing of MVC
would increase stress on injured tissues and may slow the
recovery process.
Our findings support the hypothesis that both approaches, with
and without BFR, were equally effective in preserving the studied
parameters during the minimal training period. However, it is
important to note that physiology of these adaptations may differ
during exercise at 30% of MVC compared to higher intensity
exercise (23,46,47). Therefore, BFR training at a lower intensity
(30% of MVC) appears to be a viable substitute for HIT during
recovery periods and may offer advantages, particularly for
climbers recovering from injuries, although it is more
discomforting and less enjoyable compared to HIT (48).
In conclusion, this study demonstrates that low volume of non-
exhaustive BFR training at a lower intensity can be as effective as
HIT in preserving sport-specific strength and endurance. These
findings suggest that LIT + BFR training may be a viable
alternative for climbers recovering from injuries.
Data availability statement
The original contributions presented in the study are included
in the article/Supplementary Material, further inquiries can be
directed to the corresponding author.
Ethics statement
The studies involving humans were approved by Ethics
Committee of Charles University, Faculty of Physical Education
and Sport. The studies were conducted in accordance with the
local legislation and institutional requirements. The participants
provided their written informed consent to participate in this
study. Written informed consent was obtained from the
individual(s) for the publication of any potentially identifiable
images or data included in this article.
Author contributions
TJ: Writing –original draft, Conceptualization, Investigation,
Data curation. AS: Writing –review & editing, Methodology.
VA: Methodology, Writing –review & editing. JB:
Conceptualization, Methodology, Supervision, Writing –review
& editing.
Javorský et al. 10.3389/fspor.2023.1256136
Frontiers in Sports and Active Living 07 frontiersin.org
Funding
The author(s) declare financial support was received for the
research, authorship, and/or publication of this article.
The study was supported by the grant of Charles University,
Cooperatio Program, research area Sport Sciences—Biomedical
and Rehabilitation Medicine. As a university-grant, the funding
did not influence the design, results, or interpretation of this study.
Conflict of interest
The authors declare that the research was conducted in the
absence of any commercial or financial relationships that could
be construed as a potential conflict of interest.
Publisher’s note
All claims expressed in this article are solely those of the
authors and do not necessarily represent those of their affiliated
organizations, or those of the publisher, the editors and the
reviewers. Any product that may be evaluated in this article, or
claim that may be made by its manufacturer, is not guaranteed
or endorsed by the publisher.
Supplementary material
The Supplementary Material for this article can be found
online at: https://www.frontiersin.org/articles/10.3389/fspor.2023.
1256136/full#supplementary-material
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