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O2 slow component
for severe exercise depends on type of exercise and is not
correlated with time to fatigue
1 Laboratoire Science du Sport, The purpose of
this study was to examine the influence of the type of exercise
(running vs. cycling) on the O2
uptake (
oxygen slow component; fatigue; running; cycling
PREVIOUS STUDIES PERFORMED on cycling have reported
that oxygen uptake ( Although the mechanism underlying the continuous rise in
Data comparing the magnitude of the
In addition, it has been suggested that bicyclists could minimize
peripheral muscle fatigue by pedaling at a rate that produces a higher
than the optimum metabolic rate (the most economical) but that lowers
crank forces (torque). Indeed, Patterson and Moreno (19) demonstrated
that when the pedaling rate was increased from 60 to 120 rpm, the
resultant force on the pedals averaged over a crank cycle was
isometric-like and produced no external work. Their results suggested
that pedaling at 90 rpm might minimize peripheral forces and therefore
peripheral muscle fatigue, even though this rate might result in higher
Whether these differences between running and cycling have a potential
effect on the We hypothesized that the type of exercise could influence the changes
in the efficiency, i.e., the
The purpose of this study was to examine
1) the influence of exercise,
running vs. cycling, on the
Subjects.
Ten well-trained triathletes gave their informed consent and
volunteered to participate in this study, which was approved by the
Paris Ethical Committee. The physical characteristics of the subjects
are presented in Table 1. All subjects were
highly motivated and familiar with treadmill running, ergometer
cycling, and with the sensation and symptoms of fatigue during heavy
exhaustive cycling and running exercise.
ABSTRACT
Top
Abstract
Introduction
Methods
Results
Discussion
References
O2) slow component.
Ten triathletes performed exhaustive exercise on a treadmill and on a
cycloergometer at a work rate corresponding to 90% of maximal
O2 (90% work rate maximal
O2). The duration of the
tests before exhaustion was superimposable for both type of exercises
(10 min 37 s ± 4 min 11 s vs. 10 min 54 s ± 4 min 47 s for
running and cycling, respectively). The
O2 slow component (difference between O2 at
the last minute and minute 3 of
exercise) was significantly lower during running compared with cycling
(20.9 ± 2 vs. 268.8 ± 24 ml/min). Consequently, there was no
relationship between the magnitude of the
O2 slow component and the
time to fatigue. Finally, because blood lactate levels at the end of the tests were similar for both running (7.2 ± 1.9 mmol/l) and cycling (7.3 ± 2.4 mmol/l), there was a clear dissociation between blood lactate and the O2
slow component during running. These data demonstrate that
1) the
O2 slow component depends
on the type of exercise in a group of triathletes and
2) the time to fatigue is
independent of the magnitude of the
O2 slow component and blood
lactate concentration. It is speculated that the difference in muscular
contraction regimen between running and cycling could account for the
difference in the
O2 slow component.
INTRODUCTION
Top
Abstract
Introduction
Methods
Results
Discussion
References
O2) can
attain a steady state above the lactate threshold but only in the
work-rate range where lactate remains constant. Indeed, it is only
above what has been termed "critical power" (17) that
O2 continues to
rise until the end of the test or until exhaustion (22, 29).
Furthermore, the magnitude of this slow component increases with the
level of work rate and eventually takes
O2 to the maximal
O2
(O2 max),
even during submaximal exercise (22, 23).
O2 during suprathreshold
exercise remains poorly understood, we have recently reported (5) that
the O2 slow component was extremely small during an 18-min exhaustive run compared with that
reported during cycling (23). Indeed, in that study, all the distance
runners attained only 90% of their
O2 max during the last
minute before exhaustion, without ever reaching
O2 max.
O2 slow component for
different types of dynamic exercises are not available in the literature. Kyle and Caiozzo (16) reported that various types of
exercise exclusively involving the legs yielded very similar power
output, as long as the motion was similar and the same muscle groups
were involved. However, cycling and running differ greatly in terms of
muscular contraction regimen and, therefore, mechanical efficiency. For
instance, the concentric work of cycling may account for a lower
mechanical efficiency than running, which relies on a
stretch-shortening cycle (6). The higher efficiency of
stretch-shortening movements has been attributed to the elastic
behavior of the muscles during contact with the ground. Cavagna et al.
(8) estimated elastic contributions to be 40-50% of the total
power generated during running. The gastrocnemius and soleus muscles
also function during cycling on stretch-shortening cycles, although the
stretching phases are not as apparent as in running or jumping (14).
O2. In running, however, Cavanagh and Williams (9) reported that the freely chosen stride length
allows for the most economical run.
O2 slow
component is unclear. No studies have compared the
O2 slow component for cycling
and running in subjects trained for both exercises and with the same
O2 max and the
same fraction of
O2 max at the blood
lactate threshold (2).
O2 slow component, during exhaustive suprathreshold exercise.
O2 slow component during exhaustive exercise in triathletes equally trained in cycling and
running and 2) whether the magnitude
of the O2 slow component influences the duration of exercise (time limit:
tlim) at a
velocity (90%
) or a work rate (90%
) corresponding to 90% of
O2 max. Finally, the
relationship between the O2
slow component and blood lactate accumulation for cycling and running
exercise was analyzed.
METHODS
Top
Abstract
Introduction
Methods
Results
Discussion
References
Table 1.
Physical characteristics, triathlon experience, and training regimen
data of 10 triathletes
Materials. The running tests were performed on a motorized treadmill (Gymrol 1800, Techmachine, St. Etienne, France) kept at a 0% slope for all of the tests, the speed being controlled with a resolution of 0.5-mi./h (controller provided by the Centre d'Enseignement et de Développement pour le Montage en Surface, Université Joseph Fourier, Grenoble, France). The cycling tests were carried out on an electronically braked cycle ergometer (ERG 600 Bösch, Berlin, Germany). Respiratory and pulmonary gas exchange variables were measured by using a MedGraphics CPMax cart (Medical Graphics, St. Paul, MN), which was calibrated before each test according to the manufacturer's instructions. Breath-by-breath data were averaged every 15 s. An electrocardiograph was monitored from a three-lead configuration (Jaeger cardioscope), and the output signal was fed to the CPX Medical Graphics system for computing heart rate. The blood samples were analyzed for blood lactate concentration (YSI 27 analyzer, Yellow Springs Instruments, Yellow Springs, OH).
Preliminary measurements.
The tests were performed on each subject at the same time of day in a
climate-controlled laboratory (21-22°C). The subjects were
instructed not to train hard or to ingest food and beverages containing
caffeine for 3 days before testing. Each subject underwent two
preliminary incremental tests on both the treadmill and the cycloergometer to determine 1)
O2 max,
2) the work rate associated with
O2 max
(
, and 3) the fraction of
O2 max at which the
lactate threshold appeared. These two incremental tests (running and
cycling) were performed 3 days apart and in a randomized order.
Subjects performed a continuous incremental test (3-min stages) to
exhaustion. Duration and workload increments were standardized for
running and cycling as follows. The workload increments were estimated
to demand a O2 response
equal to 2 × rest (2 Mets, i.e., 2 × 3.5 ml · kg
1 · min1).
Each work increment ranged between 35 and 50 W for cycling, depending
on the weight of the subjects [on the basis of 12 ml O2 · min1 · W1
according to the recommendations of Astrand and Rodahl (1)]. For
example, for a 60-kg subject, the workload increments were 35 W
[60 (kg) * 7 (ml · kg1 · min1)/12
(ml
O2 · min1 · W1)].
For running, the speed was increased by 2 km/h (33.3 m/min) at each
stage, except for the last stage where the increment was only 1 km/h
(16.7 m/min). The initial work was set at between 70 and 100 W for
cycling and at 10 km/h for running.
O2 max was the
highest 30-s O2 reached at
the end of an incremental test. The power output or velocity associated
with O2 max was
defined for running and cycling as the minimal workload at which
O2 max occurred (5).
All subjects gave a maximum effort and were encouraged to do so.
Blood samples were obtained from the fingertip at the end of each
stage, immediately after the end of the exercise test, and then 8 min
into the recovery period.
In this study, the lactate threshold was defined as the
O2 corresponding to the
starting point of an accelerated lactate accumulation of ~4 mmol/l
and expressed in
%O2 max (2).
Although a ramplike increase in work rate cannot allow precise
determination of this blood lactate threshold, the incremental test
provides a useful index to delineate the domain at which blood lactate starts to accumulate in the blood, as illustrated for one subject in
Fig. 1. Analysis of Fig. 1 shows that,
below 70% of the peak work rate, trivial changes in lactate occurred,
whereas above 80%, blood lactate increased above 3 mmol/l. This
"threshold" was in accordance to that proposed by Aunola and
Rusko (2), which was closer to the onset of blood lactate accumulation
as defined by Sjödin and Jacobs (24) rather than to the lactate threshold of Farrel et al. (10).
|
Experimental design and protocols.
The subjects ran and cycled to exhaustion at 90%
.
These two tests were separated by 1 wk. After a 15-min warm-up period
at 50%
, which was below the lactate threshold for all the subjects, the work
rate was increased within 20 s to 90%
. All subjects were given verbal encouragement throughout each trial. For
running, the time to fatigue at 90%
was the time at which the subject's feet left the treadmill as he
placed his hands on the guardrails. For cycling, this time corresponded
to the time at which the subject was no longer able to pedal at a given
power output. This time was defined as the time limit at 90%
(tlim 90%).
Data analysis.
Statistical analysis was performed by using
t-tests for paired comparisons to
determine whether
O2 max, blood lactate
concentration, respiratory exchange ratio, heart rate, blood lactate
threshold (in
%O2 max), and times
to exhaustion were different in the cycling and running
tests. The O2 slow
component was computed as the difference between
O2 at the last and the
third minute of the exercise. A two-way analysis of variance for
repeated measurements tested the overall effect of time on
cardiorespiratory and blood parameters during the first minute (onset),
the minute at the mid-time to fatigue, and the last minute of
tlim (end) during the constant-power test. Scheffé's post hoc anaylsis was then used to locate the diferences. The results are presented as means ± SD. Statistical significance was set at
P < 0.05.
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RESULTS |
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Top
Abstract
Introduction
Methods
Results
Discussion
References
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Incremental tests.
Table 2 shows the cycling and running
O2 max, heart rate, and
blood lactate values reached at the end of the incremental tests. There
was no significant difference between running and cycling for
O2 max,
maximal heart rate, blood lactate level, and lactate threshold.
|
O2
elicited at 90%
represented 90.7 ± 5.2 and 88.2 ± 3.1% of
O2 for cycling and running,
respectively, which was not significantly different
(P = 0.22). Consequently, the
intensity of the exhaustive exercise (i.e., 90%
) was well above the lactate threshold and similar for both type of
exercises in absolute (O2)
and relative intensity (%lactate threshold and
%O2 max). In addition,
the selected warm-up period was well below the severe-exercise domain
delineated by the lactate threshold. In fact, the end of the 15-min
warm-up period was performed at 56 ± 2 and 54 ± 3%
of O2 max for running
and cycling, respectively (50%
).
Constant work rate tests.
Table 3 shows the
O2 slow component, the
cycling and running
O2 max, heart rate, and
blood lactate reached at the end of the all-out constant work rate
tests performed at 90%
.
|
Time to fatigue.
The 90%
and 90%
were, respectively, 17.7 ± 1.1 km/h and 347 ± 43 W. Running
and cycling times to fatigue at this velocity or power output were not
significantly different at ~10 min (Table 3).
The O2 slow component.
As shown in Table 3, compared with cycling, the
O2 slow component for running
was significantly lower (268.8 ± 24 vs. 20.9 ± 2 ml/min,
P = 0.02). During cycling,
O2 at exhaustion was not
significantly different from
O2 max determined
during the incremental test. In contrast, at the end of the
exhaustive run test, the triathletes did not, on average, reach
their O2 max (Fig.
2). However, examination of
individual responses revealed different patterns of responses
(Fig. 3,
A-C). Four subjects
(subjects 4,
5, 7,
and 8) reached their
O2 max in the cycling
but not in the running test (Fig.
3A); four subjects
(subjects 2,
3, 6, and 9) reached their
O2 max both in cycling
and running (Fig. 3B), the remaining
two subjects (subjects 1 and 10)
reaching neither their cycling nor running
O2 max (Fig.
3C). In other words, of the 10 triathletes
studied, 8 reached their O2
during cycling and only 4 during running.
|
|
Blood lactate and the O2
slow component.
The subjects ended their warm-up periods with a blood lactate level of
2.9 ± 0.9 mmol/l for running and 2.4 ± 0.5 mmol/l for cycling.
Blood lactate level at the end of the exhaustive exercises was not
significantly different from that at the end of the incremental test,
both for running (7.2 ± 1.9 vs. 7.1 ± 1.7 mmol/l,
P = 0.79) and cycling (7.3 ± 2.4 vs. 7.7 ± 1.3 mmol/l, P = 0.85).
The magnitude of the O2 slow
component and the level of blood lactate accumulation were correlated
for both cycling and running (r = 0.48, P = 0.03, n = 20), for cycling only
(r = 0.66, P < 0.05, n = 10) but not for running only
(r = 0.12, P = 0.74, n = 10) (Fig.
4).
|
The O2 slow component and
time to fatigue.
No significant correlation was found between the magnitude of the
O2 slow component and the
duration tolerated in this suprathreshold exercise
(r = 
0.15 for both running and
cycling combined and r = 0.23
and 0.18 for running and cycling, respectively, considered separately). However, the duration of suprathreshold exercise was
positively correlated with the blood lactate accumulation for running
(r = 0.79, P < 0.01) but not for cycling
(r = 0.32, P > 0.05).
O2 slow
component between cycling and running (247.9 ± 83.6 ml of
O2/min) was not correlated with the difference of time to fatigue for running (18 ± 82 s) at the supra-lactate threshold workload (r = 0.25, P = 0.50).
Time to fatigue and blood lactate accumulation.
Blood lactate was not correlated with the time to fatigue for both
running and cycling (r = 0.07, P = 0.78, n = 20); however, when each type of exercise is considered in isolation, maximal blood
lactate and time to fatigue were correlated for running (r = 0.79, P = 0.005) but not for cycling
(r = 0.33,
P = 0.41).
The O2 slow component and
cardioventilatory variables.
The maximal heart rate at the end of the constant test was not
significantly different between cycling and running (180.4 ± 5.4 vs. 175 ± 7.1 beats/min, P = 0.81). Minute ventilation, however, was significantly higher in cycling
(153.6 ± 18.6 vs. 137 ± 21 l/min,
P = 0.002) but with a minute
ventilation-to-O2 ratio similar to that during cycling.
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DISCUSSION |
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Top
Abstract
Introduction
Methods
Results
Discussion
References
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In the present study, an exhaustive test performed at 90%
was used to examine the influence of the type of exercise (cycling vs.
running) on the O2 slow
component and its relationship with the time to fatigue (i.e., failure
to sustain a required power output).
Our results were twofold. First, for a similar relative and absolute
intensity, the O2 slow
component depends on the type of exercise. Second, the
O2 slow component was
correlated neither with the time to fatigue nor with blood lactate
accumulation, when running and cycling were considered separately.
Moreover, in contrast to cycling, the time to fatigue was unrelated to
the blood lactate accumulation for running.
The O2 slow component
depends on the type of exercise.
Despite the same relative intensity of exercise, the
O2 slow component was higher
for cycling than for running. The level of work rate chosen for the
test was well above each triathlete's lactate threshold because the
intensity of exercise was above the blood lactate accumulation of
between 3 and 5 mM (2). The constant work rate exercise corresponded to
a "severe-exercise" domain, defined as the intensity at which it
is no longer possible to reach a new
O2 steady state, consistent
with exercise above the critical power (12).
O2 slow component were
performed during cycling (12). However, the
O2 slow component has also
been reported during running but only for prolonged tests (20-30
min) (18, 25). No data are available on the behavior of the
O2 slow component during a
submaximal running test leading to fatigue. We observed that 8 of the
10 subjects reached their O2 max in cycling and
only 4 in running. A precise analysis of the main differences between
cycling and running may help us to understand some of the mechanisms
responsible for the O2 slow component.
By simultaneously measuring pulmonary and leg
O2 during cycle
ergometry, Poole et al. (21) demonstrated that 86% of the increment in
pulmonary O2 beyond the third
minute of exercise (i.e., the
O2 slow component) could be
accounted for by the increase in leg
O2. These data
indicate that the majority of the
O2 slow component is
attributable to factors within the working limbs (12). Different
mechanisms may account for such additional increase in
O2. It has been suggested
that the O2 slow component
may primarily be related to motor unit recruitment patterns during
exercise, depending on the contribution of lower efficiency,
fast-twitch motor units (15). More recently, Barstow et al. (3) and
Poole et al. (20) have linked the
O2 slow component
with the type of fibers, hypothesizing that both slow- and fast-twitch
fiber types are recruited simultaneously at the onset of heavy
exercise. However, because of their slower kinetics, the
O2 requirement of the
fast-twitch fibers may become manifest only after several minutes.
The hypothesis that the O2
slow component arises from the recruitment of a fast-twitch fiber
population with slow kinetics is consistent with the notion that
O2 kinetics are limited by fiber mitochondrial content (20). Moreover, it has been shown that
isolated mitochondria from type II fibers exhibit a 18% lower phosphate-to-oxygen ratio (30). Similarly, Whipp (29) considers that a,
if not the, major contributor to the
O2 excess is likely to be the
high energy cost of contraction of the type II fibers recruited at a
proportionally higher level of work rate and requiring a large
high-energy phosphate cost for force production.
In the present study, because triathletes exercised at the same
relative intensity for cycling and running (between their lactate
threshold and the work rate associated with
O2 max), there was no
reason to attribute the difference in the
O2 slow component between
these two types of exercises to a difference in the percentage of
fast-twitch fibers recruited in cycling and in running, unless it is
assumed that the contraction regimen recruits different types of fibers
at a given work rate, a result actually supported by many studies. For
instance, Gaesser (11) reported that the
O2 slow component was
significantly higher for cycling at 100 rpm than at 50 rpm. In our
study, the subjects were free to choose their most comfortable rate and
usually adopted a frequency of ~80 rpm, which may have increased the
magnitude of the slow component. Recently, Takaishi et al. (27)
demonstrated that the optimal pedaling rate estimated from
neuromuscular fatigue in working muscles was coincident not with the
pedaling rate at which the smallest
O2 was obtained but with the
preferred pedaling rate of the subjects. They suggested that the reason
that cyclists preferred a higher pedaling rate was closely related to
the development of neuromuscular fatigue in the working muscles.
In contrast, during running, the most efficient step rate is virtually
the same as the freely chosen step rate (9).
Finally, the isometric component of the contractions during cycling
should be considered. Indeed, the type of muscle contraction is not
homogeneous during the pedaling cycle, and an isometric-like component
occurs during various phases of this cycle. This could indeed greatly
affect the actual cost of pedaling in terms of O2 at a high level of work rate.
The O2 slow component,
time to fatigue, and blood lactate.
Our second finding was that the
O2 slow component was
correlated neither with time to fatigue nor with blood lactate
accumulation. Whipp (29) suggested that the more rapidly the slow
component projects toward O2,
the shorter the tolerable duration of the exercise test. In the study
by Nagle et al. (18), the subjects did not reach their
O2 max and were not
completely exhausted. In the present study as well, many subjects did
not reach their O2 max
during running but were completely exhausted at the end of the test. In
cycling, however, more subjects reached their O2 max but
were exhausted as well after the same duration. Jones (13) has
suggested that the sensation of effort presumably reflects the
magnitude of the voluntary motor command generated. The second source
of sensory information, described as a sense of force or tension, is
derived from peripheral receptors in muscles, tendons, and the skin and
is assumed to reflect the actual force exerted by the muscle.
O2 slow component and the
fatigue process remains unclear (20). The relationships between lactate
and the O2 slow component
have also been the focus of attention. Barstow and Molé (4), for
example, suggested that the magnitude of the
O2 slow component correlated
temporally with the changes in blood lactate. Roston et al. (23) also
reported a significant correlation between changes in blood lactate and the O2 slow
component during heavy exercise. It has been hypothesized that the
O2 slow component could be
induced by the Hb-O2 dissociation curve shifted to the
right by acidosis (Bohr effect) (28). Therefore, the Bohr effect
allowed further unloading of
O2 from Hb for uptake by the
muscle cells at a constant (minimum) capillary
PO2 (26). However, the temporal
relationship between blood lactate and the
O2 slow component has been
convincingly shown not to be one of cause and effect (see Ref. 20 for
review). Our results corroborate this conclusion because they clearly
show a dissociation between blood lactate and the
O2 slow component. However,
one should consider that because blood lactate concentration is the difference between rates of blood lactate production and disappearance (7), it might be possible that the same blood lactate concentration for
cycling and running corresponds to a different lactate turnover reflected by the higher O2
slow component in cycling.
In conclusion, the type of dynamic exercise performed at
the same intensity
(%
) and absolute O2 was
found to significantly affect the characteristics of the
O2 response during
heavy exercise. Not only was the
O2 slow component
dependent on the type of exercise, it was also not correlated with the
time to fatigue.
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FOOTNOTES |
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Address for reprint requests: V. Billat, Centre de Médecine du Sport CCAS, 2 Ave. Richerand, 75010 Paris, France.
Received 17 January 1997; accepted in final form 31 March 1998.
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Top
Abstract
Introduction
Methods
Results
Discussion
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