ORIGINAL ARTICLE

Respiratory muscle endurance training: effect on normoxicand hypoxic exercise performance

Michail E. Keramidas • Tadej Debevec • Mojca Amon •

Stylianos N. Kounalakis • Bostjan Simunic •

Igor B. Mekjavic

Accepted: 27 October 2009

� Springer-Verlag 2009

Abstract The aim of this study was to investigate the

effect of respiratory muscle endurance training on endur-

ance exercise performance in normoxic and hypoxic con-

ditions. Eighteen healthy males were stratified for age and

aerobic capacity; and randomly assigned either to the

respiratory muscle endurance training (RMT = 9) or to the

control training group (CON = 9). Both groups trained on

a cycle-ergometer 1 h day-1, 5 days per week for a period

of 4 weeks at an intensity corresponding to 50% of peak

power output. Additionally, the RMT group performed a

30-min specific endurance training of respiratory muscles

(isocapnic hyperpnea) prior to the cycle ergometry. Pre,

Mid, Post and 10 days after the end of training period,

subjects conducted pulmonary function tests (PFTs),

maximal aerobic tests in normoxia ( _VO2maxNOR), and in

hypoxia ( _VO2maxHYPO; FIO2 = 0.12); and constant-load

tests at 80% of _VO2maxNOR in normoxia (CLTNOR), and in

hypoxia (CLTHYPO). Both groups enhanced _VO2maxNOR

(CON: ?13.5%; RMT: ?13.4%), but only the RMT group

improved _VO2maxHYPO Post training (CON: -6.5%;

RMT: ?14.2%). Post training, the CON group increased

peak power output, whereas the RMT group had higher

values of maximum ventilation. Both groups increased

CLTNOR duration (CON: ?79.9%; RMT: ?116.6%), but

only the RMT group maintained a significantly higher

CLTNOR 10 days after training (CON: ?56.7%; RMT:

?91.3%). CLTHYPO remained unchanged in both groups.

Therefore, the respiratory muscle endurance training

combined with cycle ergometer training enhanced aerobic

capacity in hypoxia above the control values, but did not in

normoxia. Moreover, no additional effect was obtained

during constant-load exercise.

Keywords Pulmonary ventilation � Dyspnea �Isocapnic hyperpnoea � Detraining

Introduction

It is well established that ventilatory limitations cause a

reduction in maximal exercise performance (Boutellier

et al. 1992; Harms et al. 2000). Accordingly, numerous

investigations have tested the ability of specific voluntary

isocapnic hyperpnoea–respiratory muscle endurance train-

ing protocols in improving respiratory muscle endurance,

and consequently exercise performance (Sonetti et al.

2001; Spengler et al. 1999; Stuessi et al. 2001; Wylegala

et al. 2007). Findings regarding the effects of respiratory

muscle endurance training on exercise performance remain

equivocal. Some studies have observed enhanced exercise

performance (Holm et al. 2004; Leddy et al. 2007; Stuessi

et al. 2001; Wylegala et al. 2007), while others have not

(Sonetti et al. 2001; Verges et al. 2007). These contentious

results may be due, in part, to the fact that very few studies

Communicated by Susan Ward.

M. E. Keramidas (&) � T. Debevec � M. Amon �S. N. Kounalakis � I. B. Mekjavic

Department of Automation, Biocybernetics and Robotics,

Jozef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia

e-mail: mkeram@phed.uoa.gr

M. E. Keramidas � T. Debevec � M. Amon

Jozef Stefan International Postgraduate School,

Ljubljana, Slovenia

M. Amon

Biomed d.o.o, Ljubljana, Slovenia

B. Simunic

Science Research Center, University of Primorska,

Koper, Slovenia

123

Eur J Appl Physiol

DOI 10.1007/s00421-009-1286-0

have used an appropriate control or placebo group (Sheel

2002). In particular, they have either omitted a control

group (Boutellier et al. 1992; Boutellier and Piwko 1992;

Spengler et al. 1999), used a control group that conducted

sham training (Holm et al. 2004; Leddy et al. 2007; Sonetti

et al. 2001; Verges et al. 2007; Wylegala et al. 2007), or a

control group that performed no intervention (Holm et al.

2004; Stuessi et al. 2001).

Although the physiological mechanisms underlying the

purported improvements in exercise performance after

respiratory muscle endurance training remain unresolved

(Sheel 2002), factors that have been implicated include

improved ratings of breathing perception (Holm et al.

2004), improved ventilatory efficiency (Boutellier et al.

1992; Boutellier and Piwko 1992; Wylegala et al. 2007),

delay of respiratory muscle fatigue (Verges et al. 2008,

2007) and a change in the distribution of the blood flow to

respiratory and locomotor muscles (Dempsey et al. 2006;

St Croix et al. 2000).

Since hypoxic exercise enhances the ventilatory

response (Babcock et al. 1995; Cibella et al. 1996; Vogi-

atzis et al. 2007) and the associated metabolic demand of

the respiratory muscles (Roussos 1985), we reasoned that

respiratory muscle training might contribute to the

enhancement of exercise performance at altitude. This is

supported, to a degree, by the findings of Downey et al.

(2007), who reported that inspiratory muscle training

improves structural (increased maximal inspiratory mouth

pressure and diaphragm thickness) and functional (reduced

oxygen uptake, cardiac output, ventilation; and increased

arterial oxygen saturation) characteristics of pulmonary

function, but does not affect the time to exhaustion during

hypoxic exercise. However, the effect of voluntary iso-

capnic hyperpnoea mode of respiratory muscle training on

altitude exercise remains unresolved, although extrapola-

tion of the above findings would suggest that it might

enhance aerobic hypoxic performance via the mechanism

of increased breathing endurance.

Therefore, the purpose of this study was to evaluate

whether respiratory muscle endurance training combined

with a submaximal-exercise training regimen could improve

normoxic and hypoxic endurance performance. The meth-

odological novelty of the present study is the combined

respiratory and exercise training in one training session. It

was designed to simulate the manner in which athletes would

incorporate respiratory training within their overall training

programme. In addition, the respiratory training preceded the

exercise training, as this would logistically be the optimal

manner in which it could be supervised in the field.

Methods

Subjects

Eighteen healthy males participated in this study. All

subjects were free of heart and lung disease and had normal

resting pulmonary function, as assessed by standard pul-

monary function tests (PFTs). The subjects were informed

about the nature and risks involved and gave their written

consent. All were recreationally active (no more than 3–5 h

of exercise per week) and were instructed to adhere to their

usual diet and physical activity; and not to engage in any

strenuous activity and not to drink coffee or other caffei-

nated beverages on testing days. The experimental protocol

was approved by the National Committee for Medical

Ethics at the Ministry of Health of Republic of Slovenia

and conformed to the Declaration of Helsinki.

Experimental design

During the Pre, Mid, Post and 10 days after (After) testing

periods (Fig. 1), all subjects performed: (a) hematological

tests, (b) PFTs, (c) an incremental exercise test to exhaus-

tion under normoxic conditions ( _VO2maxNOR), (d) an

incremental exercise test to exhaustion under hypoxic

TRAINING PERIOD

(11th-20th session)

AFTER-TRAINING

POST-TRAINING

TRAINING PERIOD

(1st-10th session)

OFF TRAINING

(10 days)

PRE-TRAININGTESTS

MID-TRAININGTESTS

1st Day. Hematological Tests

2nd Day. % BF, PFTs, •

V O2maxNOR, CLTHYPO

3rd Day.•

V O2maxHYPO, CLTNOR

% BF was measured only pre and post training The CLTNOR and CLTHYPO were performed in counterbalanced order

2 31 2 31 2 3 321 1

Fig. 1 Overview of the experimental protocol

Eur J Appl Physiol

123

conditions ( _VO2maxHYPO), (e) a constant-load test in nor-

moxia (CLTNOR), and (f) a constant-load test in hypoxia

(CLTHYPO). The performance tests outlined above (c–f)

were conducted on 2 days, separated by a rest day. On each

test day, subjects performed a maximal aerobic capacity test

( _VO2max) in the morning and an endurance test (CLT) in the

afternoon. All the tests were conducted at the same time of

the day for each subject to avoid diurnal variations. More-

over, during the exercise cessation period, subjects

refrained from any physical training, but otherwise fol-

lowed their normal daily routines (individual physical

activity diary was recorded).

Blood sampling and analysis

Overnight-fasted subjects reported to the hematological lab-

oratory (Adria Laboratories, Ljubljana, Slovenia) on the first

day of each testing period. Blood samples were drawn from the

antecubital vein and analyzed for red blood cell count (RBCs),

hemoglobin (Hb), and hematocrit (Ht) with a cytochemical

impedance method (ABX Pentra 120, Horiba, Japan) (coef-

ficient of variation: \2, \1, and \2%, respectively). The

apparatus was calibrated before each measurement.

Pulmonary function

PFTs were conducted on the rest day in each of the per-

formance test periods (Pre, Mid, Post, After). PFTs were

conducted using a Cardiovit AT-2plus (Schiller, Baar,

Switzerland) spirometer, according to the criteria published

by Miller et al. (2005). The spirometer was calibrated

before every test with a 3-L syringe. Each subject per-

formed each test three times and the highest of the three

acceptable recorded values was used for subsequent anal-

ysis. The PFTs were used to obtain measures of forced vital

capacity (FVC), forced expiratory volume in 1 s (FEV1),

peak expiratory flow (PEF), slow vital capacity (SVC) and

maximum voluntary ventilation (MVV).

_VO2max Testing

Subjects performed two incremental exercise tests to

exhaustion on an electrically braked cycle-ergometer (ERG

900S, Schiller, Baar, Switzerland): on one occasion

inspiring normal room air (FIO2 = 0.21; _VO2maxNOR) and

on the other, a hypoxic gas mixture (FIO2 = 0.12;_VO2maxHYPO). Each test consisted of a 10-min rest period,

followed by a 2-min warm-up on a cycle-ergometer at

work rate of 60 W. In the hypoxic test, the 10-min rest

period comprised a 5-min normoxic period, followed by a

5-min hypoxic rest period. Upon completion of the warm-

up, the load was increased each minute by 30 W until the

subject was unable to maintain the pedaling frequency of

60 rpm. Attainment of _VO2max, defined as the highest _VO2

averaged over 60 s, was confirmed according to the fol-

lowing classical criteria: (a) a respiratory exchange ratio

(RER) [ 1.10, (b) cycling cadence decreased below the

60 rpm, and/or (c) a plateau in oxygen uptake.

During the exercise tests, oxygen uptake ( _VO2), venti-

lation ( _VE) and carbon dioxide production ( _VCO2) were

measured on-line with a metabolic cart (CS-200, Schiller,

Baar, Switzerland), while subjects breathed through a low

resistance two way-respiratory valve (Model 2, 700 T-

Shape, Hans Rudolph, Inc., Shawnee, USA). During the

hypoxic exercise tests, the inspiratory side of the respira-

tory valve was connected via respiratory corrugated tubing

to a 200-L Douglas bag filled with the pre-mixed humidi-

fied hypoxic test breathing mixture (12% O2, 88% N2). The

gas analyzers and pneumotachograph were calibrated

before each test with two different gas mixtures and a 3-L

syringe, respectively. Data were averaged each minute.

Peak power output (PPO) was calculated by the equation:

PPO = POFINAL ? (t/60 9 30 W), where POFINAL refers

to the last workload completed, and t is the number of

seconds.

During the test, subjects were requested to provide rat-

ings of perceived exertion (RPE; scale 0–10) dyspnea–

respiratory discomfort (D-RPE; Wilson and Jones 1991),

and leg effort (L-RPE) at min intervals. Heart rate (HR)

was measured using a HR monitor (Vantage NVTM, Polar

Electro, Kempele, Finland). The arterial oxygen saturation

(SpO2) was monitored with a finger pulse oxymeter

(Nellcor, BCI 3301, Boulder, USA), with an accuracy of

±2 units across the range of 70–100% and an acceptable

resilience to motion artifact (Langton and Hanning 1990).

Constant-load performance test

Results from the Pre _VO2maxNOR test were used to cal-

culate the workload corresponding to 80% of _VO2maxNOR

for each subject. Two constant-load tests were then per-

formed in a random and counterbalanced order either under

normoxic (FIO2 = 0.21; CLTNOR) or hypoxic conditions

(FIO2 = 0.12; CLTHYPO). Subjects completed a 2-min

warm-up on a cycle ergometer at a work rate of 60 W.

Thereafter, they cycled at 80% _VO2maxNOR until exhaus-

tion. Time to fatigue was recorded as the number of sec-

onds each subject maintained a cadence C60 rpm. During

the tests, _VO2, _VE, _VCO2, D-RPE, L-RPE and SpO2 values

were recorded, as they were described previously.

Training

After completing all baseline testing, participants were strat-

ified for age and aerobic capacity, and randomly assigned to

either the respiratory muscle endurance training (RMT = 9;

Eur J Appl Physiol

123

age: 22.1 ± 4.0 years; stature: 179.6 ± 5.2 cm; body mass:

72.8 ± 9.9 kg; BMI: 22.7 ± 2.2 kg cm2; _VO2max: 47.0 ±

5.0 mL kg-1 min-1) or the control training (CON = 9;

age: 22.7 ± 4.1 years; stature: 178.7 ± 4.6; body mass:

72.9 ± 9.7 kg; BMI: 22.5 ± 2.4 kg cm2; _VO2max: 45.8 ±

6.2 mL kg-1 min-1) group (P [ 0.05).

Both groups trained 5 days per week for 4 weeks and

were supervised by the same investigators during the entire

training period. Subjects exercised 1-h on a cycle ergom-

eter, but the RMT group performed an additional endur-

ance training of the respiratory muscles (described below)

prior to the cycle ergometry. The environmental conditions

were normoxic and kept constant and thermoneutral during

the entire training procedure.

Respiratory muscle training

The RMT group used a specific training device (Spiroti-

ger�, Idiag, Fehraltorf, Switzerland), which consisted of a

hand-held unit with a pouch and a base station. A two-way

piston valve connected to a rebreathing bag permitted the

inhalation of fresh inspired air into the rebreathing bag in

order to maintain a constant isocapnic end-tidal CO2

fraction (Sartori et al. 2008). Personal target values were

entered into the base unit and were used to monitor the

breathing frequency (fR), and tidal volume during the

training. The base station in the hand-held computer

monitored the fR, set threshold limits for breathing patterns,

and displayed visual and acoustic feedback to allow the

subject to breathe within the threshold values for isocapnia.

The respiratory muscle training protocol was based on

Wylegala et al. (2007). The duration of the respiratory muscle

training was 30 min. The volume of the bag (VBAG) was

initially set at a value representing approximately 55% of the

subject’s SVC. The fR was then determined by dividing 50%

of MVV by the bag volume such that fR = MVV (0.50)/

VBAG. In each session, the subjects were instructed to increase

the fR by 1–2 breaths per min after 20 min of training, if they

felt it would not exhaust them by 30 min. They then contin-

ued at this higher frequency for the last 10 min of the training.

The next training session began at the highest frequency

achieved in the previous session, and this was then main-

tained for 20 min followed by a further increase of 1–2

breaths min-1 for the remaining 10 min. In the event that

subjects perceived the training to be extremely difficult, the

setting of VBAG and fR, were adjusted accordingly.

Exercise training

Both CON and RMT groups conducted identical 1-h training

sessions on a cycle ergometer at a work rate equivalent to

50% of their normoxic PPO. For each subject, the control of

exercise training was carried out through HR (Hosand

TMpro�, Verbania, Italy), and rating of perceived exertion

(Borg’s scale; scale 0–10). Training progression was imple-

mented regularly by increasing a subject’s workload, such

that the exercise HR was maintained at a level corresponding

to that observed at 50% of normoxic PPO. All subjects were

instructed to pedal at a cadence of 70 rpm. Each training

session commenced with a standardized 5-min warm-up

period at a work rate equivalent to 20% of normoxic PPO.

Statistical analysis

Statistical analyses were performed using Statistica 5.0

(StatSoft, Tulsa, USA). All data are reported as mean (SD)

unless otherwise indicated. Due to a technical problem, the

Post CPTNOR performance time of one subject in the CON

group was excluded from the analysis (CON = 8 for Post

CPTNOR). Statistical significance of maximum values was

assessed with a three-way analysis of variance (ANOVA) for

repeated measures (group 9 condition 9 testing session). A

four-way ANOVA for repeated measures was used for relative

submaximal values of both incremental exercise and constant-

power tests (group 9 condition 9 testing session 9 relative

intensity). A t-test analysis was used for the training variables.

The Tukey post hoc test was employed to identify specific

differences between means when main effects were observed.

The alpha level of significance was set a priori at 0.05.

Results

Despite an identical exercise training protocol, as reflected

in the mean values of HR, PO and the mean increment in

PO for each training week (Table 1), the RMT group

perceived the submaximal exercise training harder than the

CON group as indicated by the significantly (P B 0.05)

higher values of D-RPE and L-RPE (Table 1).

Moreover, the participants did not alter their body mass

during the testing periods (CON: Pre = 72.8 ± 9.5 kg,

Mid = 74.6 ± 9.9 kg, Post = 73.6 ± 8.9 kg, After =

74.3 ± 9.8 kg; RMT: Pre = 72.8 ± 9.9 kg, Mid = 73.4 ±

9.9 kg, Post = 72.8 ± 9.1 kg, After = 72.2 ± 9.3 kg). In

addition, the body fat did not change Post training in both

groups (CON: Pre = 10.4 ± 3.0%, Post = 10.5 ± 3.8%;

RMT: Pre = 10.2 ± 3.0, Post = 9.3 ± 2.7%).

Pulmonary function

With the exception of the significant increase (P B 0.05) in

PEF Mid and After training in the CON group, no other

significant changes were noted in any of the pulmonary

function variables measured in either group (Table 2). PEF

in the CON group was also significantly lower (P B 0.05)

Pre training compared to the RMT group (Table 2).

Eur J Appl Physiol

123

Hematology

Hct (CON: Pre = 44.9 ± 3.1%, Post = 44.1 ± 3.3%,;

RMT: Pre = 46.9 ± 1.6%, Post = 45.9 ± 1.5), Hb (CON:

Pre = 15.0 ± 1.0 g L-1, Post = 14.2 ± 1.2 g L-1; RMT:

Pre = 15.2 ± 0.4 g L-1, Post = 14.6 ± 0.9 g L-1) and

RBCs (CON: Pre = 5.0 ± 0.3 9 1012 L-1, Post = 4.8 ±

0.2 9 1012 L-1; RMT: Pre = 5.2 ± 0.2 9 1012 L-1,

Post = 5.1 ± 0.1 9 1012 L-1) did not change throughout

the training intervention.

Exercise performance

Maximum and peak values during the maximal incremental

exercise tests are summarized in Table 3. Both groups

enhanced their _VO2maxNOR, but only the RMT group

exhibited an improvement also in _VO2maxHYPO. The later

improvement in the RMT group was observed only Post

training, without any changes Mid and After training. The

CON group had significantly lower _VO2maxHYPO during

Mid, Post and After training compared to the RMT group.

The CON group had a higher normoxic PPO.

During the _VO2maxNOR test, the RMT group exhibited a

significantly (P B 0.05) higher _VEmax (Table 3) during the

Mid and Post compared to Pre testing period. During the_VO2maxHYPO test, the RMT group had also significantly

(P B 0.05) higher _VEmax (Table 3) compared to the CON

group during the Mid, Post and After testing periods,

respectively. The RMT group also had significantly

(P B 0.05) higher _VEmax during the Post training hypoxic

tests compared to the Pre training values (Table 3).

The RMT group had a lower HRmax (approximately 7

beats min-1) in _VO2maxNOR during Post and After tests,

whereas the CON group did not (Table 3). Furthermore,

Table 1 Training characteristics of control (CON) and respiratory muscle training (RMT) groups

Variable CON group (n = 9) RMT group (n = 9)

Respiratory muscle training

_VEMEAN (L min-1) – 111.4 ± 30.4

HRMEAN (beats min-1) – 103.9 ± 18.8

Mean increment of _VE Week-1 (%) – 30.4 ± 19.0

Exercise training

HRMEAN (beats min-1) 142.7 ± 5.2 141.8 ± 9.9

POMEAN (Watts) 159.4 ± 17.3 167.1 ± 18.9

D-RPEMEAN (1–10) 1.9 ± 0.7 2.7 ± 0.9�

L-RPEMEAN (1–10) 2.3 ± 0.8 3.9 ± 0.7�

Mean increment of W Week-1 (%) 5.6 ± 4.0 5.9 ± 4.5

Values are mean ± SD

_VEMEAN mean minute ventilation, HRMEAN mean heart rate, POMEAN mean power output, D-RPEMEAN mean dyspnea, L-RPEMEAN mean leg

sensation, _VEMEAN mean values of overall respiratory muscle training, HRMEAN, POMEAN, D-RPEMEAN and L-RPEMEAN mean values from 5th to

60th min of exercise training� Statistically significant difference between CON and RMT groups (P B 0.05)

Table 2 Values for pulmonary function Pre, Mid, Post and After the training regimen for control (CON) and respiratory muscle training (RMT)

groups

Variable CON group RMT group

Pre Mid Post After Pre Mid Post After

FVC (L) 5.6 ± 0.9 5.5 ± 1.2 5.6 ± 1.1 5.6 ± 1.4 5.6 ± 0.3 5.6 ± 0.6 5.7 ± 0.3 5.7 ± 0.3

FEV1 (L) 4.5 ± 0.8 4.5 ± 0.9 4.6 ± 0.9 4.6 ± 1.1 4.7 ± 0.4 4.6 ± 0.5 4.6 ± 0.5 4.7 ± 0.4

PEF (L s-1) 8.5 ± 1.7 10.0 ± 1.6* 9.9 ± 1.3* 9.8 ± 1.4 10.4 ± 1.2� 10.2 ± 1.6 10.7 ± 1.3 10.8 ± 1.2

SVC (L) 5.1 ± 0.8 5.0 ± 0.8 5.6 ± 1.3 5.5 ± 1.5 5.5 ± 0.3 6.0 ± 1.7 5.6 ± 0.4 5.6 ± 0.4

MVV (L min-1) 175.9 ± 26.8 179.2 ± 30.0 181.6 ± 29.7 198.2 ± 42.0 195.9 ± 36.1 200.0 ± 42.2 200.5 ± 31.8 210.2 ± 32.2

Values are mean ± SD

FVC forced vital capacity, FEV1 forced expiratory volume in 1 s, PEF peak expiratory flow, SVC slow vital capacity, MVV maximum voluntary

ventilation

* Statistically significant difference from Pre values� Statistically significant difference between CON and RMT groups (P B 0.05)

Eur J Appl Physiol

123

Ta

ble

3P

eak

val

ues

for

card

iore

spir

ato

ryv

aria

ble

sd

uri

ng

incr

emen

tal

exer

cise

test

inn

orm

ox

ia(

_ VO

2m

axN

OR

)an

dh

yp

ox

ia(

_ VO

2m

axH

YP

O)

Pre

,M

id,

Po

stan

dA

fter

the

trai

nin

gre

gim

enfo

r

con

tro

l(C

ON

)an

dre

spir

ato

rym

usc

letr

ain

ing

(RM

T)

gro

up

s

Var

iab

leC

ON

gro

up

RM

Tg

rou

p

PR

EM

IDP

OS

TA

FT

ER

PR

EM

IDP

OS

TA

FT

ER

_ VO

2m

ax

NO

Rte

st

_ VO

2m

ax

(mL

kg

-1

min

-1)

45

.8±

6.2

52

.0±

5.4

*5

2.0

±5

.1*

52

.9±

4.7

*4

7.0

±5

.05

3.2

±6

.2*

53

.3±

6.2

*5

2.9

±9

.3*

_ VO

2m

ax

(Lm

in-

1)

3.3

±0

.53

.8±

0.5

*3

.9±

0.6

*3

.9±

0.4

*3

.5±

0.5

3.9

±0

.5*

3.8

±0

.5*

3.8

±0

.6*

_ VE

max

(Lm

in-

1)

11

5.9

±1

8.8

12

3.7

±2

1.7

12

3.1

±1

6.4

12

7.4

±1

6.1

11

7.9

±1

3.7

12

7.8

±2

2.4

*1

31

.5±

22

.1*

12

7.4

±2

3.1

PP

O(W

atts

)2

97

.9±

34

.13

18

.3±

37

.9*

32

3.5

±3

3.7

*3

27

.4±

29

.0*

30

5.3

±3

7.4

31

5.7

±4

3.0

32

1.2

±3

6.3

32

0.1

±4

2.5

HR

max

(bea

tsm

in-

1)

18

9.3

±7

.31

86

.9±

4.4

18

6.2

±5

.91

88

.7±

6.4

19

0.1

±7

.81

85

.3±

11

.51

84

.1±

12

.9*

18

3.4

±1

2.4

*

D-R

PE

peak

(1–

10

)6

.2±

2.6

7.0

±2

.07

.5±

2.0

7.5

±2

.38

.7±

1.6

7.8

±1

.57

.3±

1.5

8.8

±1

.2

L-R

PE

peak

(1–

10

)8

.0±

1.5

8.7

±1

.39

.2±

0.8

9.0

±1

.09

.3±

1.0

9.2

±1

.19

.4±

0.7

9.4

±0

.7

Sp

O2m

ax

(%)

89

.9±

8.2

95

.2±

2.0

93

.9±

5.2

94

.8±

1.8

89

.7±

3.5

90

.8±

4.9

93

.9±

2.3

92

.0±

2.7

_ VO

2m

ax

HY

PO

test

_ VO

2m

ax

(mL

kg

-1

min

-1)

38

.6±

6.7

35

.4±

3.6

36

.1±

3.9

34

.7±

3.6

36

.8±

4.9

41

.3±

4.5

�4

2.1

±5

.1*

�4

1.0

±6

.4�

_ VO

2m

ax

(Lm

in-

1)

2.9

±0

.62

.6±

0.4

2.7

±0

.42

.6±

0.3

2.7

±0

.43

.0±

0.3

3.0

±0

.3�

2.9

±0

.4

_ VE

max

(Lm

in-

1)

11

1.7

±1

4.9

10

4.8

±1

6.6

11

0.3

±1

5.1

11

4.5

±1

4.5

11

6.8

±1

4.6

11

6.2

±2

1.9

�1

27

.6±

22

.2*�

12

6.0

±2

2.9

�

PP

O(W

atts

)2

48

.9±

18

.52

48

.0±

21

.82

45

.1±

33

.12

49

.6±

24

.82

72

.0±

30

.3�

26

5.6

±3

4.3

26

6.0

±2

7.5

�2

62

.8±

28

.8

HR

max

(bea

tsm

in-

1)

18

1.0

±9

.41

78

.0±

6.7

17

4.8

±6

.0*

17

5.1

±6

.1*

18

0.0

±9

.31

74

.0±

9.2

*1

75

.9±

9.2

17

3.2

±1

1.0

*

D-R

PE

peak

(1–

10

)7

.1±

2.1

7.7

±1

.57

.5±

2.2

7.9

±2

.98

.2±

1.3

7.5

±1

.77

.7±

1.5

7.4

±2

.0

L-R

PE

peak

(1–

10

)7

.3±

2.8

7.4

±2

.47

.9±

1.4

8.5

±1

.09

.3±

0.7

9.4

±0

.59

.1±

0.9

9.4

±0

.5

Sp

O2m

ax

(%)

75

.8±

6.0

72

.1±

3.1

72

.4±

4.1

73

.3±

4.5

72

.7±

3.7

74

.0±

3.6

74

.1±

2.4

75

.4±

3.4

Val

ues

are

mea

n±

SD

*S

tati

stic

ally

sig

nifi

can

td

iffe

ren

cefr

om

Pre

val

ues

�S

tati

stic

ally

sig

nifi

can

td

iffe

ren

ceb

etw

een

CO

Nan

dR

MT

gro

up

s(P

B0

.05

)

Eur J Appl Physiol

123

both groups decreased HRmax in _VO2maxHYPO, but the

decrement was observed in different testing periods: the

CON group had a lower HRmax Post and After training,

whereas the RMT group had a significantly lower HRmax

Mid and After training. There was a main effect of group

on L-RPEpeak, since the CON had lower values than the

RMT group, but there were no post hoc differences

between them over the testing periods. There were no

differences in D-RPEpeak and SpO2max in both groups

(Table 3).

The mean PO during the CLT was 238.6 ± 28.7 W and

249.7 ± 26.7 W for the CON and RMT group, respec-

tively. CON and RMT group did not differ in any of the

performance tests during the initial testing period (Fig. 2).

Both groups exhibited no significant change in the recorded

hypoxic performance time after the training intervention

(Fig. 2b). Nevertheless, all participants increased the time

to exhaustion in normoxia, though there were no

differences between the groups (Fig. 2a). The RMT group

maintained the improvements observed in normoxia after

training, however (Fig. 2a). There was a main effect of

testing session on HRmax during CLTNOR and CLTHYPO,

since the values decreased over the testing periods. In

particular, the CON group had lower HRmax Post training

compared to Pre training tests (Table 4). There were no

differences in SpO2max for both groups (Table 4).

Moreover, in CLTNOR the RMT group had higher values

of _VE at 60, 80 and 100% of endurance time compared to

the CON group Pre and Post training (Fig. 3a). In addition,

the RMT group had higher _VE at 100% of endurance time

Pre and After training. In CLTHYPO, the RMT group

increased _VE at 20, 80 and 100% of endurance time

compared to CON group Pre and Post training (Fig. 3b).

Both groups increased _VE during CLTNOR Post training,

but at different endurance time points (Fig. 3a). The CON

group increased _VE at 20, 80, and 100% of endurance time,

Fig. 2 Mean (±SD) time to

exhaustion for the constant-load

test (a) in normoxia and (b) in

hypoxia. CON group = 8

during post-training test;

*statistically significant

difference from Pre values;§statistically significant

difference between Mid and

Post test; �statistically

significant difference between

Post and After test; (P B 0.05)

Eur J Appl Physiol

123

while the RMT group increased it at 20, 40, and 60%

(Fig. 3a). During CLTHYPO only the CON group increased_VE at 20 and 100% of endurance time (Fig. 3b).

Discussion

The principal finding of the present study is that respira-

tory muscle endurance training improves _VO2maxHYPO

without a concomitant enhancement of constant-load

exercise performance either in hypoxia or normoxia. An

interesting observation was also that subjects perceived the

exertion of exercise training conducted immediately after

respiratory muscle training more strenuous, than did the

control group.

Aerobic capacity in normoxia and hypoxia

Both the RMT and CON group equally improved_VO2maxNOR Mid, Post and After training, reflecting a

similar training-induced improvement. Previous studies

have examined the influence of respiratory muscle endur-

ance training on _VO2max and found no significant

improvement (Fairbarn et al. 1991; Holm et al. 2004;

Morgan et al. 1987) supporting the notion that _VO2max is

not limited by ventilation (Reybrouck et al. 1975). How-

ever, some researchers have suggested that the pulmonary

system may indeed limit _VO2max under certain circum-

stances (e.g. hypoxia, asthma, COPD), when a reduction in

arterial PO2 prevails (Bauerle and Younes 1995; Calbet

et al. 2003; Kinnula and Sovijarvi 1996). Exercise-induced

arterial hypoxemia enhances peripheral-locomotor muscle

fatigue (Romer et al. 2006), and may also contribute to the

fatigue of respiratory muscles, and therefore cause a ven-

tilation-induced limitation of _VO2max. In the present study,

a moderate degree of exercise-induced arterial hypoxemia

(Dempsey and Wagner 1999) was observed during_VO2maxNOR (Table 3), which was unaffected by the

improved exercise ventilation in the Post and After training

tests.

The enhanced _VO2maxHYPO in the RMT group was

accompanied by increased _VEmax at identical power output,

most likely resulting in an increased metabolic demand of

the respiratory muscles (Babcock et al. 1995; Roussos

1985; Vogiatzis et al. 2007). Downey et al. (2007) main-

tained that inspiratory muscle training significantly

decreased submaximal _VE and conjectured, that it is

probably the result of increased SpO2, indicating a reduced

input to the peripheral chemoreceptors. However, in the

present study, _VEmax is higher after training, and despite its

higher values the SpO2max was unchanged during the

incremental exercise to exhaustion in hypoxia.

It has been suggested that respiratory muscle training

reduces the perception of adverse respiratory sensations

despite the increased exercise ventilation (Holm et al.

2004; Verges et al. 2008). In the present study, the higher

values of _VE and the improved _VO2max were not related

with lower values of D-RPEpeak in both conditions. Thus,

the hypothesis that changes in perception of breathing, such

as decreased sense of respiratory exertion or breathless-

ness, might contribute, as secondary factor, to increased

performance cannot be confirmed by the present results.

Table 4 Peak values for _VO2, HR and SaO2 during constant load test in normoxia (CLTNOR) and hypoxia (CLTHYPO) Pre, Mid, Post and After

the training regimen for control (CON) and respiratory muscle training (RMT) groups

Variable CON group RMT Group

Pre Mid Post After Pre Mid Post After P

CLTNOR

_VO2peak

(mL kg-1 min-1)

45.3 ± 6.3 48.4 ± 5.6 43.8 ± 8.7 46.1 ± 5.7 45.1 ± 4.3 52.9 ± 5.2 50.6 ± 5.2 49.6 ± 4.6 n.s.

HRpeak

(beats min-1)

190.6 ± 6.1 187.6 ± 8.4 182.4 ± 5.3* 185.8 ± 7.0 181.6 ± 8.3 178.0 ± 5.5 175.2 ± 8.1 176.7 ± 7.0 0.00�

SpO2peak (%) 92.7 ± 2.4 93.9 ± 1.5 94.3 ± 0.7 93.9 ± 0.9 92.9 ± 2.8 92.7 ± 3.6 92.8 ± 2.4 92.8 ± 1.7 n.s.

CLTHYPO

_VO2peak

(mL kg-1 min-1)

44.2 ± 10.8 39.1 ± 3.6 36.5 ± 6.2 36.8 ± 4.7 42.2 ± 5.9 43.8 ± 6.0 45.4 ± 6.1 43.1 ± 6.4 n.s.

HRpeak

(beats min-1)

186.6 ± 10.7 185.4 ± 11.6 184.8 ± 8.1 182.7 ± 7.2 179.6 ± 11.7 174.6 ± 11.9 174.8 ± 12.5 174.8 ± 11.1 0.00�

SpO2peak (%) 72.1 ± 5.7 72.4 ± 3.5 73.3 ± 5.8 72.4 ± 4.1 76.3 ± 6.7 74.1 ± 4.0 74.8 ± 5.2 75.2 ± 4.1 n.s.

Values are mean ± SD

* Statistically significant difference from Pre values� Statistically significant main effect of testing session (P B 0.05)

Eur J Appl Physiol

123

Exercise performance in normoxia and hypoxia

The physiological mechanisms described above most likely

contribute to the observed improvements in CLTNOR at

80% of _VO2max (Fig. 2a). In both groups endurance per-

formance was primarily enhanced by the increased PPO

and peripheral neuromuscular adaptations, and additionally

by the contribution of the endurance respiratory muscle

training in the RMT group. It has been suggested that the

benefits of respiratory muscle endurance training are lim-

ited to submaximal exercise levels 65–85% of _VO2max

(Morgan et al. 1987; Spengler et al. 1999), since

improvements were less evident or even absent when

endurance was tested at higher exercise intensities (Morgan

et al. 1987). It is not yet clear, why respiratory muscle

endurance training is most effective at intensities lower

than 85% of _VO2max, especially since diaphragmatic fati-

gue occurs above this intensity, and its onset is delayed by

RMT. Spengler and Boutellier (2000) suggested that RMT

might mainly train the extradiaphragmatic respiratory

muscles, whose activity predominates at lower exercise

intensities.

It is also noteworthy that the RMT group maintained

their training adaptation 10 days after the end of training

regimen, whereas in the CON group aerobic capacity and

endurance performance decreased below the Post training

levels during this same time period in accordance with the

findings of Mujika and Padilla (2001). They report that

10 days of cessation of training was sufficient to reduce

endurance performance by affecting both cardiorespiratory

and metabolic/muscular function. The results in the RMT

group in the present study support the findings of Cullinane

et al. (1986), who reported that the reduction in respiratory

function was more pronounced after 10 days of insufficient

training stimulus. Our findings suggest there may be an

important role of the ventilation in performance preserva-

tion, which is accomplished by the respiratory muscle

training, after a short period of detraining and/or inactivity.

0

20

40

60

80

100

120

140

160

VE

NO

R (L

.min

-1)

CON Group

***

***

****

ARMT Group

**

**

†††**

†††

†††††††

0

20

40

60

80

100

120

140

160

REST 20 40 60 80 100

% ENDURANCE TIME

VE

HY

PO

(L.m

in-1

)

*

**

B

REST 20 40 60 80 100

% ENDURANCE TIME

PREMIDPOSTAFTER

†††

††††††

Fig. 3 Values (±SE) of minute ventilation ( _VE) during the constant-

load test (a) in normoxia (CLTNOR) and (b) in hypoxia (CLTHYPO) at

the same relative endurance time. Statistically significant difference

between *Pre and Mid test, **Pre and Post test, ***Pre and After

training test. Statistically significant difference between CON and

RMT groups ���in Post test; ����in After training test; (P B 0.05)

Eur J Appl Physiol

123

Despite the significantly increased _VO2maxHYPO, the

RMT group did not improve their hypoxic endurance

performance. This is most likely due to greater exercise

intensity performed in the hypoxic CLT test (CLTHYPO).

Although both CLT tests were performed at the same

absolute workload equivalent to 80% of normoxic Pre

training _VO2max, the relative intensity of the workload was

much higher in the CLTHYPO test (CON: 95.72 ± 7.34%;

RMT: 91.99 ± 5.86% of Pre training _VO2maxHYPO,

respectively). As noted previously, diaphragmatic fatigue

occurs at exercise intensities [ 85% of _VO2max and the

respiratory muscle endurance training is probably unable to

enhance performance in this exercise intensity range.

Training procedure

Both groups performed the same exercise training, as

confirmed by the values of HRMEAN and the weekly

increment of workload. However, the perceived dyspnea

and leg exertion differed significantly between groups

during the submaximal exercise training sessions. The

RMT group, which conducted a 30-min respiratory muscle

endurance training prior to the cycle ergometry, provided

significantly higher ratings of D-RPEMEAN and L-RPE-

MEAN. The higher perception of dyspnea probably reveals

slight training-induced fatigue of the respiratory muscles. It

is noteworthy that the respiratory muscle training induced a

greater perception of fatigue in the legs, which may be

attributed, in part, to the previously reported effect of

respiratory muscle fatigue on exercise performance (Mador

and Acevedo 1991; Martin et al. 1982). According to this

theory, the increased metabolic demand of the respiratory

muscles due to RMT (increased neural activity in type IV

afferents nerve fibers) may have enhanced sympathetic

outflow to the limb, thus affecting blood flow redistribution

(Sheel et al. 2001; St Croix et al. 2000). The ensuing

competition by the respiratory and locomotor muscles for

blood flow may have resulted in lower perfusion of the

locomotor muscles during the cycle ergometry following

RMT, than would have been experience by the CON group,

which did not conduct any prior respiratory muscle

training.

In conclusion, the present study shows that respiratory

muscle endurance training combined with cycle ergometer

training improved aerobic capacity in hypoxia above the

control training but did not in normoxia. Moreover, no

additional effect was obtained during constant-load exer-

cise. However, respiratory muscle training seems to delay

the negative effects of inactivity or reduced training load,

resulting in preservation of enhanced performance. More-

over, performing a RMT session immediately before

exercise training significantly increases the rate of per-

ceived exertion during the training session. Doing so may

limit the quality of the training session performed by an

athlete.

Acknowledgments The current project was funded, in part, by the

Olympic Committee of Slovenia, and by a ‘‘Knowledge for Security

and Peace’’ grant from the Ministry of Defense (Republic of Slove-

nia). Moreover, we would like to thank all the subjects for their time

and effort. Finally yet importantly, we would like to thank all the

personnel of Adria Lab for their technical support.

Conflict of interest statement The authors state that there is no

personal of financial conflict of interest in the present study.

References

Babcock MA, Johnson BD, Pegelow DF, Suman OE, Griffin D,

Dempsey JA (1995) Hypoxic effects on exercise-induced

diaphragmatic fatigue in normal healthy humans. J Appl Physiol

78:82–92

Bauerle O, Younes M (1995) Role of ventilatory response to exercise

in determining exercise capacity in COPD. J Appl Physiol

79:1870–1877

Boutellier U, Piwko P (1992) The respiratory system as an exercise

limiting factor in normal sedentary subjects. Eur J Appl Physiol

Occup Physiol 64:145–152

Boutellier U, Buchel R, Kundert A, Spengler C (1992) The

respiratory system as an exercise limiting factor in normal

trained subjects. Eur J Appl Physiol Occup Physiol 65:347–353

Calbet JA, Boushel R, Radegran G, Sondergaard H, Wagner PD,

Saltin B (2003) Determinants of maximal oxygen uptake in

severe acute hypoxia. Am J Physiol Regul Integr Comp Physiol

284:R291–R303

Cibella F, Cuttitta G, Kayser B, Narici M, Romano S, Saibene F

(1996) Respiratory mechanics during exhaustive submaximal

exercise at high altitude in healthy humans. J Physiol 494(Pt

3):881–890

Cullinane EM, Sady SP, Vadeboncoeur L, Burke M, Thompson PD

(1986) Cardiac size and VO2max do not decrease after short-

term exercise cessation. Med Sci Sports Exerc 18:420–424

Dempsey JA, Wagner PD (1999) Exercise-induced arterial hypox-

emia. J Appl Physiol 87:1997–2006

Dempsey JA, Romer L, Rodman J, Miller J, Smith C (2006)

Consequences of exercise-induced respiratory muscle work.

Respir Physiol Neurobiol 151:242–250

Downey AE, Chenoweth LM, Townsend DK, Ranum JD, Ferguson

CS, Harms CA (2007) Effects of inspiratory muscle training on

exercise responses in normoxia and hypoxia. Respir Physiol

Neurobiol 156:137–146

Fairbarn MS, Coutts KC, Pardy RL, McKenzie DC (1991) Improved

respiratory muscle endurance of highly trained cyclists and the

effects on maximal exercise performance. Int J Sports Med

12:66–70

Harms CA, Wetter TJ, St Croix CM, Pegelow DF, Dempsey JA

(2000) Effects of respiratory muscle work on exercise perfor-

mance. J Appl Physiol 89:131–138

Holm P, Sattler A, Fregosi RF (2004) Endurance training of

respiratory muscles improves cycling performance in fit young

cyclists. BMC Physiol 4:9

Kinnula VL, Sovijarvi AR (1996) Hyperventilation during exercise:

independence on exercise-induced bronchoconstriction in mild

asthma. Respir Med 90:145–151

Eur J Appl Physiol

123

Langton JA, Hanning CD (1990) Effect of motion artefact on pulse

oximeters: evaluation of four instruments and finger probes. Br J

Anaesth 65:564–570

Leddy JJ, Limprasertkul A, Patel S, Modlich F, Buyea C, Pendergast

DR, Lundgren CE (2007) Isocapnic hyperpnea training improves

performance in competitive male runners. Eur J Appl Physiol

99:665–676

Mador MJ, Acevedo FA (1991) Effect of respiratory muscle fatigue

on subsequent exercise performance. J Appl Physiol 70:2059–

2065

Martin B, Heintzelman M, Chen HI (1982) Exercise performance

after ventilatory work. J Appl Physiol 52:1581–1585

Miller MR, Hankinson J, Brusasco V, Burgos F, Casaburi R, Coates

A, Crapo R, Enright P, van der Grinten CP, Gustafsson P, Jensen

R, Johnson DC, MacIntyre N, McKay R, Navajas D, Pedersen

OF, Pellegrino R, Viegi G, Wanger J (2005) Standardisation of

spirometry. Eur Respir J 26:319–338

Morgan DW, Kohrt WM, Bates BJ, Skinner JS (1987) Effects of

respiratory muscle endurance training on ventilatory and endur-

ance performance of moderately trained cyclists. Int J Sports

Med 8:88–93

Mujika I, Padilla S (2001) Cardiorespiratory and metabolic charac-

teristics of detraining in humans. Med Sci Sports Exerc 33:413–

421

Reybrouck T, Heigenhauser GF, Faulkner JA (1975) Limitations to

maximum oxygen uptake in arms, leg, and combined arm-leg

ergometry. J Appl Physiol 38:774–779

Romer LM, Dempsey JA, Lovering A, Eldridge M (2006) Exercise-

induced arterial hypoxemia: consequences for locomotor muscle

fatigue. Adv Exp Med Biol 588:47–55

Roussos C (1985) Function and fatigue of respiratory muscles. Chest

88:124S–132S

Sartori R, Barbi E, Poli F, Ronfani L, Marchetti F, Amaddeo A,

Ventura A (2008) Respiratory training with a specific device in

cystic fibrosis: a prospective study. J Cyst Fibros 7:313–319

Sheel AW (2002) Respiratory muscle training in healthy individuals:

physiological rationale and implications for exercise perfor-

mance. Sports Med 32:567–581

Sheel AW, Derchak PA, Morgan BJ, Pegelow DF, Jacques AJ,

Dempsey JA (2001) Fatiguing inspiratory muscle work causes

reflex reduction in resting leg blood flow in humans. J Physiol

537:277–289

Sonetti DA, Wetter TJ, Pegelow DF, Dempsey JA (2001) Effects of

respiratory muscle training versus placebo on endurance exercise

performance. Respir Physiol 127:185–199

Spengler CM, Boutellier U (2000) Breathless legs? Consider training

your respiration. News Physiol Sci 15:101–105

Spengler CM, Roos M, Laube SM, Boutellier U (1999) Decreased

exercise blood lactate concentrations after respiratory endurance

training in humans. Eur J Appl Physiol Occup Physiol 79:299–

305

St Croix CM, Morgan BJ, Wetter TJ, Dempsey JA (2000) Fatiguing

inspiratory muscle work causes reflex sympathetic activation in

humans. J Physiol 529(Pt 2):493–504

Stuessi C, Spengler CM, Knopfli-Lenzin C, Markov G, Boutellier U

(2001) Respiratory muscle endurance training in humans

increases cycling endurance without affecting blood gas con-

centrations. Eur J Appl Physiol 84:582–586

Verges S, Lenherr O, Haner AC, Schulz C, Spengler CM (2007)

Increased fatigue resistance of respiratory muscles during

exercise after respiratory muscle endurance training. Am J

Physiol Regul Integr Comp Physiol 292:R1246–R1253

Verges S, Boutellier U, Spengler CM (2008) Effect of respiratory

muscle endurance training on respiratory sensations, respiratory

control and exercise performance: a 15-year experience. Respir

Physiol Neurobiol 161:16–22

Vogiatzis I, Georgiadou O, Koskolou M, Athanasopoulos D, Kostikas

K, Golemati S, Wagner H, Roussos C, Wagner PD, Zakynthinos

S (2007) Effects of hypoxia on diaphragmatic fatigue in highly

trained athletes. J Physiol 581:299–308

Wilson RC, Jones PW (1991) Long-term reproducibility of Borg scale

estimates of breathlessness during exercise. Clin Sci (Lond)

80:309–312

Wylegala JA, Pendergast DR, Gosselin LE, Warkander DE, Lundgren

CE (2007) Respiratory muscle training improves swimming

endurance in divers. Eur J Appl Physiol 99:393–404

Eur J Appl Physiol

123