Effects of hypercapnic-hypoxic training on respiratory muscle strength and front crawl stroke performance among elite swimmers

Abstract

The aim of the present study was to determine the effects of an 8-week hypercapnic-hypoxic (H-H or apnea) training program on respiratory muscles strength and 100 meter crawl swimming performance. The study was conducted on a sample of 26 elite Croatian swimmers (experimental group [EG] n=12, control group [CG] n=14). Both groups were subjected to the same swimming training programs and training sessions on a treadmill. The experimental group was additionally subjected to hypercapnic-hypoxic training program with increased muscular activity. Data on the following outcome variables was collected: the strength of respiratory muscles (maximal inspiratory pressure (MIP) and maximal expiratory pressure (MEP)), 100m front crawl swimming time (R100m) and breathing frequency during the same test (BF100m). A series of two way repeated measures ANOVAs has shown significant interactions between group (EG and CG) and the repeated-measure factor (pre- and post-test) (MIP: p = 0.006, MEP: p < 0.001, R100m, p < 0.001, FB100m, p < 0.001), all showing greater efficacy of the experimental program. It seems that the hypercapnic-hypoxic training program may provide substantial benefits for elite swimmers, in addition to their standard training sessions.

Full text

Turkish Journal of Sport and Exercise
http://dergipark.ulakbim.gov.tr/tsed/index
Year: 2016 - Volume: 18 - Issue: 1 - Pages: 17-24
DOI: 10.15314/tjse.83447
ISSN: 2147-5652
Effects of hypercapnic-hypoxic training on respiratory
muscle strength and front crawl stroke performance
among elite swimmers
Dajana KARAULA 1, Jan HOMOLAK 2, Goran LEKO 1
1 Department of Sport, Faculty of Kinesiology, University of Zagreb, Croatia
2 School of Medicine, University of Zagreb, Croatia
Address Correspondence to D. Karaula, e-mail: dajana.zoretic@kif.hr
Abstract
The aim of this resent study was to determine the effects of an 8-week hypercapnic-hypoxic (H-H or apnea) training program
on respiratory muscles strength and 100 meter crawl swimming performance. The study was conducted on a sample of 26
Croatian elite swimmers (experimental group [EG] n=12, control group [CG] n=14). Both groups were subjected to the same
swimming training programs and training sessions on a treadmill. The experimental group was additionally subjected to
hypercapnic-hypoxic training program with increased muscular activity. Date on the following outcome variables were
collected: the strength of respiratory muscles (maximal inspiratory pressure (MIP) and maximal expiratory pressure (MEP)),
100m front crawl swimming time (R100m) and breathing frequency during the same test (BF100m). A series of two way repeated
measures ANOVAs has shown significant interactions between group (EG and CG) and the repeated-measure factor (pre- and
post-test) (MIP: p = 0.006, MEP: p < 0.001, R100m, p < 0.001, FB100m, p < 0.001), all showing greater efficacy of the experimental
program. It seems that the hypercapnic-hypoxic training program may provide substantial benefits for elite swimmers, in
addition to their standard training sessions.
Keywords: Breathing frequency, expiratory muscles strength, hypercapnic-hypoxic training, inspiratory muscles strength, 100
m front crawl stroke.
INTRODUCTION
Breathing during swimming may interfere with
propulsion and cause disruptions in timing between
two strokes (24,37). To avoid that, elite swimmers,
especially on shorter distances, often endeavor to
reduce their breathing frequency during swimming
as much as possible. This means that they take a
breath every third or fourth, or even fifth or sixth
stroke. Such breathing rhythm enables them to swim
mechanically more effectively (24,31) and therefore
faster (5,31). Due to the occurrence of hypercapnia,
respiratory and metabolic acidosis, reduced
breathing frequency may lead to earlier occurrence
of fatigue (16,44,45,48). Increases in arterial PCO2
stimulate breathing through both, the carotid bodies
and the central chemoreceptors. This lead to the
assumption that hypercapnic training could increase
swimmers’ ability to hold their breath longer during
the race by postponing the stimulus to breathing,
and consequently allow for more efficient technique
and faster swimming (20).
Research on breath-hold divers who had
hypercapnic trainings outside the water (walking
apnea) has shown that the spleen contraction,
reduction in blood acidosis, and oxidative stress
occur during the voluntary breath holding (apnea)
(9,16). Some previous studies have discovered
presence of a weakened ventilation response in
breath-hold divers and synchronized swimmers
(2,4,10,33). However, the hypothesis that exposure
to hypercapnic condition reduces the ventilation
response to increased CO2 concentrations was
rejected.
Different methods of restricting lung
ventilation are used as part of swimming training.
For example, a common practice is swimming with
reduced breathing frequency and enlarged dead
space by using a snorkel. Hypercapnic-hypoxic (H-
H or apnea) training outside water with increased
muscular stress seems to be a less common practice.
It was shown that training of breath holding may
improve tolerance to hypoxemia, regardless of the
genetic factor or buffering muscle capacity (19).

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Turk J Sport Exe 2016; 18(1): 17–24
© 2016 Faculty of Sport Sciences, Selcuk University 18
Other studies have indicated that this may then
positively influence anaerobic and/or anaerobic
capacity (22,47). Accordingly, a recent study has
shown positive effects of hypercapnic-hypoxic
training on hemoglobin concentrations and
maximum oxygen uptake among elite swimmers
(49). Several researchers tried to take the advantage
of physiological adaptations associated with
hypercapnia and hypoxia. The most common way to
induce hypercapnia and hypoxia described in
literature is hypoventilation and reduced breathing
frequency (RBF). Since there is a great resemblance
between RBF training and H-H training, we believe
that some of the training effects that have been
repeatedly reported with RBF can also be
accomplished with H-H training. RBF training can
be done either at low or high lung volumes with
RBF at high lung volume being more similar to H-H
training. Studies indicate that RBF training at high
lung volumes can increase tidal volume during
incremental exercise and decrease the ventilatory
response to exercise induced hypercapnia (14,18,20).
Moreover, Kapus et al. also reported the effects of
RBF training in swimmers. Swimmers in the
experimental group showed higher lactate
concentrations and greater PCO2 after the training.
Because of the long lasting stimulus, swimmers
adapted to swimming with fewer breaths (19).
According to Sharp et al. (38,39), the greater
respiratory muscle strength is a factor which
contributes to a positive relation between the
strength of the torso and the swimming
performance. Moreover, Kilding et al. (21) have
demonstrated that inspiratory muscle training can
improve swimming performance in club-level
trained swimmers in events shorter than 400 m (21).
It could be assumed that increased strength of
respiratory muscles may positively affect swimming
performance through increased volume of air
exchanged within each inhalation and exhalation,
and consequently reduced breathing frequency
during the race. Greater amount of air in the lungs
may also have positive effect on the buoyancy of
swimmers. Evidence about the effects of respiratory
muscle strength on breathing frequency during
swimming and swimming performance is scarce.
Furthermore, no previous studies have examined
the effects of hypercapnic-hypoxic training on
muscle-strength of elite swimmers.
MATERIALS & METHOD
Participants
The study included a sample of 26 elite male
swimmers (age range 17 to 25 years). All
participants trained regularly for eight or more
consecutive years, with a minimal training load of 1
two-hour session six days a week. Stratified
randomization was applied to form the
experimental (EG; n=12) and control group (CG;
n=14), where the prior stratification of the sample
was done according to the International Point Scores
(IPS) for 100 m front crawl stroke.
Based on the medical examination, all
participants obtained physician consents for
participating in competitive swimming events. All
participants signed a written informed consent,
before taking part in the study.
Experimental procedures
Data was collected on two occasions (pre und
post), eight weeks apart, using standardized
procedures. The maximal strength of inspiratory
muscles (MIP) and maximal strength of expiratory
muscles (MEP) were assessed via the respiratory
pressure meter MicroRPM™ Software (Puma PC,
Micro Medical, Kent, England). The 100m front
crawl stroke swimming performance times were
measured in a short-course (25m) swimming pool by
Omega OCP5 touchpad. During the same test, the
breathing frequency (BF100m) was assessed on the
100m front crawl stroke swimming performance.
Between the baseline and follow up
measurements, both EG and CG engaged in their
standard training programs, including swimming
and treadmill sessions. Additionally, the EG
underwent hypercapnic-hypoxic (H-H) training
sessions on a treadmill 3 times a week for 8 weeks.
Each H-H session was approximately 30-45 minutes
long. The heart rate (HRmax) at the maximum oxygen
uptake (VO2max) was used to determine the treadmill
speed. The treadmill speed remained the same
during the whole training program. Oxygen blood
saturation (SaO2), the carbon dioxide amount in the
exhaled breath (CO2) and the heart rate (HR), which
was 60% from the maximum HR, were constantly
monitored. Each test subject was instructed to hold
his breath for as long as possible. Hypercapnia was
monitored with capnometer (Model C300, External
Sidestream ETCO2 Module, Beijing National
Medical Co) and oxygen saturation was controlled
with Edan H100B oximeter (Edan Instruments,

Karaula et al., 2016
Turk J Sport Exe 2016; 18(1): 17–24
© 2016 Faculty of Sport Sciences, Selcuk University 19
China). Targeted values for blood gas carbon
dioxide levels were over 45 mmHg which is the
laboratory diagnostic criteria for hypercapnia.
Breath-holding time necessary to induce
hypercapnia was determined for each test subject. .
The breaks between two breath-holding cycles were
defined as one full ventilation cycle (inhalation,
holding the breath + exhalation, inhalation -
exhalation, inhalation + holding the breath).
In addition to regular swimming training
sessions, the control group underwent aerobic
training on a treadmill at 60% of the maximal heart
rate 3 times a week for 8 weeks. Each training
session was approximately 30-45 minutes long.
Statistical analysis
Means and standard deviations were calculated
for each dependent variable. One-simple the
D'Agostino-Pearson test, Histogram test,
Probability-probability plot (P-P plot) and
Kolmogorov-Smirnov test confirmed normal
distributions (13). Maximal strength of inspiratory
muscles (MIP) and maximal strength of expiratory
muscles (MEP), breathing frequency on 100 m front
crawl stroke (BF100m) and results on 100 m front
crawl stroke (R100m) were compared using a one-
way ANOVA with repeated measures. An alpha
value of p<0.05 was assumed to check statistical
significance. The effect size of the obtained
hypercapnic-hypoxic training differences in the
dimension of the initial and final measurements in
all parameters was calculated with the Cohen d-
index. The data were analyzed using that statistical
software Statistica (ver.11.0).
RESULTS
The mean, standard deviation and Cohen d for
both group in initial and final measurement are
summarized in Table 1. Based on the numerical
parameters (Table 2) of the t-test for independent
specimens through p-values (p < 0.05) it is noticeable
that there is no statistically significant difference of
all variables in the initial measurement between the
experimental and the control group.
The results of a series of ANOVA for the
repeated measurements show that there were
statistically significant difference in maximal
strength of inspiratory muscles, maximal strength of
expiratory muscles, breathing frequency on 100 m
front crawl stroke and results on 100 m front crawl
stroke. The differences are shown separately for
each variable in Figures 1, 2, 3 and 4.
Figure 1 shows the results of the results on 100
m front crawl stroke (R100m). As shown, the EG and
the CG are homogenous in the initial state (p=.626).
In the final measurement the swimmers who were
subjected to the hypercapnic-hypoxic training
program scored better than the swimmers who were
not subjected to the program. Based on the results
on 100 m front crawl stroke in the initial
measurement (R100m) (56.75±2.09) and in the final
measurement (54.72±2.75) it can be concluded that
the R100 has decreased for 3.6%. Control group
improved result for 1.1%.
Figure 2 shows the results in the breathing
frequency on 100 m front crawl stroke (BF100m). EG
swimmer reduced the BF100m and Cohen d shown the
large effect (Cohen d= 1.83).
Figure 3 shows the results of maximal strength
of inspiratory muscles and Figure 4 shows the
results of maximal strength of expiratory muscles. In
the final measurement the swimmers who were
subjected to the hypercapnic-hypoxic training
program scored better than the swimmers who were
not subjected to the program. During the training
period the MIP at EG increased for 14.9% and MEP
for 1.9%.
Table 1. Results of descriptive statistics for the experimental and control group in initial and final measurement.
Variable
Experimental group –
Initial measurement
Experimental group –
Final measurement
Cohen
d
Control group – Initial
measurement
Control group –
Final measurement
Cohen
d
Mean. ± SD
Mean ± SD
Mean ± SD
Mean ± SD
MIP
119.86±19.33
146.21±31.32
1.07
126.42±27.72
124.50±29.78
0.07
MEP
130.36±27.78
149.71±30.82
0.69
153.25±36.59
146.92±30.80
0.20
BF100m
19.83±2.52
16.08±1.68
1.83
20.36±2.65
18.79±1.93
0.70
R100m
56.75±2.09
54.72±2.75
0.86
56.30±2.52
55.70±2.34
0.26
Legend: Mean - mean, SD – standard deviation, Cohen d- effect size used to indicate the standardized diffrence between txo means, an effect size is a
measure of the strength of a phenomenon, MIP – maximal strength of inspiratory muscles, MEP – maximal strength of expiratory muscles, BF100m –
breathing frequency on 100 m front crawl stroke, R100m – results on 100 m front crawl stroke

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© 2016 Faculty of Sport Sciences, Selcuk University 20
Table 2. Results of t-test for independent samples in the initial measurement.
Variable
Experimental group – Initial measurement
Control group – Initial measurement
p
Mean ± SD
Mean ± SD
MIP
119.86±19.33
126.42±27.72
0.486
MEP
130.36±27.78
153.25±36.59
0.082
BF100m
19.83±2.52
20.36±2.53
0.611
R100m
56.75±2.09
56.30±2.52
0.626
Legend: Mean - mean, SD – standard deviation, p – statistically significant at level p<.05, MIP – maximal strength of inspiratory muscles, MEP – maximal
strength of expiratory muscles, BF100m – breathing frequency on 100 m front crawl stroke, R100m – results on 100 m front crawl stroke.
Figure 1. The diference between the experimental and control
group in the initial and final measurement in the variable of
result on 100 m front crawl stroke.
Figure 2. The diference between the experimental and control
group in the initial and final measurement in the variable of
breathing frequency on 100 m front crawl stroke.
Figure 3. The diference between the experimental and control
group in the initial and final measurement in the variable of
maximal strength of inspiratory muscles.
Figure 4. The diference between the experimental and control
group in the initial and final measurement in the variable of
maximal strength of expiratory muscles.
experimental group, control group, 1- initial measurement, 2- final measurement, p – statistically significiant at level p<.05, F – variance of the
group means / mean of the within group variances.
DISCUSSION
Since pulmonary system can limit maximal
exercise performance (1, 3, 7) and accessory
respiratory muscles greatly contribute to functioning
of the respiratory system during maximal sport
performance, their role in performance and potential
benefit of respiratory training implementation in
exercise program should be considered. Fatigue of
the respiratory musculature is important factor that
can limit sport performance and eventually even
cause cessation of sport activity. Several studies
have reported potential benefit of respiratory muscle
training in various sports. Respiratory muscle
training improves recovery time during high
intensity, intermittent exercise in repetitive sprint
athletes (34), anaerobic capacity in cyclists (15) and
rowing performance (12). Theoretical benefit of

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© 2016 Faculty of Sport Sciences, Selcuk University 21
increased inspiratory and expiratory muscle
strength we observed could be huge.
The increased strength of inspiratory and
expiratory muscles of swimmers in the EG could
have resulted in the enlarged volume of air
exchanged with each breath. Thus, the content of
dead space in breath (150 mL) was perceptually
reduced. A larger amount of air in lungs has a
positive effect on the amount of the available
oxygen, elimination of excess CO2 and flotation of
swimmers. Mentioned factors could have caused
observed reduction in number of breaths as well as
improvement in result in 100-meter front crawl race
(19). Breathing during swimming interferes with the
swimming technique quality and causes a time
disbalance between two strokes (24,37). Therefore,
swimmers are advised to swim with as few
inhalations as possible when swimming shorter
distances. The time spent on every inhalation is 0.2
seconds on average (26).
In the context of competitive swimming,
measuring of respiratory muscle strength has not
been widely analyzed. The swiftness of exhalation
during swimming and the additionally increased
hydrostatic pressure to the thorax can result in
pressure to the chest wall towards inside when the
inspiratory muscles are relaxed and weak (6). When
muscles are relaxed, water pressure presses the lung
wall from the outside and antagonizes respiratory
muscles during inhalation (11), which results in a
pronounced exhalation in water and larger stress
during inhalation. The somewhat difficult
functioning of the respiratory muscles results in
arterial vasoconstriction which is the consequence of
the reduced concentration of CO2 caused by
hyperventilation (42).
It must be pointed out that all the tested
swimmers have been practicing for twelve years on
average. Since this analysis includes top swimmers,
the large homogeneity in obtained respiratory
muscle function measures is understandable. In
course of their long-time practice the swimmers
have become familiar with the fact that the increased
breathing frequency and irregular breathing in
water causes the time disbalans between two
strokes, which renders achievement of the desired
result impossible. Years of repetitive hypercapnic
episodes during swimming training caused
adaptations to higher quantities of CO2 by reducing
chemo-receptor sensitivity. Improvement in 100
front crawl swimming result can also be partially
explained by change in breathing pattern frequency
seen in experimental group subjects during final 100
front crawl swim. Figure 3 and 4 shows that the
group of swimmers, who were subjected to the
hypercapnic-hypoxic regimen, has significantly
improved strength of their inspiratory and
expiratory muscles in comparison to swimmers in
the control group. The swimmers from the
experimental group have improved the inspiratory
muscle strength values (MIP) for 14.9% and the
expiratory muscle strength values (MEP) for 1.9% in
relation to the control group. There is a statistically
significant difference between the performance
values of the experimental group (Table 1).Based on
the previously conducted analyses (1, 25, 22, 35) it
can be concluded that the Croatian swimmers have
similar values of initially measured respiratory
muscle strength in relation to swimmers and free
divers mentioned in these analyses.
Based on the results of this study it can be
assumed that the hypercapnic-hypoxic practice has
significantly increased the respiratory muscle
strength. Statistically significant differences can be
attributed to the eight-week exposure to
hypercapnia and hypoxia combined with increased
muscle activity. Such practice must have enlarged
the diaphragm thickness which plays an important
role in respiratory system and sports performance
(8). Voluntary holding of breath may have resulted
in involuntary contractions of intercostal muscles
during the hypercapnic-hypoxic practice. It is also
assumed that above mentioned contraction
occurrence has resulted in hypertrophy of
intercostal muscles. According to the available
literature, mobility of breastbone and costal joints
and changes in lung and breast muscle elasticity (30)
are also possible changes that occur during
voluntary breath holding. Maximum inspiratory
pressure is of great importance in swimming due to
the limited inhalation time – the stronger the
inspiratory muscles, the more air can be inhaled into
the lungs in a shorter period of time.
Athletes with lower MIP values are more
sensitive to fatigue during practice (27,28,29).
Literature also suggests that escalation of MIP
values correlates with greater lung diffusion
capacity. Because of the increase in respiratory
muscle strength, the critical swimming speed (41)
and the swimming durability (49) were improved. It
should be noted that the change in relative
diaphragm contributions (movement of abdominal
cavity) in relation to the movements of the thorax
and contractions of intercostal muscles in relation to

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Turk J Sport Exe 2016; 18(1): 17–24
© 2016 Faculty of Sport Sciences, Selcuk University 22
the respiratory volume are of great importance. The
diaphragm prevails in stand-still condition and
during the chemical breathing stimulation, while the
intercostal muscles prevail during the intense
practice.
Sprinters who have specialized for 100-meter
races should be subjected to certain hypercapnic-
hypoxic workouts and often do sprint racing
without breathing (26). To swim a 100-meter race,
distance swimmers must have highly developed
anaerobic metabolic system strength. Endurance
training increases the efficiency of aerobic
metabolism by increasing quantity of myoglobin
and hemoglobin concentration and thus enabling
more oxygen to be transported into muscle cells (26).
A 100-meter front crawl stroke distance takes 50
to 60 seconds. Energy needs are covered by: ATP-KP
with 10%, anaerobic glycolysis system with 55% and
aerobic metabolism with 35% (26). Since the group
of swimmers who were subjected to hypercapnic-
hypoxic practice have developed statistically
significant differences such as: higher hemoglobin
concentration and maximum oxygen uptake (50) as
well as respiratory muscle strength, it can be
assumed that observed improvement in swimming
performance can mainly be attributed to aerobic
energy system adaptations. The increase of oxygen
supply to the muscles enables metabolizing larger
quantities of pyruvate and hydrogen ions delaying
occurrence of acidosis and thus allowing swimmers
to swim faster. It can be assumed that buffering
capacities are improved, which then in turn
influences the acid-base balance (local level/cellular)
and buffer capacities of hemoglobin (general
level/extracellular) as well as the lactate transporter
MCT4 (40). Some other researched also suggest that
such type of (interrupted) hypoventilation practice
influences arterial desaturation, induces
hypercapnia and thus encourages development of
higher buffering capacities (43,44,46,47).
Aforementioned adaptations allow swimmer to
prolong usage of energy sources needed for extreme
requirements during race. Activation of anaerobic
glycolysis occurred in top swimmers subjected to
hyprecapnic-hypoxic practice, most probably due to
better buffering which enables a more efficient
energy production and thus a longer anaerobe
glycolysis period. Based on the increased
hemoglobin concentration (50), swimmers who were
subjected to hypercapnic-hypoxic practice may also
have increased their aerobic capacity, ability to
recuperate faster and endure longer and more
intense workouts.
Besides the stated assumptions, swimmers who
were subjected to hypercapnic-hypoxic practice had
lesser number of inhalations than the control group
(Figure 2). One of the reasons may be the increased
respiratory muscle strength which contributes to a
positive relation between the torso and the
swimming performance (38,39) and enables
swimmer to (can) inhale more air in one inhalation.
The second reason could be the chemoreceptors
have become less sensitive to increased levels of CO2
in blood due to the hypercapnic-hypoxic practice.
The result analysis (Figure 1) of a 100-meter
front crawl distance (R100m) has established a
statistically significant difference in progress
between the experimental and the control group.
Both groups of swimmers have obtained better
results in final measurements, which can be
explained by training program both groups took
part in. The experimental group has improved its
result for 3.6 % compared to the initial measurement
and the control group improved its result for 1.1%.
The difference in progress can be attributed to
positive influence of hypercapnic-hypoxic training
on swimmers’ buffering capacity. Hypercapnic-
hypoxic training also causes spleen contraction,
which improves oxygen transmission due to
increase in number of erythrocytes and hemoglobin
concentration (17,23,32,36).
According to obtained results it can be assumed
that the hypercapnic-hypoxic practice has resulted
in stretching of all muscles surrounding the thorax
and thus increasing the total lung capacity and
respiratory muscle strength. Muscles which
surround the thorax are one of two primary
generators of force which shoves the swimmer
through the water. Based on the obtained results it
can be assumed that top swimmers who have been
subjected to the hypercapnic-hypoxic practice
developed better buffering capacity due to stronger
respiratory muscles and increase in hemoglobin
concentration. Also, better tolerance to the increased
level of CO2 in blood has been observed due to
decrease in chemoreceptor sensitivity.
Understanding the adjustment to normobaric
hypercapnia and hypoxia is of great importance in
kinesiology and sport physiology. Better
understanding of mechanisms behind physiological
adaptation to hypercapnia and hypoxia such as
changes in buffering capacity, metabolic and

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© 2016 Faculty of Sport Sciences, Selcuk University 23
hematological adaptations can immensely influence
theoretical sport science.
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