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EFFECTS OF INSPIRATORY MUSCLE TRAINING ON 
PHYSICAL PERFORMANCE DURING BACKPACK 
CARRYING
Monika JERUC TANŠEK1, Andrej ŠVENT2, Alan KACIN1
1 University of Ljubljana, Faculty of Health Sciences, Physiotherapy Department, Slovenia
2 Intact d.o.o., Slovenia
Corresponding Author:
Alan KACIN
University of Ljubljana, Faculty of Health Sciences, Physiotherapy Department, 
Zdravstvena pot 5, 1000 Ljubljana, Slovenia
Phone: +386 1 300 11 19
Email: alan.kacin@zf.uni-lj.si
ABSTRACT
Purpose: Restricting chest movement when carrying a loaded backpack reduces ef-
ficiency and increases the work of the respiratory muscles. The aim of the present study 
was to investigate the effects of six weeks of inspiratory muscle training (IMT) on respira-
tory muscle strength and endurance and on physical performance when carrying a load 
Methods: Twenty male (age: 32.2 ± 3.4 years) members of the Special Operati-
ons Unit of the Slovenian Army volunteered to participate. The experimental group 
(n=10) trained their respiratory muscles for six weeks against an incremental inspi-
ratory resistance with a breathing apparatus. The placebo group (n=10) performed 
the same IMT protocol but with a sham inspiratory resistance. Assessment of the su-
bjects before and after IMT included measurements of the maximal inspiratory and 
expiratory pressures, heart rate measurements, and ratings of perceived physical 
and respiratory exertion before and after a 60-min walk test with a 25-kg backpack 
Results: After six weeks of IMT, the maximum inspiratory pressure measured before 
and after the 60-minute walk test increased significantly (p < 0.001) in the experi-
mental group by 47 ± 13% and 58 ± 20%, respectively. Inspiratory fatigue was also 
significantly lower in the experimental group. No changes were observed in the he-
art rate and the rating of perceived exertion during the walking test. In the placebo 
group, no significant changes were observed in the measured parameters after IMT. 
Original scientific article                       DOI: https://doi.org/10.35469/ak.2022.335
received: 2022-07-25              UDC: 796.015:612.2
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Conclusion: Six weeks of IMT with progressive breathing resistance improves strength 
and reduces fatigue of the respiratory muscles. Individuals who perform tasks that requi-
re them to carry a heavy backpack for extended periods of time may benefit from IMT. 
Keywords: load-carrying, respiratory muscle fatigue, respiratory muscle training, 
thoracic motion restriction.
UČINKI VADBE INSPIRATORNIH MIŠIC NA TELESNO 
ZMOGLJIVOST  MED NOŠENJEM NAHRBTNIKA
IZVLEČEK
Cilj: Omejitev gibanja prsnega koša med nošnjo obteženega nahrbtnika zmanjša 
učinkovitost in poveča delo dihalnih mišic. Cilj pričujoče študije je bil raziskati učinke 
šesttedenske vadbe inspiratornih mišic (VIM) na jakost in vzdržljivost dihalnih mišic 
ter telesno zmogljivost med prenašanjem bremena.
Metode: Prostovoljno je sodelovalo 20 moških (starost: 32,2 ± 3,4 let) pripadnikov 
Enote za specialno delovanje Slovenske vojske. Eksperimentalna skupina (n=10) je 
šest tednov neprekinjeno vadila proti naraščajočemu uporu pri vdihu s posebno dihal-
no napravo. Placebo skupina (n=10) je izvedla enak protokol VIM z napravo, vendar 
le z navideznim inspiratornim uporom. Začetno in končno testiranje preiskovancev je 
vključevalo meritve največjih inspiratornih in ekspiratornih tlakov, meritve frekvence 
srčnega utripa in oceno občutenja telesnega in dihalnega napora pred in po 60-minu-
tnem testu hoje s 25-kg nahrbtnikom.
Rezultati: Po šesttedenski VIM se je največji inspiratorni tlak izmerjen pred in po 
testu hoje pomembno (P < 0,001) povečal v eksperimentalni skupini, in sicer za 47 ± 
13 % pred testom in za 58 ± 20 % po testu. Značilno se je zmanjšala tudi inspiratorna 
utrujenost v eksperimentalni skupini. Odziv frekvence srčnega utripa in ocena občute-
nja napora med testom hoje se po VIM ni spremenila. V placebo skupini po VIM nismo 
opazili pomembnih sprememb v nobenem izmerjenem parametru.
Zaključek: Šesttedenska VIM s progresivnim inspiratornim uporom izboljša jakost 
inspiratornih mišic in zmanjša njihovo utrudljivost. Tovrstna dihalna vadba ima lahko 
pozitivne učinke za ljudi med opravljanjem nalog, ki zahtevajo dolgotrajno nošenje 
težkega nahrbtnika.
Ključne besede: prenašanje bremen, utrujenost dihalnih mišic, vadba dihalnih mi-
šic, omejitev gibanja prsnega koša.
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INTRODUCTION
Carrying a load with a backpack is defined as the movement of a person with an 
additional mass on the trunk supported by shoulder straps (Knapik, Harman, Steelman 
& Graham, 2012). It is a common form of occupational physical activity, especially for 
soldiers, firefighters and rescue workers, as well as for various forms of sports and rec-
reational activities. The load carried in a backpack restricts the movement of the chest 
and the amount of air a person can inhale, increasing the work of breathing (Dominelli, 
Sheel & Foster, 2012; Faghy & Brown, 2014b). In this case, the respiratory muscles 
are working outside the optimal limits of their length-tension curve (Romer & Polkey, 
2008). Altered respiratory mechanics lead to accelerated fatigue of the respiratory mu-
scles (Faghy & Brown, 2014a; Faghy & Brown, 2014b), which can reduce physical 
performance by reducing the blood flow to other skeletal muscles and increasing the 
perception of physical and respiratory effort (Dempsey, Romer, Rodman, Miller, & 
Smith, 2006; Harms, et al., 1997).
The actual effect of carrying an extra load in a backpack on respiratory muscle 
function under different working conditions has not been studied in detail. Butcher, 
Jones, Eves and Petersen (2006) reported a significant reduction in the maximum air-
way pressure in professional firefighters wearing a heavy backpack and a respiratory 
mask during both long-duration low-intensity physical activities and short-duration hi-
gh-intensity physical activities. However, it is difficult to assess the individual effects 
of wearing a backpack and using a breathing mask because the mask itself increases 
the work of breathing (Eves, Jones & Petersen, 2005). In subjects of varying fitness 
levels, wearing a 25 kg backpack without a breathing mask has been shown to reduce 
the maximum inspiratory pressure (MIP) by 11% during 60 min walking (58% V̇O2max) 
on a treadmill and by a further 5% during subsequent high-intensity running (Faghy & 
Brown, 2014a; Faghy & Brown, 2014b; Faghy, Blacker & Brown, 2016).
The physical training of members of military special forces includes both low- 
intensity activities (e.g. military patrols) and high-intensity activities (e.g. military 
interventions and combat operations) while wearing a backpack, so it might be use-
ful to include respiratory muscle training in their training routine. By using special 
training aids and training programs, one can strengthen the inspiratory or expiratory 
respiratory muscles in isolation or both at the same time. Inspiratory muscle training 
(IMT) adds resistance to the inspiratory flow and primarily strengthens the inspira-
tory muscles, especially the abdominal diaphragm, which creates an intrathoracic 
negative pressure during inspiration. IMT is commonly used to reduce respiratory 
effort during physical activity in elite athletes (HajGhanbari et al., 2013) and healthy 
individuals (Illi, Held, Frank & Spengler, 2012; Sales et al., 2016) as it significantly 
increases the strength of the inspiratory intercostal muscles, as well as the diaphragm 
(Verges, Lenherr, Haner, Schulz, & Spengler, 2007; Romer & Polkey, 2008). This has 
been shown to improve athletic performance (HajGhanbari et al., 2013), e.g. cycling 
(Romer, McConnell & Jones, 2002; Johnson, Sharpe & Brown, 2007; McConnell, 
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2011), running (Tong et al., 2008), swimming (Yañez-Sepulveda et al., 2021) and 
rowing (Griffiths & McConnell, 2007).
Respiratory muscle performance can be improved with various training protocols, 
most of which result in increased respiratory muscle strength and, to a lesser extent, 
improved muscle endurance (Fernández-Lázaro et al., 2021). IMT protocols vary and 
depend on the respiratory device used, the characteristics of the exerciser and the de-
sired effects. In general, a minimum of four weeks of regular IMT, usually performed 
twice daily for at least 5 days per week at 50-70% of the maximum inspiratory pressure 
(MIP), is required for improvement in respiratory muscle strength (McConnell, 2013). 
Moderate- to high-intensity IMT (∼60% MIP) can increase maximal contraction velo-
city and inspiratory muscle strength. Faghy & Brown (2016) reported that six weeks 
of IMT performed twice daily increased the maximal inspiratory pressure at rest by 
31% and significantly attenuated the cardiovascular and perceptual responses to 60 
minutes of walking with a 25-kg backpack at a steady pace, while improving perfor-
mance by 8% during high-intensity timed runs. Because the relative intensity of their 
IMT protocol was kept constant at 50% of the maximal inspiratory muscle pressure, we 
hypothesize that even higher training effects can be achieved with a more progressive 
inspiratory resistance protocol. The progressivity of inspiratory resistance is likely to 
be of critical importance for training individuals with high levels of physical fitness, 
such as members of military special forces.
The aim of our study was therefore to investigate the effects of six weeks of IMT 
with progressive breathing resistance in members of military special forces on respi-
ratory muscle strength and endurance, heart rate response and the perception of effort 
when walking with a heavy backpack.
METHODS
We conducted a controlled prospective intervention study on a sample of members 
of the Special Unit of the Slovenian Army (SOU SA). The study was approved by the 
Medical Ethics Committee of the Republic of Slovenia (No. 0120-494/2017/7), the 
Ministry of Defence of the Republic of Slovenia and the General Staff of the Slovenian 
Army.
Study Sample
All the potential SOU SA candidates were first given important information regar-
ding the purpose and procedure of the study. Twenty male (mean age: 32.2 ± 3.4 years, 
age range: 27-38 years) members of the SOU SA site completed a questionnaire on 
their general health and signed a declaration in which they volunteered to participate 
in the study. Exclusion criteria for the subjects were cardiovascular, respiratory or me-
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tabolic diseases, a history of peripheral or central thromboembolism, radiculopathy or 
other peripheral neurological disorders of the lower limbs.
Assessment of General Fitness
The assessment of general physical fitness was based on regular military tests per-
formed by the subjects in the three months prior to the start of the study. The tests 
included two minutes of push-ups, two minutes of abdominal crunches and a fast run 
of 3200 meters. The result achieved by the subject in each test task was converted into 
points using a motor test scoring system defined by gender and age categories (Ivšek 
& Pograjc, 2014).
Study Design
The twenty subjects were randomly divided by lot into two groups of equal size. 
The experimental group (EG) performed a six-week IMT program with a breathing 
device that provided a progressive increase in inspiratory resistance. The placebo group 
(PG) performed the same program with a device that did not add inspiratory resistance 
but had a virtual resistance regulator installed. Both groups were tested before and after 
the completion of the six-week IMT program, as described below. 
Tests and Measurements 
All the tests and measurements were carried out in the sports hall of Vojašnica Ed-
varda Peperka, Moste-Polje Ljubljana. On the day of the test, the subjects ate a small 
meal two to three hours before the exercise and abstained from coffee or alcoholic 
beverages for at least 24 hours before the exercise. The subjects were familiarised with 
all the tests and measurement protocols before the first data collection.
Heart Rate and Aerobic Capacity
The subjects were placed in a stationary, semi-recumbent position on the exami-
nation table for 10 minutes and their heart rate was measured using a monitor with a 
chest strap (Polar M430 POLAR Electro, Europe AG, Val-de-Travers, Switzerland). 
The Polar Fitness Test™, which assesses a person’s maximum oxygen consumption 
(V̇O2max) based on resting heart rate variability, gender, age, height, body weight and 
self- assessed physical activity level, was used to assess aerobic capacity.
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Respiratory Muscle Strength and Fatigue
Measurements were taken using an inspiratory and expiratory MicroRPM® 
pressure measurement device (VYAIRE Medical Inc., Illinois, USA). Respiratory 
muscle strength was measured using the maximum inspiratory pressure (MIP) and 
maximum expiratory pressure (MEP) in cm H2O (Faghy & Brown, 2016). The MIP 
measurement was derived from the residual volume (maximal expiration) and the MEP 
measurement of the total lung capacity (maximal inspiration). Each measurement was 
taken five times. The minimum and maximum values were excluded and the average 
of the remaining three measurements was calculated (McConnell, 2013). The respira-
tory muscle fatigue was determined from the difference (∆) between the MIP and MEP 
values obtained before (pretest) and immediately after (posttest) the 60-minute walking 
test. 
Walking Test with a Backpack
Before the 60-minute walking test, the subjects put on a military backpack (V2 
Plus System, Tasmanian Tiger GmbH, Dasing, Germany) that was evenly filled with 
a 25-kg load. Each subject adjusted and fastened the straps of the backpack individu-
ally before performing the test. The subjects completed the 60-minute walking test on 
a leveled Technogym Skillrun™ treadmill (Technogym, Cesena, Italy). Before they 
started walking, subjective ratings of overall perceived exertion (RPE) were assessed 
using the 15-item RPE scale and respiratory effort was assessed using the CR10 RPE 
scale (Borg, 1982). The heart rate was measured and recorded continuously during the 
test. The test began with a three-minute warm-up period so that the subjects gradually 
reached a target speed of 6.5 km/h, which they then maintained for 60 minutes (Faghy 
et al., 2016). Every ten minutes, the subjects rated the overall physical effort and respi-
ratory effort. After completing the test, the subjects removed their backpacks, cooled 
down by walking slowly on a treadmill for 5 minutes, and then rested under control for 
another 20 minutes.
Inspiratory Muscle Training Intervention
The experimental group (n=10; EG) exercised with the POWERbreathe® (medium 
resistance) inspiratory muscle strengthening device (Powerbreathe International Ltd., 
Southam, UK) with an initial resistance of 60% MIP. The placebo group (n=10; PG) 
performed the same IMT program using the same respirator with the internal inspira-
tory valve removed, minimizing the respiratory resistance. Every two weeks, the sub-
jects’ MIP was measured again and inspiratory resistance was adjusted accordingly 
with an additional 10% increase to reach the final training target of 80% MIP. The MIP 
was reassessed in PG at the same time points and the breathing resistance was adjusted 
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virtually. All subjects performed 30 consecutive maximal breaths twice daily. Each 
maximal inspiration was followed by an active forced expiration to expiratory residual 
volume (McConnell, 2011). Both groups kept an exercise diary that encouraged them 
to perform IMT regularly; compliance with the program was checked weekly by the 
investigators 
The POWERbreathe® respirator works on the principle of suprathreshold 
inspiratory loading. This requires the subject to generate an inspiratory pressure that 
exceeds the pressure threshold set on the device in order to open the inspiratory valve. 
The inspiratory threshold is increased by tensioning the spring attached to the air valve 
of the device. The advantage of suprathreshold loading is that the increase in inspiratory 
resistance does not affect the actual airflow through the unit. Furthermore, the load re-
sistance can be assessed objectively (McConnell 2011; McConnell, 2013).
Statistical Analysis and Data Processing
The normality of the data distribution was analyzed using the Shapiro-Wilk test, 
which showed the adequacy of the parametric tests. The effect of training was assessed 
by comparing the mean values of heart rate and maximum respiratory pressures me-
asured at rest prior to the 60-minute walk test (pretest), before and after the six-week 
IMT period. The effect of IMT on fatigue during walking was assessed by comparing 
the heart rate and RPE at the end of the walking (posttest) and ∆ MIP and ∆ MEP. The 
means were compared using the independent samples t-test and the two-way factorial 
ANOVA (group × time) with repeated measures for the time factor. If the factor interac-
tion was statistically significant, a pairwise comparison was performed using Tukey’s 
HSD post-hoc test. The threshold for statistical significance was set at p < 0.05 for all 
analyses. Results are presented as means ± standard deviations unless otherwise stated. 
RESULTS
Subjects from EG and PG did not differ significantly in age, anthropometric charac-
teristics, general physical fitness, and aerobic capacity. The detailed analysis of group 
characteristics is shown in Table 1.
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Table 1: Comparison of the basic subjects’ characteristics across groups.
EG PG p-value
Age (years) 33.3 ± 4.1 31.2 ± 2.8 0.258
Body height (cm) 180.6 ± 7.4 179.3 ± 4.5 0.641
Body weight (kg) 85.7 ± 5.6 82.7 ± 5.6 0.246
BMI (kg/m2) 26.4 ± 2.3 25.6 ± 2.8 0.396
3200 m run (min) 13.3 ± 1.1 13.2 ± 1.1 0.843
Abdominal crunches (no./2 
min) 87.3 ± 9.3 86.4 ± 9.0 0.843
Push-ups (no./2 min) 81.5 ± 7.2 79.4 ± 8.1 0.547
V̇O2max (ml/kg/min) 47.8 ± 3.08 49.3 ± 2. 79 0.485
EG – experimental group; PG – placebo group; BMI – body mass index; 2max – maximal 
pulmonary oxygen consumption
Respiratory Muscle Strength
The interaction of factors during the six-week IMT was significant (p < 0.001) for 
the pretest MIP. The pretest MIP increased significantly (p < 0.001) in the EG by 63 ± 
15 cm H2O (47 ± 13%) after training. There was no significant change (p = 0.162) in 
the pretest MIP in the PG after training. The pretest MIP was significantly higher (p < 
0.001) in the EG after training (Figure 1). 
The interaction of factors during the six weeks of IMT was significant (p < 0.001) 
for the posttest MIP. The posttest MIP increased significantly (p < 0.001) in the EG by 
71 ± 20 cm H2O (58 ± 20%) after training. There was no significant change (p = 0.306) 
in the posttest MIP in the PG after training. The posttest MIP was significantly higher 
(p < 0.001) in the EG after training (Figure 2) 
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*** indicates the pretest to posttest difference in the EG at p < 0.001. ### indicates the difference 
between the groups at p < 0.001.
Figure 1: Comparison of the mean (standard deviation) maximum inspiratory pressure 
(MIP) measured before the 60-minute walk test between the experimental group (EG) 
and the placebo group (PG) before and after the inspiratory muscle training program.
*** indicates the pretest to posttest difference in the EG at p < 0.001. ### indicates the difference 
between the groups at p < 0.001.
Figure 2: Comparison of the mean values (standard deviation) of the maximum inspira-
tory pressure (MIP) measured after the 60-minute walk test between the experimental 
group (EG) and the placebo group (PG) before and after the inspiratory muscle train-
ing program. 
EG before    EG after     PG before   PG after
EG before    EG after     PG before   PG after
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The interaction of factors during six weeks of IMT was not significant for the pretest 
MIP (p = 0.556) and the posttest MIP (p = 0.279) (Figures 3 and 4).
Figure 3: Comparison of the mean (standard deviation) maximum expiratory pressure 
(MEP) measured before the 60-minute walk test between the experimental group (EG) 
and the placebo group (PG) before and after the inspiratory respiratory muscle train-
ing program.
Figure 4: Comparison of the mean (standard deviation) maximum expiratory pressure 
(MEP) measured after the 60-minute walk test, between the experimental group (EG) 
and the placebo group (PG) before and after the inspiratory respiratory muscle train-
ing program.
EG before    EG after     PG before   PG after
EG before    EG after     PG before   PG after
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Respiratory Muscle Fatigue
The interaction of factors during the six-week IMT was also significant (p < 0.001) 
for ∆ MIP during the 60-minute walk test. The ∆ MIP decreased (p < 0.01) from -12 ± 
4 cmH2O before the training to -4 ± 4 cmH2O afterward. In contrast, the ∆ MIP in PG 
before (-12 ± 3 cmH2O) and after (-12 ± 4 cmH2O) the training was no different (p = 
0.991). The ∆ MIP after IMT was significantly lower in EG (p < 0.001). 
The interaction of the factors for ∆ MEP during the 60-min walk test was also sig-
nificant (p < 0.01). The ∆ MEP decreased from -15 ± 5 cmH2O before the training to 
-7 ± 5 cmH2O afterward. In contrast, the ∆ MIP in the PG before (-19 ± 7 cmH2O) and 
after (-16 ± 4 cmH2O) the training was no different (p = 0.549). The ∆ MEP after IMT 
was significantly lower in EG (p < 0.01).
Heart Rate and Perceived Exertion
The interaction of the factors during the six-week IMT was not significant for the 
heart rate (p = 0.215), respiratory effort (p = 0.327), and overall body exertion (p = 
0.644) (Figure 5).
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Figure 5. The mean (standard deviation) heart rate response (A) and ratings of overall 
body effort (B) and respiratory effort (C) during the 60-minute walk test before and 
after the six-week program of inspiratory muscle training for the experimental (EG) 
and placebo groups (PG).
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DISCUSSION
The main objective of the present study was to evaluate the effects of six weeks of 
IMT with progressive inspiratory resistance in members of military special forces on 
respiratory muscle strength and endurance, heart rate response and the perception of 
exertion when walking with a heavy backpack. The results show that the experimental 
group increased inspiratory muscle strength (MIP) by 47% as a result of the training, 
while there was no effect on the expiratory muscle strength (MEP). In addition, a si-
gnificant reduction in inspiratory and expiratory fatigue during the 60-minute walk test 
with a 25-kg backpack was found in the experimental group, but this was not reflected 
in the RPE or HR values during the test. In contrast, there was no significant improve-
ment in any of the measured physiological parameters in the placebo group. Thus, our 
results fully confirm the findings of the meta-analysis on the effects of IMT in various 
types of athletes (Karsten, Ribeiro, Esquivel & Matte, 2018) and highlight the impor-
tance of the progressivity and specificity of breathing resistance for optimal training 
adaptation (McConnell, 2011).
A direct comparison with the results of the methodologically most similar study 
by Faghy and Brown (2016) confirms our initial assumption that progressive inspira-
tory resistance further enhances the effects of IMT. Indeed, Faghy and Brown (2016) 
achieved a 31% increase in MIP in the pretest and 19% in the posttest after six weeks 
of IMT in moderately physically fit healthy subjects, while our study achieved a 47% 
increase in MIP in the pretest and as much as 58% in the posttest in very physically fit 
subjects during the same training period. The training effect was substantially higher in 
our subjects despite their higher baseline level of physical fitness.
 IMT also had a positive effect on the endurance of the respiratory muscles of the 
experimental group. The initial inspiratory and, interestingly, expiratory fatigue indu-
ced by the 60-minute walk test were reduced by 8 cmH2O. However, these changes 
were too small to reduce the subjective ratings of whole-body exertion or respiratory 
effort. Considering that the weight of a backpack is critical to respiratory fatigue under 
given exercise conditions, the weight of the backpack in our study appears to have 
been too low to cause noticeable respiratory fatigue and impair physical performance. 
Indeed, Dominelli, Sheel and Foster (2012) have shown that a backpack weighing less 
than 35 kg has no effect on respiratory mechanics and thus on the demand for respi-
ratory effort during short periods of walking. Consistent with this, Shei, Chapman, 
Gruber & Mickleborough (2017) reported that six weeks of flow-resistive IMT impro-
ved physical performance in recreational athletes but did not attenuate diaphragmatic 
fatigue during constant-load running to volitional exhaustion with a 10-kg backpack. 
Indeed, Faghy and Brown (2014a; 2014b) have shown that prolonged low-intensity 
physical activity with a backpack weighing less than 25 kg does not cause premature 
fatigue of the inspiratory respiratory muscles, although it does cause certain changes in 
the cardiovascular and metabolic responses and perceived physical exertion. They also 
highlight other factors that may mitigate premature inspiratory respiratory muscle fati-
gue, namely previous regular exercise, male gender, higher body mass, higher skeletal 
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muscle strength and higher aerobic capacity (Faghy & Brown, 2014a; Faghy & Brown, 
2014b). All of these factors were present in our subjects, which most likely contributed 
to the low perceived respiratory muscle fatigue during the 60-minute walking test. Re-
spiratory muscle fatigue due to the altered movement mechanics and excessive load on 
the chest when carrying a heavy backpack is clearly progressive (Butcher, et al., 2006; 
Faghy & Brown, 2014a; Faghy & Brown 2014b), so training-induced increases in re-
spiratory muscle strength cannot completely prevent it. Wearing a backpack also requi-
res a greater activity of the abdominal diaphragm to stabilize the thoracolumbar spine, 
which further accelerates diaphragm fatigue and worsens the mechanics and economy 
of breathing. It is therefore not surprising that IMT can also improve postural control 
when carrying loads and consequently reduce lower back pain (Janssens et al., 2015).
The main limitation of our study protocol was that it failed to produce detectable 
changes in the heart rate and ratings of perceived physical and respiratory exertion be-
tween the experimental and placebo groups. The most likely reason for this is that the 
intensity of the 60-minute walk test or the weight of the backpack was too low for the 
given population sample. To effectively stress the respiratory muscles, subjects must 
be exposed to a combination of prolonged moderate- to high-intensity physical activity 
while carrying a backpack weighing at least 25 kg. A two-stage testing protocol, such 
as that used by Faghy and Brown (2016), or a multi-stage protocol with progressive 
walking speeds and treadmill incline, such as that used by Armstrong, Ward, Lomax, 
Tipton, and House (2019), would most likely be more appropriate 
The recommended intensity of IMT is between 50 and 70 per cent of the MIP, 
exercise duration is up to 30 breathing cycles and exercise frequency is twice daily, 
every day per week (McConnell, 2013). To optimize the training protocol for the ne-
eds of our study, we considered the basic principles of physical training: progressive 
overload and specificity. The progressivity of exercise overload is primarily achieved 
through incremental intensity (resistance), but increasing the time and frequency of 
exercise can have an additional effect. In our study, we only ensured the progressivity 
of the overload during the six-week training period by increasing intensity, i.e. by 
increasing the inspiratory resistance threshold on the respirator by 10% every two 
weeks. It could be that an additional modulation of the breathing exercise time, i.e. 
the number of breathing cycles, would further improve the training effect, especially 
the endurance of the inspiratory muscles. This should be tested in future studies. On 
the other hand, an additional increase in daily exercise frequency would not be feasi-
ble in our subjects, as performing IMT twice a day, seven days a week, was already 
at the upper limit of their busy daily schedule.
CONCLUSION
The aim of this study was to determine whether six weeks of IMT could increase 
the strength and endurance of the respiratory muscles, thereby delaying their fatigue 
when carrying a backpack during physical activity. The IMT protocol used in previ-
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Monika JERUC TANŠEK, Andrej ŠVENT, Alan KACIN: EFFECTS OF INSPIRATORY MUSCLE TRAINING ON PHYSICAL PERFORMANCE ..., 5–21
ous studies was improved on by progressively increasing the inspiratory resistance to 
ensure a more optimal overload of the respiratory muscles. Our results show that MIT 
significantly increases inspiratory strength and reduces muscle fatigue during a 60-mi-
nute walk with a 25-kg backpack. However, the positive changes in inspiratory pressure 
were not reflected in the perception of whole-body exertion and respiratory effort or in 
cardiovascular responses. Various groups of physically demanding occupations (e.g. 
military, firefighters, rescue workers, etc.), as well as people engaged in recreational 
activities that involve carrying heavy backpacks (hikers, alpinists, skiers, etc.), could 
benefit from MIT.
Acknowledgments
We would like to thank the Ministry of Defence of the Republic of Slovenia and 
the Slovenian Army for their support and logistical assistance in our study. Our special 
thanks go to the former Major General of the Slovenian Army Dr Andrej Osterman. We 
would also like to thank the management of Vojašnica Edvarda Peperka, Moste-Polje 
Ljubljana, for allowing us to use their sports hall for tests and measurements. Above 
all, we would like to thank all the test subjects who participated in the study, i.e. the 
members of the Special Operations Unit of the Slovenian army.
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