Faculty of Sport, University of Ljubljana, ISSN 1318-2269 53 Kinesiologia Slovenica, 18, 1, 53–64 (2012)
IZVLEČEK
Namen raziskave je bil ugotoviti učinek šest tedenske 
vadbe za moč vdišnih mišic na dihalni volumen med 
ve č stop enjsk i m obremen i l n i m te stom z n i ž jo f rek venco 
dihanja. Enaindvajset merjencev (devet fantov in 
dvanajst deklet) je bilo razdeljenih v dve skupini: 
poskusno (skupina E) in referenčno (skupina C). Skupina 
E je dvakrat dnevno vadila s 30 dinamičnimi vdihi in 
izdihi s pomočjo dihalnega trenažerja, pri čemer je bil 
upor vdiha približno 50% največjega vdišnega pritiska. 
Skupina C opisane vadbe ni opravljala. Pred vadbo in po 
njej sta obe skupini opravili večstopenjski obremenilni 
test na kolesarskem ergometru z nižjo frekvenco 
dihanja. Le-ta je bila določena z 10 vdihi na minuto. 
Skupina E je z vadbo povečala moč vdišnih mišic (+35 
± 16%; p<0.01) in dihalni volumen (+13 ± 14%; p<0.01) 
med večstopenjskim obremenilnim testom z nižjo 
frekvenco dihanja. Slednje je omogočilo tudi daljše 
opravljanje tega testa (čas trajanja se je z vadbo podaljšal 
za 17 ± 15%; p<0.01). Pri skupini C opisanih sprememb 
ni bilo. Glede na dobljene rezultate lahko zaključimo, 
da vadba za moč vdišnih mišic poveča dihalni volumen 
med večstopenjskim obremenilnim testom z nižjo 
frekvenco dihanja. Kljub vsemu pa verjamemo, da 
nam bodo verodostojnejše in uporabnejše podatke dale 
bodoče  raziskave, ki bodo ugotovile učinek tovrstne 
vadbe kot dodatek običajni vadbi z nižjo frekvenco 
dihanja ali brez nje.
Ključne besede: vdišne mišice, vadba, zmanjšano diha-
nje, večstopenjska obremenitev, plavanje
ABSTR ACT
The purpose of this study was to investigate the 
influence of inspiratory muscle training (IMT) on tidal 
volume (V
T 
or inspiratory volume) during incremental 
exercise where breathing frequency is restricted. 21 
healthy subjects (9 males, 12 females) were divided 
into two groups: experimental (Group E) and control 
(Group C). Group E performed 30 dynamic inspiratory 
efforts twice daily against a pressure-threshold load of 
~50% maximal inspiratory pressure. A spring-loaded 
threshold inspiratory muscle trainer was used for this 
IMT. Group C received no IMT. Prior to and following a 
6 week intervention both groups performed incremental 
cycle ergometry until volitional exhaustion with 
reduced breathing frequency (RBF), which was defined 
as 10 breaths per minute. After training, the inspiratory 
muscle strength in Group E (+35 ± 16%; p<0.01) and 
submaximal and maximal V
T
 was higher (+13 ± 14%; 
p<0.01) during incremental cycle ergometry with RBF 
compared with pre-training. The latter adaptation was 
the reason for the extended time to exhaustion (17 ± 15%; 
p<0.01) following training. V
T
 and time to exhaustion 
were unchanged in Group C. It could be concluded that 
IMT increased V
T
 during incremental exercise where 
breathing frequency was restricted. However, future 
research is needed to establish the efficacy of this 
training as a supplement to regular exercise training 
with or without RBF. 
Keywords: inspiratory muscles, training, reduced 
breathing, incremental exercise, swimming
University of Ljubljana, Faculty of sport 
Laboratory of biodinamics, Slovenia
Corresponding author:
Jernej Kapus, PhD
University of Ljubljana
Faculty of Sport
Gortanova 22
1000 Ljubljana, Slovenia
e-mail: nejc.kapus@fsp.uni-lj.si
INSPIRATORY MUSCLE TRAINING INCREASES TIDAL 
VOLUME DURING INCREMENTAL EXERCISE WITH 
REDUCED BREATHING – A PILOT STUDY
V ADBA ZA MOČ VDIŠNIH MIŠIC POVEČA DIHALNI 
VOLUMEN MED VEČSTOPENJSKO OBREMENITVIJO Z 
NIŽJO FREKVENCO DIHANJA – PILOTSKA RAZISKA V A
Jernej Kapus 
Anton Ušaj

54 Inspiratory muscle training Kinesiologia Slovenica, 18, 1, 53–64 (2012) 
INTRODUCTION
Perhaps the best example in sport of where breathing frequency is naturally reduced is during 
front crawl swimming. While performing the front crawl swimmers may use different breathing 
patterns. They usually take a breath every second stroke cycle (Maglischo, 2003). However, they 
can reduce their breathing frequency by taking a breath every fourth, fifth, sixth or eighth stroke 
cycle. During such exercise, breathing (and specifically inspiration) must be coordinated with 
stroke mechanics and as a result the tidal volume (V
T
 or inspiratory volume) is elevated to com-
pensate for the reduced breathing frequency (RBF) (Dicker, Lofthus, Thornton, & Brooks, 1980). 
However, as V
T
 increases it requires progressively stronger inspiratory muscle force to expand 
the lungs, which requires greater effort and results in breathing discomfort. Further, the time for 
inhalation during swimming is quite short. Cardelli, Lerda, & Cholet (2000) demonstrated that 
the duration of inhalation lasted less than 0.5 seconds during 800m front crawl swimming at a 
lower velocity i.e. 1.28 m/s. Swimmers must therefore inhale quickly to ensure high lung volumes 
during swimming with RBF. Therefore, it could be suggested that this breathing pattern during 
swimming could pose a great challenge for the respiratory muscles. Indeed, it was recently shown 
that a reduced breathing frequency induced a larger magnitude of inspiratory muscle fatigue 
during 200m of maximal swimming (Jakoljevic & McConnell, 2009). 
In the 1970s, Leith and Bradley (1976) demonstrated that the strength of respiratory muscles can 
be improved by inspiratory muscle training (IMT). It is now well documented that IMT enhances 
the performance of athletes across a range of endurance sports (Griffiths & McConnell, 2007; 
Romer, McConnell, & Jones, 2002; Voliantis et al., 2001), as well as during repeated sprinting 
(Romer, McConnell, & Jones, 2002). Recent evidence suggests that IMT has a small positive effect 
on swimming performance in trained swimmers in events shorter than 400 m (Kilding, Brown, & 
McConnell, 2010). Given the unique challenge for the respiratory muscles, it is surprising that so 
far no studies have examined the influence of IMT on exercise with RBF. Therefore, the purpose 
of this study was to investigate the influence of IMT on V
T
 during incremental exercise where 
breathing frequency is restricted. Considering suggestions from previous studies, we hypothesise 
that IMT will increase V
T
 during incremental exercise where breathing frequency is restricted. 
Due to the technical limitations of measuring respiratory parameters during swimming, the 
influence of IMT on subjects’ performance in reduced breathing conditions was investigated 
during cycle ergometry in the present study. During such exercise the breathing frequency can 
be modified and respiratory parameters measured with greater ease. 
MATERIALS AND METHODS
Subjects 
Nine males (age: 27 ± 3 years, height: 1.80 ± 0.07 m, body mass: 80 ± 13 kg) and twelve females 
(age: 24 ± 1 years, height: 1.67 ± 0.03 m, body mass: 61 ± 5 kg)volunteered to participate in this 
study. None of the subjects were smokers and none had any respiratory diseases. The subjects 
were fully informed of the purpose and possible risks of the study before giving their written 
consent to participate. The study was approved by the National Ethics Committee of the Republic 
of Slovenia.

Inspiratory muscle training  55 Kinesiologia Slovenica, 18, 1, 53–64 (2012)
Testing protocol
The subjects completed the following tests in the same order in pre- and post-training: 1) tests 
of pulmonary function; 2) tests of respiratory muscle strength; and 3) an incremental cycle 
ergometry with RBF. All testing took place in controlled environmental laboratory conditions 
(21°C, 40–60% RH, 970–980 mbar) and at the same time of day. The subjects were asked to 
maintain their usual eating habits and avoid consuming food 2 hours before the testing. Post-
training testing began 2 days after the last training session. 
Pulmonary function. A pneumotachograph spirometer (Vicatest P2a, Mijnhardt, Netherlands) 
was used to measure resting flow-volume profiles. The following variables were derived: vital 
capacity (VC), forced vital capacity (FVC) and forced expiratory volume in one second (FEV
1.0
). 
Pulmonary function measurements were made according to European Respiratory Society 
recommendations (Miller et al., 2005).
Respiratory muscle strength was assessed by measuring the maximal inspiratory mouth pressure 
(MIP) at residual volume and maximal expiratory mouth pressure (MEP) at total lung capacity. 
Both parameters were measured using a portable hand-held mouth pressure meter (MicroRMP, 
MicroMedical Ltd, Kent, UK) in the standing position. The assessment of maximal pressures 
required a sharp, forceful effort maintained for a minimum of 2 s. All subjects became well 
habituated with the procedure during two separate familiarisation sessions. They received 
visual feed-back of the pressure achieved during each effort by viewing the digital display on 
the hand-held device in order to maximise their inspiratory and expiratory effort. MIP and MEP 
measurements were taken repeatedly until a stable baseline of each parameter was achieved. The 
criteria for determining MIP and MEP stability was successive efforts within 5%. The highest 
value recorded was included in the subsequent analysis.
Incremental exercise test with RBF was performed on an electromagnetically braked cycle ergom-
eter (Ergometrics 900, Ergoline, Windhagen, Germany) with a pedal cadence of 60 rpm. RBF was 
defined as 10 breaths per minute and was regulated by a breathing metronome. The breathing 
metronome was composed of a gas service solenoid valve (24 VDC, Jakša, Ljubljana, Slovenia) 
and a semaphore with red and green lights. Both were controlled by micro-automation (Logo DC 
12/24V, Siemens, Munich, Germany). The subjects were instructed to exhale and inhale during 
a two second period of an open solenoid valve (the green semaphore light was switched on) and 
to hold their breath, using almost all their lung capacity (holding their breath near total lung 
capacity), for four seconds when the solenoid valve was closed (when the red semaphore light was 
switched on) (Kapus, Ušaj, & Kapus, 2010; Sharp, Williams, & Bevan, 1991; Yamamoto, Mutoh, 
Kobayashi, & Miyashita, 1987). Prior to testing and training, the subjects were familiarised with 
cycling on the cycle ergometer and breathing in time with the metronome. The test began at an 
intensity of 30 W and increased by 30 W every minute until volitional exhaustion.The subjects 
breathed through a mouthpiece attached to apneumotachograph during each cycle ergometry 
test. Expired air was sampled continuously by a metabolic cart (V-MAX29, SensorMedics Cor-
poration, Yorba Linda, USA) for a breath-by-breath determination of pulmonary ventilation(V
·
E
), 
V
T
, end-tidal partial pressure of carbon dioxide (P
ET
CO
2
), end-tidal partial pressure of oxygen 
(P
ET
O
2
), and oxygen uptake (V
·
O
2
). The pneumotachograph and O
2
 and CO
2
 analysers were 
calibrated with a standard three-litre syringe and precision reference gases, respectively. For 
further statistical analysis the breath-by-breath data were averaged for each 10 second interval. 

56 Inspiratory muscle training Kinesiologia Slovenica, 18, 1, 53–64 (2012) 
Training protocol
The subjects were categorised according to gender and their baseline MIP measures and divided 
into matched trios. Two subjects of each trio were assigned at random to the experimental group 
(Group E) by an independent observer and the third to the control group (Group C). Descriptive 
measures of the subjects and training groups are presented in Table 1.
Group E performed 30 dynamic inspiratory efforts twice daily for six weeks (84 training sessions) 
against a pressure-threshold load of ~50% MIP. A spring-loaded threshold inspiratory muscle 
trainer (POWERbreathe, Gaiam Ltd., Southham, UK) was used for this IMT. After the initial 
setting of the training loads the subjects were instructed by an independent observer to increase 
the load periodically once a week to a level that would permit them to only just complete 30 
manoeuvres. The subjects initiated each inspiratory effort from the residual volume and strove 
to maximise the tidal volume. This IMT protocol is known to be effective in eliciting an adaptive 
response (Griffiths & McConnell, 2007; McConnell & Lomax, 2006; McConnell & Sharpe, 2005; 
Romer, McConnell, & Jones, 2002; Voliantis et al., 2001). The subjects completed a training 
diary throughout the study to record their adherence to the training. IMT had ceased 48 hours 
before the post-training testing. Group C received no IMT. The subjects in both groups were 
recreationally active, although they did not perform any intense exercise training during the 
research period.
Statistical analyses
The results are presented as means and standard deviations. Intra-group differences between 
the pre- and post-training values were calculated with a paired, two-tailed t-test. An analysis 
of covariance (ANCOVA), with pre-training values as covariates and post-training values as 
dependent variables, was used to test for differences between the groups resulting from differ-
ent training interventions. Statistical significance was accepted at the p≤0.05 level. Effect sizes 
were calculated using Cohen’s d statistics to assess the magnitude of the treatment with 0.2 
being deemed small, 0.5 medium and 0.8 large (Cohen, 1988). All statistical parameters were 
calculated using the statistics package SPSS (version 15.0, SPSS Inc., Chicago, USA) and the 
graphical statistics package Sigma Plot (version 9.0, Jandel & Tübingen, Germany).
R ESULTS
Descriptive measures of the subjects and training groups are presented in Table 1.
Table 1. Descriptive characteristics of the subjects and training groups
Parameter All subjects Group E (N = 14; 6 M, 8 F) Group C (N = 7; 3 M, 4 F) 
Age (yr) 25 ± 2 24 ± 1 27 ± 3
Height (cm) 173 ± 8 172 ± 8 174 ± 9
Body mass (kg) 69± 13 67 ± 9 74± 18
VC (l) 5.08± 1.08 5.03 ± 0.94 5.19± 1.41
FEV
1.0
 (l
.
s
-1
) 3.99± 0.74 3.98 ± 0.71 4.00 ± 0.86
MIP (cm H
2
O) 125± 22 122 ± 21 130± 26
MEP (cm H
2
O) 149± 30 142 ± 24 162±38
Values are means ± SD. VC, vital capacity; FVC, forced vital capacity; FEV
1.0
, forced expiratory volume in one second; 
MIP , maximal inspiratory mouth pressure; MEP , maximal mouth expiratory pressure

Inspiratory muscle training  57 Kinesiologia Slovenica, 18, 1, 53–64 (2012)
All subjects in Group E completed the prescribed training programme. Spirometry parameters 
and parameters of respiratory muscle strength measured at the pre- and post-training testing 
are shown in Table 2.
Table 2. Spirometry parameters and parameters of respiratory muscle strength pre- and post-
training
Parameter Group Pre-training Post-training
VC (l)
E 5.03 ±0.94 5.02 ±1.03
C 5.19±1.41 5.23±1.41
FEV
1.0
 (l)
E 3.98 ± 0.71 4.06 ±0.74
C 4.00 ± 0.86 3.98± 0.88
MIP (cm H
2
O)
E 122 ±21 164 ±30 ††
**
C 130±26 133±32
MEP (cm H
2
O)
E 142 ± 24 145 ±29
C 162±38 166± 43
Values are means ± SD. VC, vital capacity; FEV1.0, forced expiratory volume in one second; MIP maximal inspiratory 
pressure; MEP , maximal expiratory pressure.Significant training effect (paired T test): †† - p<0.01. Significant differ-
ences between groups after the training (ANCOV A): ** - p<0.01.
As shown in Table 2, MIP differed among the groups in response to the training period (p<0.01). 
As expected,a higher value (p=0.01, d=1.68) was observed after training in Group E compared 
with the pre-training values. By contrast, spirometry parameters and parameters of respiratory 
muscle strength were unchanged in Group C. Time to exhaustion during incremental cycle 
ergometry with RBF was extended with the IMT in group E (p<0.01, d=0.95), whereas this 
parameter did not change in group C. Between-group differences in time to exhaustion were 
also significant for the post-training comparisons (p<0.05; see Figure 1). 
Figure 1. Time to exhaustion obtained during incremental cycle ergometry with reduced 
breathing frequency pre- and post-training. Values are means ± SD. Significant training effect 
(paired t-test): †† - p<0.01; significant differences between groups after the training (ANCOVA): 
* - p<0.05.

58 Inspiratory muscle training Kinesiologia Slovenica, 18, 1, 53–64 (2012) 
Table 3 shows the peak power output obtained at the pre- and post-training incremental tests. 
Peak power output was defined as the highest work stage completed (i.e. the last work stage 
thesubject actually sustained for one minute), and hence the power output obtained.
Table 3. Peak power output (W) obtained at the pre- and post-training incremental tests
Subject Group Pre-training Post-training
1 E 150 180
2 E 150 180
3 E 150 180
4 E 270 270
5 E 150 150
6 E 150 180
7 E 150 210
8 E 150 150
9 E 180 210
10 E 150 180
11 E 150 180
12 E 150 180
13 E 180 210
14 E 180 180
15 C 300 180
16 C 120 120
17 C 150 180
18 C 210 210
19 C 180 150
20 C 180 180
21 C 210 240
The lowest peak power output obtained at the pre- and post-training incremental tests was 150 
W and 120 W, respectively, for Group E and Group C (Table 3). Therefore, the average data of 
measured respiratory parameters to these work stages and maximal or minimal values during 
the incremental test with RBF are presented in Figures 2 and 3, respectively, for Group E and 
Group C.

Inspiratory muscle training  59 Kinesiologia Slovenica, 18, 1, 53–64 (2012)
Figure 2. Respiratory parameters (V
·
E
, minute ventilation (a); V
T
, tidal volume (b); P
ET
O
2
 end-
tidal partial pressure of oxygen (c); P
ET
CO
2
, end-tidal partial pressure of carbon dioxide (d)) and 
V
·
O
2
, oxygen uptake (e)) obtained during the incremental test with RBF pre- (white squares and 
columns) and post-training (black squares and columns) in Group E. Values are means ± SD. 
Significant training effect (paired t-test): † - p<0.05 and †† - p<0.01.

60 Inspiratory muscle training Kinesiologia Slovenica, 18, 1, 53–64 (2012) 
As shown in Figure 2, there were significant training changes in V
·
E
, V
T
,P
ET
O
2
and P
ET
CO
2
 (p 
varied from 0.01 to 0.05) obtained at submaximal work stages during an incremental test with 
RBF. In addition, maximal values of V
·
E 
(p<0.01, d=0.75), V
T
 (p<0.01, d=0.81) and V
·
O
2
(p<0.05, 
d=0.53) were also enhanced by the IMT in Group E. By contrast, respiratory parameters during 
the incremental test with RBF did not change (neither in submaximal values nor at maximal 
or minimal) throughout the training in Group C (Figure 3). Between-group differences in the 
maximal values of V
·
E 
and V
·
O
2
were also significant for the post-training comparisons (p<0.05).
Figure 3. Respiratory parameters (V
·
E
, minute ventilation (a); V
T
, tidal volume (b); P
ET
O
2
 end-
tidal partial pressure of oxygen (c); P
ET
CO
2
, end-tidal partial pressure of carbon dioxide (d)) and 
V
·
O
2
, oxygen uptake (e)) obtained during the incremental test with RBF pre- (white squares and 
columns) and post-training (grey squares and columns) in Group C. Values are means ± SD.

Inspiratory muscle training  61 Kinesiologia Slovenica, 18, 1, 53–64 (2012)
DISCUSSION
This pilot study is the first to our knowledge to have investigated the influence of IMT on 
V
T
 during incremental exercise where breathing frequency is restricted. The data indicate that 
IMT increased MIP (Table 2) and additionally V
T
 during the RBF exercise (Figure 2b). The 
later adaptation could be the reason for the significant improvement in time to exhaustion at 
incremental cycle ergometry with RBF in Group E (17 ± 15%; Figure 1). On the basis of the large 
effect sizes, it could be suggested that the training intervention per se was primarily responsible 
for the increased mentioned variables. 
The 35 ± 16% increase in MIP after six weeks of IMT (Figure 2) is in accordance with previous 
IMT studies. However, the increase obtained in some studies was greater (from 45% to 64% 
reported by Romer & McConnell, 2003; Voliantis et al., 2001) and lower in others (from 15% to 
26% reported by Griffiths & McConnell, 2007; Inbar, Weiner, Azgad, Rotstein, & Weinstein, 2000; 
Johnson, Sharpe, & Brown, 2007;Klusiewicz, Borkowski, Zdanowicz, Boros, & Wesolowski, 2008; 
McConnell & Sharpe, 2005; Lomax, 2010). Established training principles appear to apply to IMT 
(Romer and McConnell, 2003), thus these discrepancies may be related, in part, to inter-study 
differences in the IMT mode, intensity and duration. However, since the scale of physiological 
adaptation within a system depends on its baseline status (Åstrand & Rodahl, 1986), it seemed 
that the training improvement is mainly related to the pre-training level of MIP. McConnell 
(2011) suggested that changes in inspiratory muscle strength due to IMT could be attributed to 
changes in diaphragm thickness. Indeed, it was shown that diaphragm thickness increased after 
just four weeks of IMT, which confirmed the presence of rapid fibre growth (hypertrophy) in 
response to loading (Downey et al., 2007).
It is well known that during endurance exercise the respiratory muscles perform significant 
amounts of metabolic work. Indeed, Aaron, Seow, Johnson, & Dempsey (1992) showed that 
during strenuous exercise respiratory muscles can command ~10%of total oxygen consumption 
in moderately fit subjects and up to 15% in highly fit subjects. Taking this into account, previous 
studies have tried to determine the effects of IMT on respiratory muscle functions and in addi-
tion on maximal aerobic capacity (Inbar, Weiner, Azgad, Rotstein, & Weinstein, 2000; Riganas, 
Vrabas, Christoulas, & Mandroukas, 2008; Williams, Wongsathikun, Boon, & Acevedo, 2002),on 
exercise performance (Forbes, Game, & Syrotuik, 2011; Kilding, Brown, & McConnell, 2010; 
Voliantis et al. 2001; Williams, Wongsathikun, Boon, & Acevedo, 2002), and on post-exercise 
inspiratory muscle fatigue (Romer, McConnell, & Jones, 2002). However, interestingly the impact 
of IMT on breathing parameters such us V
·
E
, V
T
 and breathing frequency during exercise has not 
been examined yet. Hypoventilation is a well-documented phenomenon during exercise with 
RBF. A reduction of approximately 48% in V
E
 was demonstrated in previous studies in which 
10 breaths per minute was used for the reduced breathing conditions (Kapus, Ušaj, & Kapus, 
2010; Sharp, Williams, & Bevan, 1991; Yamamoto, Mutoh, Kobayashi, & Miyashita, 1987). A 
similar reduction in V
E
 was reported when swimmers reduced their breathing frequency during 
swimming from the usual taking of a breath every second stroke cycle to taking a breath every 
fifth (Dicker, Lofthus, Thornton, & Brooks, 1980) or sixth (Town & Wanness, 1990) stroke cycle. 
As a reduction in V
·
E
 is unlikely to benefit exercise performance, it follows that an increase in 
V
·
E
 would be advantageous. Due to the prescribed and unchanged breathing frequency in the 
present study, an increase in V
T
 was the only mechanism available for increasing V
·
E 
(Figure 2a). 
Indeed, our results indicate that IMT does increase V
T
 during incremental exercise with RBF. 

62 Inspiratory muscle training Kinesiologia Slovenica, 18, 1, 53–64 (2012) 
Specifically, maximal values of V
T
 increased by 13 ± 14% following the IMT (Figure 2b). Despite 
breathing with large V
T
, a 2 second period of an opened valve was enough for the subjects’ 
expiration and for deep inspiration. The obtained improvement in V
T
 a s re su lt of t he I M T wa s le s s 
than the 41% reported after training with RBF (Kapus, 2008). However, it should be emphasised 
that increased V
T
was obtained without any additional exercise or RBF training in the present 
study.The higher V
T
at the post-training testing allowed the better regulation of blood gases, i.e. 
it induced a higher submaximal P
ET
O
2 
anda lower submaximal P
ET
CO
2 
in comparison with the 
pre-training testing (Figure 2). These measurements could indirectly show the level of O
2
 a nd CO
2
 
in blood during rest and exercise (Williams and Babb, 1997). Previous studies demonstrated that 
hypercapnia and/or hypoxia in blood could be the reasons for earlier fatigue during exercise with 
RBF (Kapus, Ušaj, Kapus, & Štrumbelj, B., 2003; Sharp, Williams, & Bevan, 1991). Therefore, it 
seemed that a higher V
T
 (via better regulation of blood gases) enabled higher peak power output 
during the post-training incremental exercise with RBF (Table 3). This adaptation was the reason 
for the higher maximal values of V
·
O
2
 during the post-training testing in comparison with the 
pre-training testing (Figure 2). 
Considering this, it could be suggested that IMT may offer some potential benefits (via increasing 
V
T
) for a swimmer’s ability to swim with fewer breaths and to hold their breath for longer during 
the underwater phases (flip turns, gliding, and underwater strokes). This could create an im-
portant biomechanical advantage for a swimmer’s performance (Chatard, Collomp, Maglischo, 
& Maglischo, 1990; Kolmogorov & Duplisheva, 1992; Lerda, Cardelli, & Chollet, 2001; Pedersen 
& Kjendlie, 2006).
Possible study limitations. The presented pilot study has some limitations and the results should 
be interpreted in the context of its design. Firstly, because the subjects were not blinded for the 
training intervention one may argue that this might have influenced our results. Although the 
subjects were naive and were not informed about the potential effect of the IMT on exercise 
performance with RBF, a placebo-controlled design should be used. Indeed, in most of the previ-
ous studies concerning the influences of IMT the experimental group and sham-training control 
group were compared. Subjects were told that they were participating in a study to compare 
the influence of strength (like Group E in the present study, which performed IMT) versus 
endurance protocols (the placebo group, which also used the pressure threshold device, however, 
performed 60 slow protracted inhalations against minimum pressure threshold once daily for six 
weeks) (Inbar, Weiner, Azgad, Rotstein, & Weinstein, 2000; Kilding, Brown, & McConnell, 2010; 
McConnell & Sharpe, 2005; Voliantis et al., 2001). Further, increased V
T
 during the post-training 
exercise with RBF could be the learning effect of the IMT. Indeed, at each training session 
the subjects in Group E performed several rapid inhalations in which they strove to maximise 
their V
T
. To avoid this effect, IMT should be added as an experimental group’s supplement to 
training with RBF which would be otherwise performed by both groups, i.e. experimental and 
control. Nonetheless, we find these preliminary data to be encouraging as part of the growing 
literature supporting the use of IMT as a supplement to regular swimming training. Future 
research is needed to establish the efficacy of this training in a randomised, controlled trial with 
the experimental design suggested above.
It could be concluded that IMT increased V
T
during incremental exercise where breathing 
frequency was restricted. However, further research is needed to establish the efficacy of this 
training as a supplement to regular exercise training with or without RBF.

Inspiratory muscle training  63 Kinesiologia Slovenica, 18, 1, 53–64 (2012)
ACKNOWLEDGMENTS
The research was supported by a grant from the Fundacija za šport, a foundation for financing 
sport organisations in Slovenia (RR-11-608).
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