The Swedish Journal of Scientific Research
Original Article
V̇O2 decrease Before Exhaustion During Constant
Load Exercise. Role of Respiratory Muscles
Abderraouf Ben Abderrahman1*, Lotfi Bouguerra1, Fatma Rhibi2, Armel Cretual2,
Mohamed Ansari4, Amel Chebbi3, Jacques Prioux2
1
Higher Institute of Sport and Physical Education of Ksar-Said, Tunisia, 2Movement, Sport, and Health Sciences Laboratory,
University of Rennes 2, Rennes, France, 3Faculty of Medicine of Tunis, Tunisia, 4Dubaï Sports Council, United Arab Emirates
ABSTRACT
VO
Objectives: The aim of our work was to analyse the VO
2
2 kinetic during a constant load exercise, to check the existence of a
decrease at the end of this kind of exercise and finally to study the respiratory muscles strength evolution, before and after this kind of
exercise. Patients and Methods: Eight endurance trained athletes (20.6 ± 2.7 yrs) performed three field-tests until exhaustion: firstly
a maximal graded test to determine their maximal oxygen uptake ( VO
2max ) and maximal aerobic velocity (MAV) and secondly two
constant velocity exercises on track at 100% (tlim100) and 95% of MAV (tlim95) until exhaustion. Results: Our study outcomes revealed a
VO
2 decrease before the end of exercise for three subjects. The mean decrease duration was 51.3 ± 13.4 s and represented 8.3 ± 2.1 %
of the total exercise duration. Maximal inspiratory and expiratory pressures (PImax and PEmax), measured before and after exercise were
considered as respiratory muscle strength indices and were not significantly different before or after the exercise. Conclusion: The
existence of a VO
2 decrease before the end of the exercise, already highlighted in the literature is confirmed. Our results indicated
that respiratory muscle fatigue was not explicative for VO
2 decrease. However, further studies are necessary to confirm these results.
Keywords: Continuous exercises, VO
2 decrease, respiratory muscle strength, maximal respiratory pressures
BACKGROUND
During exercise, muscle and pulmonary VO
uptake
2
) increase approximately exponentially to a steady
( VO
2
state until the end of exercise [1]. However, Perrey
et al. [2] during a continuous exercise realized on
treadmill at 95% of v VO
2max (velocity associated with
VO
),
observed
a
VO
2max
2 decrease before exhaustion
for 7 of their 13 subjects (54 %). This VO
2 decrease
before exhaustion had been also found by Astrand
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and Saltin [3], Nummela and Rusko [4] and Heubert
et al. [5] during a maximal constant load exercise.
Nevertheless, this decrease is merely limited to an
observation in these studies. More recently, Thevenet
et al. [6] during intermittent exercise (105% of maximal
aerobic velocity (MAV) alternated with passive recovery)
decrease
with trained adolescents, also showed a VO
2
before exhaustion. According to these authors, this
result could be explained by a minute ventilation ( V E )
decrease. In their study, Perrey et al. [2] also suggested
that ventilatory system deterioration could explain
VO
2 decrease before exhaustion. Unfortunately, these
authors did not highlight specific characteristics for
decrease. Considering respiratory
subjects with a VO
2
muscles fatigue as a condition in which there is a loss
in the capacity for developing force of muscle, which
is reversible by rest [7], we hypothesized that the
respiratory muscle strength loss could be the origin of
Address for correspondence:
Abderraouf Ben Abderrahman, Higher Institute of Sport and Physical Education of Ksar-Said, Tunisia.
Tel: (00 216) 20 316 494, E-mail: benabderrahmanabderraouf@yahoo.fr
The Swedish Journal of Scientific Research • Vol. 6 • Issue 1 • June 2019
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Ben Abderrahman, et al.: VO
decrease before exhaustion during constant load exercise
2
decrease during continuous exercise. Maximal
VO
2
respiratory pressures are considered a good index of
respiratory muscle strength [8]. Moreover, at the end
of the exercise, respiratory frequency (fr) increase could
be insufficient compared to tidal volume (VT) decrease
and hence, according to the relationship V E = VT × fr,
could be responsible for V E decrease [9].
breath-by-breath portable metabolic system (Cosmed
K4b2, Rome, Italy; [11]) in order to determine V E , VT, fr
. Further details about the system are provided
and VO
2
elsewhere [12]. The K4b2 was calibrated before the
beginning of each test according to the manufacturer’s
guidelines. Heart rate (HR) was continuously monitored
(Polar Electro, Kempele, Finland).
Maximal Static Mouth Pressure Measurements
OBJECTIVES
Then, the aim of our work was to analyse the VO
2
kinetic during a constant load exercise, to check the
existence of a VO
2 decrease at the end of this kind
of exercise and finally to study the respiratory muscles
strength evolution, before and after this kind of exercise.
PATIENTS AND METHODS
Subjects
Eight male physical education students (mean age
20.6 ± 2.7 yrs) volunteered to participate in this study.
All were from the same athletic club and regularly
practised athletics for at least 3 years. Subjects were 19
to 27 years old. Their mean ± SD for mass, height and
percentage of fat were 70.5 ± 3.1 kg, 180.2 ± 6.2 cm
and 12.4 ± 2.2 %, respectively. Before testing, the
subjects underwent a medical examination and were
fully informed of the experimental procedures and a
signed consent was obtained from the subjects. The
inclusion criteria required for subjects was the absence
of cardiovascular diseases; pre- or diabetes risk and
hypertension (i.e., blood pressure > 140/90 mmHg)
and absence of electrocardiogram abnormalities. This
study had been approved by the University of Nantes
Research Ethics Committee.
Overview
Subjects performed three field-tests until exhaustion
on a 400-m outdoor tartan track at the same time of
the day, with at least 48h rest between each test [10].
Atmospheric conditions were checked before each test
ensuring that all sessions were carried out under similar
environmental conditions (wind speed lower than
2.5 m.s-1; temperature between 18 and 23°C; humidity
between 40 and 70%). Athletes first performed a maximal
graded test to determine VO
2max and MAV. Then,
they performed in a randomized order two continuous
exercises until exhaustion at 100% (tlim100) and 95% (tlim95)
of MAV. During all tests, we used the Cosmed K4b2
Maximal respiratory pressures, considered as a good
index of respiratory muscle strength, can be used in
order to appreciate respiratory muscle fatigue [13].
Maximal inspiratory (PImax) and expiratory (PEmax)
pressures were respectively measured at residual volume
(RV) and total pulmonary capacity (TPC) with a ZAN
betterflow portable device (Flowhandy ZAN 100,
Messgeraete Gmbh, Germany) using the technique of
Black and Hyatt [14]. This measure was realized in the
athletics stadium, just next to the athletics tracks, by
the same experimenter at rest and 3 min after the end
of the test. In each case, PImax and PEmax were measured
5 times respectively. The highest and lowest values were
rejected and the three others were averaged for data
processing [15]. Maximal pressures were generated at
the mouth as previously detailed [16].
Maximal Graded Test
Red cones were set at 20 m intervals along the track
(inside the first line). The initial speed of the maximal
graded test was 8 km.h-1 and was increased by 1 km.h-1
every 2 min [17], to determine VO
2max , MAV, peak
minute ventilation ( VE max) and peak respiratory
frequency (frmax). V E max and frmax were determined at
the corresponding time associated with VO
2max . The
determination methods of MAV and VO2max have been
extensively described elsewhere [6].
Constant Load Exercises and Breathing Pattern
Measurement
Athletes performed two constant load exercises until
exhaustion (tlim): a 100% of MAV constant exercise
(tlim100) to confirm the MAV values and a constant load
exercise at 95% of MAV (tlim95) to study VO
2 decrease
and its link with maximal respiratory pressures. For
tlim95, V E , VT and fr values were averaged over a 2s
period. Then, the values were averaged over 20 periods,
each corresponding to 5% of the individual tlim duration.
The time course of V E , VT, fr are presented on figure 1
for a representative subject. We also calculated the last
minute V E and fr values for tlim95.
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Ben Abderrahman, et al.: VO
decrease before exhaustion during constant load exercise
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Speed Control
The tests were performed on an athletic track equipped
with cones every 20 m. During both constant load
exercise and maximal graded test, running speed was
maintained constant thanks to an experimenter on
bicycle that the subject followed. Firstly, the latter
experimenter was provided with a mp3 device (located
in a bag carried across his shoulder by the experimenter
and connected to him by headphones) giving an
imposed time signal every 20 m and took care of the
subject position on a level with the aft wheel. Secondly,
another experimenter also paid attention to the subject
position and to lap time.
Statistical Analysis
Figure 1: VO
2 , breathing pattern and maximal respiratory pressures
for tlim95 in subject S2
VO
2 : Oxygen uptake, VT: tidal volume, fr: respiratory frequency,
: minute ventilation; PI
V
and PEmax: Maximal inspiratory and
E
max
decrease begining
expiratory pressures. The vertical line shows the V
E
(95% tlim)
V̇O 2 Kinetic Modelling
Exercise data before recovery were analysed using Matlab®
(Mathworks, Natick, MA). The cardio-respiratory values
were averaged on a 2 s periods and then smoothed thanks
to a gaussian sliding mean processing along a 10 s wide
window. The second order model also usually called a
kinetic
mono-exponential function that best fits the VO
2
curve obtained was identified. Finally, a Kalman filter and
a algorithm of abrupt changes detection [18] were used
in order to detect a local loss of adequacy between the
kinetic. If such
model estimated and the measured VO
2
kinetics, the
a change was detected at the end of VO
2
algorithm computed the best linear approximation of this
slope), meaning the part of the curve after
phase ( VO
2
slope values, we calculated
the change. Based on VO
2
-1
amplitude (ml.min .kg-1) and duration (s) values to
decrease [2].
characterize the VO
2
Mean PImax and PEmax values were compared using a
paired t-test. A linear regression model was used to assess
the relationship between PImax, PEmax both measured
at the end of tlim95 and the duration of tlim95. Normal
Gaussian distribution of the data was verified with the
Kolmogorov-Smirnov test (with Lilliefor’s correction).
For all statistical analyses, the level of significance was
, V , V and fr
set at p<0.05. Statistical analyses for VO
2
E
T
were not provided since they are not consistent regarding
the weak number of subjects. Effect sizes (ES) were
evaluated from the Cohen’s d. ES of ≤ 0.2, 0.21-0.60,
0.61-1.20, 1.21-2.0, ≥2.0 were respectively considered as
trivial, small, moderate, large and very large [19].
RESULTS
Maximal Graded Test
Mean values for MAV, VO
2max , Rmax, HRmax, VE max and
frmax were: 18.4 ± 0.6 km.h-1, 58.1 ± 3.5 ml.min-1.kg-1,
1.2 ± 0.1, 192.1 ± 6.2 bpm, 151.8 ± 11.1 l.min-1 and
55.1 ± 6.5 min-1 respectively (Table 1). In subjects
decrease, mean values of MAV (18.5 ±
with VO
2
-1
-1
0.9 km.h-1), VO
2max (56.3 ± 4.3 ml.min .kg ), Rmax
(1.2 ± 0.0), HRmax (190.7 ± 7.6 bpm), VE max (148.4 ±
5.7 l.min-1) and frmax (57.9 ± 5.6 min-1) did not present
any particularly higher or lower values compared with
mean values of MAV (18.4 ± 0.5 km.h-1), VO
2max (59.2
-1
-1
± 2.8 ml.min .kg ), Rmax (1.2 ± 0.1), HRmax (193.0 ±
5.9 bpm), V E max (153.9 ± 13.6 l.min-1) and frmax (53.4
decrease.
± 7.0 min-1) in subjects without VO
2
Constant Load Exercises
Mean values for tlim100 were 363.4 ± 56.8 s and ensured
the subjects MAV values. Mean values of tlim95 was
The Swedish Journal of Scientific Research • Vol. 6 • Issue 1 • June 2019
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Ben Abderrahman, et al.: VO
decrease before exhaustion during constant load exercise
2
564.7 ± 75.3 s. Table 2 shows last minute V E and
fr values during tlim95. The higher last minute V E
(149.4 ± 10.3 l.min-1 vs 147.3 ± 12.4 l.min-1) and fr
(54.4 ± 79 min-1 vs 55.0 ± 6.9 min-1) values were not
decrease.
systematically observed in subjects with VO
2
V̇O 2 Decrease Characteristics
The model used did reveal a slope at the end of
exercise for subjects 2, 3 and 5 during tlim95 (Table 3),
decrease for these subjects. The
illustrating a VO
2
decrease (DD) was 51.3 ±
mean duration of the VO
2
13.4s and corresponded in mean to 8.3 ± 2.1% of total
exercise duration.
Respiratory Muscle Fatigue
, V , fr and V expressed relatively to time
VO
2
E
T
to exhaustion for tlim95 are shown in figure 1. Only
representative subject S2 is represented in this figure.
This figure also shows individual values of PImax and
PEmax measured before and at the end of the exercise.
Table 1: Maximal graded test parameters
Subjects
MAV (km.h‑1)
VO2max (ml.min‑1.kg‑1)
Rmax
HRmax (bpm)
VEmax (l.min‑1)
frmax (min‑1)
S2
19.0
61.1
1.2
196.0
145.1
61.1
S3
17.5
52.9
1.2
194.0
145.0
51.5
S5
19.0
54.8
1.2
182.0
155.0
61.3
Mean±SD (n=3)
18.5
0.9
56.3
4.3
1.2
0.0
190.7
7.6
148.4
5.7
57.9
5.6
S1
19.0
58.0
1.2
185.0
156.7
55.8
S4
18.5
63.3
1.2
196.0
141.7
45.6
S6
18.5
59.6
1.0
200.0
150.6
57.8
S7
17.5
59.6
1.3
195.0
144.5
46.5
S8
18.5
55.7
1.2
189.0
176.0
61.3
Mean±SD (n=5)
18.4
0.5
59.2
2.8
1.2
0.1
193
5.9
153.9
13.6
53.4
7.0
Mean±SD (n=8)
18.4
0.6
58.1
3.5
1.2
0.1
192.1
6.2
151.8
11.1
55.1
6.5
ES (Cohen’s d)
0.17
0.83
0.00
0.37
0.50
0.69
V
MAV: maximal aerobic velocity; VO
E
2max : maximal oxygen uptake; Rmax: maximal respiratory exchange ratio; HRmax: maximal heart rate;
ventilation; frmax: maximal respiratory frequency. ES: effect size (Cohen’s d)
max
: maximal minute
Table 2: Minute ventilation and respiratory frequency values during tlim95
Subjects
tlim95 (s)
Last minute VE (l.min‑1)
%VEmax
fr (min‑1)
%frmax
S2
510.0
151.3
104.3
61.5
100.7
S3
624.0
138.3
95.4
45.8
88.9
S5
742.0
158.6
102.3
55.9
91.2
Mean±SD (n=3)
625.3
116.0
149.4
10.3
100.7
4.7
54.4
7.9
93.6
6.3
S1
474.0
143.9
91.8
52.4
93.9
S4
586.0
127.7
90.1
44.3
97.1
S6
632.0
159.9
106.2
60.4
104.5
S7
648.0
154.9
107.2
57.7
124.1
479.0
149.9
85.2
62.2
101.5
Mean±SD (n=5)
S8
563.8
82.9
147.3
12.4
96.1
9.9
55.4
7.2
104.2
11.8
Mean±SD (n=8)
564.7
75.3
148.1
10.9
97.8
8.3
55.0
6.9
100.2
11.0
ES (Cohen’s d)
0.82
0.19
0.55
0.14
0.96
: mean last minute ventilation expressed in l.min‑1 and relatively to V
V
V
E
); fr: mean last minute
E max (determined during maximal graded test ‑ %
E
max
respiratory frequency expressed in min‑1 and relatively to frmax (determined during maximal graded test ‑ % frmax). ES: effect size (Cohen’s d)
The Swedish Journal of Scientific Research • Vol. 6 • Issue 1 • June 2019
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Ben Abderrahman, et al.: VO
decrease before exhaustion during constant load exercise
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Table 3: VO
decrease characteristics during tlim95 for subjects S2, S3 and S5
2
Subjects
tlim (s)
TD (s)
DD (s)
%DD
∆ (ml.min‑1.kg‑1)
%∆
S2
510.0
474.0
36.0
7.1
2.7
5.6
S3
624.0
546.0
78.0
12.5
2.5
5.8
S5
742.0
702.0
40.0
5.4
5.6
12.8
Mean±SD (n=3)
625.3
67.0
574.0
67.3
51.3
13.4
8.3
2.1
3.6
1.0
8.1
2.4
tlim95: time to exhaustion performed at 95% of MAV; TD: time delay of VO2 decrease beginning; DD: decrease duration; %DD: percentage of decrease duration
VO
tlim95; ∆: decrease amplitude; %∆: percentage of
2 decrease amplitude relatively to the total VO 2 amplitude A, considered as the difference between VO
2
plateau and VO
2 at rest (t=0)
The vertical line in the figure represents the onset of V E
decrease and is linked to the other parameters. Despite
the lack of statistical analyse in the three subjects with
decrease, V and V seemed to decrease and fr
a VO
2
E
T
seemed to increase at the end of tlim95.
Mean values of maximal inspiratory and expiratory
pressures values (PImax and PEmax) measured at rest
(rest) and at the end of the exercise (end) are shown
in figure 2. Statistical results did not highlight any
significant evolution of PImax (d = 0.21) and PEmax
(d = 0.20) values between the rest and the end of
the exercise during tlim95. No significant relationship
was found between PImax [=83.8 + (0.063 × tlim95),
r2 = 0.05, p=0.587], PEmax [=-6.96 + (0.27 × tlim95),
r2 = 0.19, p=0.27], both measured at the end of tlim95,
and the duration of tlim95.
DISCUSSION
The aim of our work was to analyse the VO
2 kinetic
during a constant load exercise, to check the existence
of a VO
2 decrease at the end of this kind of exercise
and finally to study the respiratory muscles strength
evolution, before and after this kind of exercise. Our
study outcomes reveal a VO
2 decrease before the end
of exercise for three subjects. From a methodological
point of view, firstly, VO
2 data were averaged on a
two seconds period. Data were analysed with Matlab®
software and a second order model was applied to VO
2
kinetics. An ad-hoc filtering process, based on Kalman
filter was then used in order to detect the changes of
model relatively to VO
2 kinetic. When, and only when,
a series of changes was detected at the end of VO
2
kinetic, the software applied a linear phase. This latter
was only applied after a VO
2 steady state detection. As
the linear phase is based on several decreasing plots,
we conclude that the decrease observed is not due to
measurement artefacts. Secondly, the VO
2 decrease
observed in our three subjects also cannot be explained
by running speed variations. Indeed, running speed was
maintained constant for each intensity thanks to four
controls. Therefore, according to these methodological
decrease
considerations, we concluded that the VO
2
could be only explained by physiological process.
Moreover, the proportion of subjects with a VO
2
decrease (47%) are close to those reported by Perrey
et al. [2].
Respiratory Muscle Fatigue
One hypothesis put forward in order to understand this
before exhaustion during continuous
decrease in VO
2
exercise concerns respiratory muscle fatigue [2].
During short intermittent exercise (30s at 105% of
MAV alternated with 30s passive recovery) we have
decrease before exhaustion. We also
shown [6] a VO
2
decrease was partly connected
suggested that this VO
2
with respiratory muscle fatigue. Maximal respiratory
pressures, considered as a good index of respiratory
muscle strength, can be used in order to assess respiratory
muscle fatigue [13]. However, according to Hayot et
Matecki [20] maximal respiratory pressures used as
a fatigue index should be coupled with other fatigue
evaluation methods. Thus, in our study, two approaches
were used to appreciate the respiratory muscle fatigue.
The first was related to maximal respiratory pressures
(PImax and PEmax) measurements and the second one
depended on the use of VT, fr and V E kinetics [9].
Maximal respiratory pressure measurement fell within
the range described by Leech et al. [21] and Chen et
al. [22], and our results (Figure 2) did not present a
significant difference between these two parameters at
rest (rest) and at the end of the exercise (end). Since
was not present
the phenomenon of a decrease in VO
2
in more than half of the subjects, it seems logical that
there was no systematic fatigue of the respiratory muscles
in the total group. But that does not necessarily mean
The Swedish Journal of Scientific Research • Vol. 6 • Issue 1 • June 2019
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Ben Abderrahman, et al.: VO
decrease before exhaustion during constant load exercise
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Figure 2: Mean values (± SD) for maximal inspiratory and expiratory
pressures for the whole population during tlim95
PImax and PEmax: Maximal inspiratory and expiratory pressures.
NS : no significant difference
that respiratory muscles fatigue was not the cause of
in those three subjects. Moreover,
the decrease in VO
2
PImax and PEmax values seemed to be lower at the end of
the exercise (Figure 2). We could hypothesize that even
without a significant decrease, 95% of MAV intensity
seems to induce respiratory muscle fatigue. Thevenet
et al. [6] during intermittent exercise observed a longer
decrease (30% of time to exhaustion) during a longer
exercise duration (around 35min). Whereas our mean
tlim95 was shorter than ten minutes, mean time to
exhaustion during intermittent exercise in Thevenet
et al. [6] study was much longer because of the exercise
modality. If decrease is linked to respiratory muscle
fatigue, it could confirm results of Jonhson et al. [23]
who recommended exercise intensity and duration at less
of 85% of MAV and ten minutes respectively to induce
respiratory muscle fatigue. We thought that 95% of MAV
was an appropriate intensity in order to run longer than
ten minutes, but it seems to be necessary to choose lower
intensities or other exercise modality to run longer and
then support the respiratory muscle fatigue hypothesis.
These observations are consistent with conclusions of
Romer et Polkey who suggested that exercise intensity is
important but also that exercise duration plays a major
role in diaphragm fatigue [24].
The second approach was based on the use of VT, fr
and V E kinetics [9]. Let us first note that the filtering
process used to analyse VO
2 kinetics cannot be used
in those cases. Indeed, VE and fr kinetics do not fit a
second order model. For example, no stable state is
reached before exhaustion. Our results showed that
the decrease in V E began at around 95% of tlim95. This
result could be explained by a VT decrease. Indeed, after
95% of tlim95, the increase in fr seemed to be insufficient
to prevent the VT decrease, that can be considered
as an indirect sign of respiratory muscle fatigue [9]
and can partly explain the VO
2 decrease [6] before
exhaustion. Nevertheless, the absence of significantly
different results on respiratory muscle fatigue and
the impossibility of statistical processing V E , VT and
fr values, lead us to consider our results with caution
and to partly reject our hypothesis. Another hypothesis
has been advanced by Perrey et al. [2] to explain the
origin of the VO
2 decrease before exhaustion. This
hypothesis is related to a cardiac output decrease and
to an O2 arterio-veinous difference decrease. The latter
hypothesis seems to be more likely during a maximal
exercice and could have an influence on the locomotor
muscles perfusion and VO
2 for trained athletes.
Indeed, during a maximal exercise, respiratory muscles
O2 consumption corresponds to 10-15% of VO
2max . It
induces a greater respiratory muscle blood flow, which
could in turn induce locomotor muscle vasoconstriction.
Then, it could compromise the blood flow, necessary for
a good perfusion of locomotor muscles, and decrease
the O2 arterio-veinous difference [25]. An exerciseinduced hypoxaemia (EIH) could also explain an O2
arterio-veinous difference decrease. EIH is defined
as a reduction in the arterial pressure O2 (PaO2) by
more than 1kPa and/or a haemoglobin O2 saturation
(SaO2) below 95% [26]. The ability to maintain a
high alveolar O2 pressure (PAO2) is critical for blood
oxygenation and this appears to be difficult in large
individuals. A large lung capacity and, in turn, diffusion
capacity seem to protect PaO2. A widening of the
PAO2-PaO2 difference does indicate that a diffusion
limitation, a ventilation-perfusion mismatch and/
or a shunt influence the transport of O2 from alveoli
to the pulmonary capillaries. A marked increase in
cardiac output induces a faster transit time. When the
latter is combined with diffusion limitation previously
described, the O2 transport problem is accentuated.
decrease before
To conclude, the existence of a VO
2
the end of the exercise, already highlighted in the
literature [2, 3, 4], seems to be confirmed. However, the
respiratory muscle fatigue hypothesis seems to be partly
rejected to explain our results. We suppose, indeed,
that the exercise duration was insufficient to induce a
respiratory muscle fatigue in the subjects. It could be
interesting hence, to test other exercise intensities or
modalities, in order to study respiratory muscle fatigue
and its link with VO
2 decrease over a longer period.
The Swedish Journal of Scientific Research • Vol. 6 • Issue 1 • June 2019
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Ben Abderrahman, et al.: VO
decrease before exhaustion during constant load exercise
2
Limitations
Whilst this study examined only eight male physical
education students, studying more high-level middle
and long-distance runners is certainly warranted to get
a better understanding of the nature of the associations
decrease and the respiratory muscle
between the VO
2
fatigue.
Aknowledgement
In memory of Delphine Thevenet.
Conflict of Interests
The authors have no conflicts of interest that are
directly relevant to the content of this article.
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