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Institute of Exercise and Sport Sciences, Copenhagen Muscle Research Centre, The August Krogh Institute, University of Copenhagen, DK-2100 Copenhagen, Denmark
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ABSTRACT |
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 TOP
 ABSTRACT
 INTRODUCTION
SUBJECTS AND METHODS
RESULTS
DISCUSSION
REFERENCES
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The present study examined the onset and the rate of rise of muscle oxidation during intense exercise in humans and whether oxygen availability limits muscle oxygen uptake in the initial phase of intense exercise. Six subjects performed 3 min of intense one-legged knee-extensor exercise [65.3 ± 3.7 (means ± SE) W]. The femoral arteriovenous blood mean transit time (MTT) and time from femoral artery to muscle microcirculation was determined to allow for an examination of the oxygen uptake at capillary level. MTT was 15.3 ± 1.8 s immediately before exercise, 10.4 ± 0.7 s after 6 s of exercise, and 4.7 ± 0.5 s at the end of exercise. Arterial venous O2 difference (a-vdiff O2) of 18 ± 5 ml/l before the exercise was unchanged after 2 s, but it increased (P < 0.05) after 6 s of exercise to 43 ± 10 ml/l and reached 146 ± 4 ml/l at the end of exercise. Thigh oxygen uptake increased (P < 0.05) from 32 ± 8 to 102 ± 28 ml/min after 6 s of exercise and to 789 ± 88 ml/min at the end of exercise. The time to reach half-peak a-vdiff O2 and thigh oxygen uptake was 13 ± 2 and 25 ± 3 s, respectively. The difference between thigh oxygen delivery (blood flow × arterial oxygen content) and thigh oxygen uptake increased (P < 0.05) after 6 s and returned to preexercise level after 14 s. The present data suggest that, at the onset of exercise, oxygen uptake of the exercising muscles increases after a delay of only a few seconds, and oxygen extraction peaks after ~50 s of exercise. The limited oxygen utilization in the initial phase of intense exercise is not caused by insufficient oxygen availability.
blood flow; mean transit time; oxygen delivery
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INTRODUCTION |
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TOP
ABSTRACT
INTRODUCTION
SUBJECTS AND METHODS
RESULTS
DISCUSSION
REFERENCES
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AT THE START OF EXERCISE, energy liberation in skeletal muscle increases instantaneously and becomes more than 50 times higher during intense exercise than at rest. The contribution of nucleotides and creatine phosphate as well as the role of glycogenolysis and glycolysis are well described (22). It is, however, unclear how early muscle oxidation increases at the onset of exercise and to what extent aerobic metabolism contributes to the elevated energy turnover at the muscular level in the initial phase of exercise.
A few human studies have examined muscle oxygen uptake
(
O2) in the first period of exercise
with the use of the Fick principle, i.e., in addition to measuring the
blood flow, the oxygen content was determined in arterial blood and in
the venous blood that drains the active muscle (3,
10, 14). On the basis of such measurements in
the transition from unloaded cycling to moderate intensity cycling, it
has been suggested that muscle oxidation does not increase until 10 to
15 s of exercise (10). However, in neither this nor
in the two other studies, the transit time from the artery to the
capillaries and further to the collection point of the venous blood
samples was determined. These transit times are essential to know,
because O2 rises progressively in the
initial phase of exercise. In addition, in the studies by Grassi et al.
(10) and Hughson et al. (14), the response in
venous oxygen content was probably blunted by a significant amount of
blood coming from inactive tissues in the area drained by the vein. By
using an isolated muscle-exercise model and obtaining measurements of
blood transit times, it is possible to accurately determine the
O2 of the contracting muscles.
A controversial issue is whether oxygen delivery limits muscle oxygen
utilization at onset of exercise (25). There are a number
of studies supporting that local factors cause the delay in oxygen
utilization in the initial phase of exercise (8, 9, 27). For example, in recent studies using
an isolated in situ canine gastrocnemius muscle preparation, Grassi et
al. (8, 9) observed that elevated
O2 diffusion or O2 delivery did not change
oxygen kinetics in the initial phase of electrically induced muscle
contractions. On the other hand, there appear to be conditions where
O2 is related to oxygen supply
(7, 13, 14, 16). As
an example, Hughson et al. (14) studied intermittent static handgrip and found that both blood flow and
O2 of the underarm increased more
rapidly at the onset of exercise with the arm below compared with above
heart level. It is therefore still an open question as to whether
oxygen availability to contracting muscles influences
O2 kinetics at onset of dynamic exercise.
Thus the aims of the present study were to examine the onset and the
rate of rise of muscle O2 in the
exercising muscles in the initial phase of intense exercise and to
evaluate whether oxygen availability limits muscle
O2 in the initial phase of intense
exercise. Femoral venous blood flow as well as arterial and venous
oxygen content were measured in subjects performing intense
knee-extensor exercise. In addition, thigh-blood transit times were
determined to estimate the delay from the femoral artery to capillaries
and to the venous site of blood sampling. In the last 5 s before
exercise, the experimental leg was moved passively to increase blood
flow and oxygen availability of the contracting muscle
(20).
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SUBJECTS AND METHODS |
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TOP
ABSTRACT
INTRODUCTION
SUBJECTS AND METHODS
RESULTS
DISCUSSION
REFERENCES
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Subjects. Six healthy male subjects ranging in age from 21 to 24 yr, with an average height of 178 (range: 172-183) cm and an average body mass of 72.5 (67.9-78.3) kg, participated in the experiment. All subjects were habitually physically active, but none trained for competition. The subjects were fully informed of any risks and discomforts associated with these experiments before giving their informed consent to participate. The study was approved by the Frederiksberg, Copenhagen, Ethics committee.
Methods. Subjects performed a one-legged knee-extensor exercise in the supine position on an ergometer that permitted the exercise to be confined to the quadriceps muscle (1, 3). Before the experiment, the subjects had practiced the exercise on more than three separate occasions.
About 3 h before the experiment, the subjects had a light breakfast and they reported to the laboratory ~2 h before the experiment. After a period of rest in the supine position, a catheter was placed in a femoral artery under local anesthesia. The tip was positioned 1-2 cm proximal to the inguinal ligament. A catheter was also placed in the femoral vein of the experimental leg ~1-2 cm distal to the inguinal ligament. A thermistor for measurement of venous blood temperature was inserted through the catheter and was advanced 8-10 cm proximal to the tip. After the placement of the catheters, the subjects were moved to the experimental room (ambient temperature 20-22°C), and after ~1 h of rest, the subjects performed a 3-min knee-extensor exercise period with the experimental leg [65.3 ± 3.7 (± SE) W; kicking frequency 60 rpm]. On the basis of a number of preexperiments, the work intensity was selected so that the subject would have been exhausted within 4 min corresponding to a relative intensity ~120% of peak thighO2 (3).
Before the exercise, the leg was passively moved for 5 s to
accelerate the flywheel to obtain a constant power output from onset of
exercise and to increase blood flow. This resulted in a lowering of the
arterial-venous difference (a-vdiff) in O2
content, but no other effects of this procedure were observed, as can
be seen by comparing values obtained at rest with data obtained during
the passive exercise (
2 s) in the figures presented. After 60 min of
rest, the exercise protocol was repeated with the same leg to measure
thigh blood flow in the initial phase of exercise, because blood flow
could not be determined during the initial minute of the first exercise
bout due to the frequent sampling of venous blood. Blood was drawn from
the femoral artery 10 ± 1 and 5 ± 2 s before the
exercise and 2 ± 1, 6 ± 0, 10 ± 1, 14 ± 2,
29 ± 2, 45 ± 3, 58 ± 3, 89 ± 3, 119 ± 4, 145 ± 3, and 165 ± 2 s during exercise. Femoral venous blood was collected 10 ± 1 and 3 ± 1 s before the
exercise and 2 ± 1, 6 ± 1, 9 ± 1, 14 ± 1,
29 ± 1, 42 ± 2, 59 ± 4, 89 ± 4, 112 ± 3, 145 ± 2, and 167 ± 2 s during the exercise. All blood
samples were collected in 2-ml syringes and immediately placed in
ice-cold water until analyzed. In addition, femoral venous blood flow
was measured by the thermodilution technique (2)
approximately every 30 s after ~1 min of the first exercise bout
as well as 2 ± 0 s before and after 2 ± 0, 7 ± 1, 29 ± 2, 34 ± 2, 61 ± 1, 66 ± 1, 92 ± 2, and 162 ± 2 s of the second exercise bout. An occlusion
cuff placed just below the knee was inflated (220 mmHg) during the
exercise to avoid contribution of blood from the lower leg.
Blood analysis. Oxygen saturation and hemoglobin concentration of blood were determined spectrophotometrically (Radiometer OSM-3 Hemoximeter). The Hemoximeter was calibrated spectrophotometrically by the cyanomethemoglobin method (6). Hematocrit determinations were made in triplicate with the use of microcentrifugation. PO2, PCO2, and pH were measured with the Astrup technique (ABL 30, Radiometer, Copenhagen, Denmark).
Muscle mass. The mass of quadriceps femoris muscles was estimated by the use of magnetic resonance imaging. Briefly, for each subject 30-33 parallel axial T1-weighed sections of the right thigh were obtained with a multislice spin-echo FLASH sequence (TR = 500 ms, TE = 15 ms) with the use of a body coil. Slice thickness was 3 mm with a 12-mm interslice gap. Pixel size was 1.2 mm2. This setting was selected to optimize image quality to clearly separate muscle, bone, fat, and connective tissue. Image analysis was performed with the use of NIH Image software. The mean knee-extensor mass of the experimental leg was 2.35 kg, with a range of 1.94-2.79 kg.
Blood transit times. To determine the transit time of the blood from the femoral artery to the collecting site in the femoral vein, on a separate day, four of the subjects in the original experiment and three additional subjects carried out the same 3-min exercise with the experimental setup being identical to the main experiment. Before and frequently during the exercise, 2 mg indocyanine green (ICG, Becton Dickinson) in a concentration of 5 mg/ml was injected rapidly into the femoral artery, immediately followed by a flush with isotonic saline (5 ml). Blood was withdrawn from the femoral vein at a speed of 30 ml/min for measurements of ICG concentration with a linear densitometer. The densitometer output was sampled with a computer (5 Hz). The time from injection to the time when the curve peaked, corrected by transit time of catheter (the dead space of the catheters divided by the pump flow), was used as the mean transit time (MTT).
In one of the experiments and in three additional experiments in which MTT was also measured, the transit time from arterial infusion of ICG to appearance in the muscle microcirculation was determined. A NIRO300 (Hamamatsu Photonics) with dual channel near infrared laser diodes was used for optical measurements at four positions of the quadriceps muscle, namely at a proximal and distal portion of m. vastus lateralis as well as at a medial portion of m. rectus femoris and a distal portion of m. vastus medialis. The optodes were placed over the long axis of the muscles, and an algorithm incorporating the specific extinction coefficients for ICG and the modified Beer-Lambert Law (5) at wavelengths of 775, 826, 850, and 910 nm was used to calculate the change in the concentration of ICG from light attenuation. Measurements were performed during passive exercise and after 6, 16, 30, 60, and 120 s of exercise and expressed relatively to MTT. An example is shown in Fig. 1. With the use of the relative values of transit time from the femoral artery to muscle microcirculation and MTT for each individual, the average time to which the collected artery and venous blood represented capillary blood was estimated. Average values of MTT were used for the two subjects where MTT was not determined. All blood variables are presented in relation to mean time at the capillary level.
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Calculations.
Thigh O2 was calculated by multiplying
the thigh blood flow with the difference between femoral artery and
venous (a-vdiff) O2 content. Blood flows
obtained at the same time during the first and second exercise were in
agreement (67 s: 4.36 ± 0.30 vs. 4.45 ± 0.23 l/min; 94 s: 4.64 ± 0.52 vs. 4.91 ± 0.31 l/min; 159 s: 6.03 ± 0.76 and 5.81 ± 0.73 l/min), and values for the second exercise period were used in the calculations. A continuous blood flow
curve was constructed for each subject by linear connection of the
consecutive data points, and the blood flow at the time of obtaining
blood samples was estimated on the basis of simple proportional calculations.
Statistics. One-way ANOVA with repeated measures was used for evaluation of changes during the exercise. If a significant value was observed, then the Newman-Keuls post hoc test was used to locate the differences. A significance level of 0.05 was chosen. Standard error of the mean (± SE) is only given in the text where this value cannot be obtained from a figure.
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RESULTS |
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TOP
ABSTRACT
INTRODUCTION
SUBJECTS AND METHODS
RESULTS
DISCUSSION
REFERENCES
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Blood transit times and thigh blood flow.
MTT (n = 7) was 15.3 ± 1.8 s immediately
before the exercise and decreased (P < 0.05) to
10.4 ± 0.7 s after 6 s and 5.9 ± 0.4 s after
35 s of exercise with a small further decline (P < 0.05) to 4.7 ± 0.5 s at the end of exercise (Fig.
2). Transit time from the femoral artery
to the muscle microcirculation (n = 4) was 5.8 ± 0.6 s (39 ± 4% of MTT) immediately before the exercise and
decreased (P < 0.05) to 3.2 ± 0.3 (35 ± 3% of MTT), 2.4 ± 0.2 (40 ± 4% of MTT), 1.9 ± 0.1 (39 ± 2% of MTT), and 1.8 ± 0.1 s (45 ± 2% of
MTT) after 7 ± 1, 18 ± 1, 46 ± 3, and 113 ± 9 s of exercise, respectively. With the use of these values and
MTT for each subject, the average time to which the collected artery and venous blood represented capillary blood was estimated (Fig. 2).
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O2.
Arterial oxygen content of 189 ml/l immediately before exercise
increased (P < 0.05) during the exercise reaching 201 ml/l (Fig. 4A). Femoral venous
oxygen content increased (P < 0.05) from 145 ml/l at
rest to 172 ml/l immediately before the exercise, and it decreased
(P < 0.05) within the first 6 s of exercise being 146 ml/l after 6 s, 67 ml/l after 42 s, and 55 ml/l at the
end of exercise (Fig. 4A).
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O2 was 32 ml/min
immediately before the exercise, and it increased (P < 0.05) to 102 ml/min after 6 s, reaching 536 ml/min after 42 s
of exercise and 789 ml/min at the end of exercise (Fig. 4C).
The time to reach 50 and 90% of peak thigh
O2 was 25 ± 3 and 101 ± 10 s, respectively.
The difference between thigh oxygen delivery (thigh blood flow × arterial oxygen content) and O2 of 291 ml/min before exercise increased (P < 0.05) to 367 ml/min at onset of exercise (Fig. 5). It
decreased (P < 0.05) to preexercise level after
14 s of exercise and remained constant throughout the rest of
exercise.
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Blood gasses and pH.
Venous blood PO2 was 38.3 mmHg at rest, and it
increased (P < 0.05) to 53.8 mmHg before the exercise.
During the exercise, it decreased (P < 0.05) to 19.5 mmHg after 29 s; thereafter it remained constant (Fig.
6A). Arterial
PO2 rose (P < 0.05) during the
first 6 s of exercise and then declined (P < 0.05).
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DISCUSSION |
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TOP
ABSTRACT
INTRODUCTION
SUBJECTS AND METHODS
RESULTS
DISCUSSION
REFERENCES
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The present study shows that O2 by
the contracting muscle increases within only a few seconds upon onset
of exercise and that muscle oxygen delivery markedly exceeds
O2, suggesting an intracellular
limitation in muscle oxygen extraction (Figs. 4 and 5). These findings
suggest that oxygen utilization of the contracting muscles is much
faster than suggested previously, and it may even be faster than
revealed in the present study, because oxygen bound to myoglobin is a
likely source of oxygen in the very early phase of exercise.
Muscle O2 at onset of exercise.
It was observed that oxygen extraction (femoral a-vdiff
O2) was elevated after a few seconds (<6 s) of exercise.
In contrast, Grassi et al. (10) found femoral
a-vdiff O2 being unaltered during the first
12 s in the transition from very low to low intensity cycle
exercise. The difference can be explained by the fact that the transit
time of the blood in the exercising leg was not taken into account in
the latter study. When blood samples collected from the femoral artery
and vein at the same time in the present study were used for the
calculation of a-vdiff O2, as done by Grassi et
al. (10), it was observed that a-vdiff
O2 and O2 were not different
from rest until 13 s of exercise (Fig.
7). This is a value similar to that
obtained by Grassi et al. (10), and it shows the
importance of making the correction. Thus the present data suggest that
muscle oxidation starts within a few seconds of exercise and most
likely even faster because there is probably also a significant
utilization of oxygen bound to myoglobin in the first phase of
exercise. In support of the latter suggestion, it has been observed
that half of the stored myoglobin-associated oxygen, determined by
1H nuclear magnetic resonance spectroscopy, was used within
20 s of dynamic exercise onset (21).
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O2 of the
nonmuscle tissues and hamstring/adductor muscles are maintained throughout the exercise. It is clear that Q a-vdiff
O2 is greater than a-vdiff O2 for
the entire thigh, but it follows the same pattern of increase (Fig.
8), resulting in a similar time to
half-peak extraction (11 ± 2 s). Thus, after a delay of only
3 s, there is a pronounced increase in the extraction of oxygen
from hemoglobin by the exercising muscles, but it takes ~50 s
before the extraction is maximal.
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O2 also increased
rapidly, and the time to reach half-peak thigh
O2 was 25 s (Fig. 4). Values of the
same magnitude were obtained by Grassi et al. (10) and
also by Hughson et al. (14) using rhythmic intermittent
static handgrip contractions (work:rest ratio 1:2). A direct comparison
among these studies cannot be made, however, because in the latter
studies the transit time of blood was not determined and the degree of
perfusion to nonactive tissues could not be evaluated. Furthermore, in
the study by Hughson et al. (14), it was unclear whether
the blood collected from a forearm vein in the antecubital fossa area
represented the venous blood from the active muscles (4).
Their finding of a rather low maximal oxygen extraction of ~131 ml/l
could suggest that there was a considerable contribution from nonactive
tissues during the contraction.
Limitation in muscle respiration in the initial phase of exercise.
It is commonly discussed whether oxygen supply limits muscle
O2 in the initial phase of exercise
(25). In the present study, thigh blood flow was elevated
before the exercise due to the passive movements (without increasing
thigh O2), and it increased rapidly
during the first phase of exercise leading to a high delivery of oxygen
to the exercising muscle from the onset of exercise. It should also be
noted that the femoral venous blood flow represents a delayed response
of the perfusion of the thigh. This can explain why the time to reach
half peak of 12 s was somewhat greater than the 9 s observed
when measuring arterial blood flow at the start of one-legged
knee-extensor exercise at a similar intensity (20). Thus
the rise in perfusion of the thigh was probably even faster than that
reflected in the femoral venous blood flow. The difference between
thigh oxygen delivery and O2 was
greatest in the initial phase of exercise, and it became reduced to a
constant level after 14 s of exercise (Fig. 5). These findings indicate that oxygen supply is in excess of demand in the initial phase
of dynamic exercise and that oxygen delivery is not limiting for
O2 of the contracting muscles. It cannot
be excluded, however, that a nonmaximal oxygen extraction by the
contracting muscle in the initial phase of exercise is due to an
inefficient flow distribution, i.e., hyperperfusion in areas of the
muscle that were inactive. A spatial and temporal heterogeneity of
blood flow within contracting muscles has been observed in animal
studies (18). However, the difference between oxygen
delivery and O2 reached preexercise
level after 14 s, before thigh O2
was maximal (Figs. 4 and 5). This is in accordance with the observation
in dog muscle that capillary recruitment is almost complete after 15 s of exercise (11). Therefore, it is likely that
the pattern of blood flow is not the only cause of the reduced
oxygen extraction in the initial phase of exercise.
O2 kinetics at the onset of dynamic exercise. It should be noted that there appear to be conditions where
oxygen availability can reduce oxygen utilization. MacDonald et al.
(16) observed that leg blood flow and pulmonary
O2 increased at a slower rate when knee
extensor/flexor exercise was performed in the supine position compared
with an upright position. Altered oxygen availability to working
muscles has also been shown to cause a change in pulmonary
O2 kinetics under other conditions such
as hypoxia (7, 15, 19) or
-blockade (13). However, the present data show that,
under normal conditions, oxygen supply does not limit oxygen
utilization during concentric exercise. Furthermore, blood flow in the
exercising thigh becomes adjusted in the first phase of exercise maybe
to reduce perfusion of nonactive tissues.
Thus these findings suggest a limitation in the oxygen utilization of
the contracting muscle cells. The cause of this phenomenon remains to
be elucidated. One mechanism potentially involved is an insufficient
provision of acetyl-CoA for the tricarboxylic acid (TCA) cycle as a
result of a delayed increase in the activity of pyruvate dehydrogenase
(PDH; 24). The end result is a delayed provision of reducing
equivalents to the electron transport chain. A role for PDH as a
limiting factor for muscle O2 has been
suggested by several recent studies, which employed the pharmacological agent dichloroacetate (DCA). DCA administration was shown to markedly increase the active fraction of PDH in skeletal muscle and to attenuate
markers of anaerobic ATP provision during the rest to work transition
in both animals and humans (12, 24). However, the effect of elevating the activity of PDH on muscle
O2 has not been investigated.
In summary, the present data show that
O2 of the contracting muscles increases
after only a few seconds of exercise and that the time to reach 50 and
90% of peak oxygen extraction is 13 and 51 s, respectively. The
limited oxygen utilization in the initial phase of intense exercise
does not appear to be caused by insufficient oxygen availability, but
it may rather be due to a nonoptimal distribution of blood flow in the
exercising muscles and to a limited extraction of oxygen by the
contracting muscle cells.
Perspectives
The present study illustrates the importance of being able to determine blood transit times when muscle oxygen kinetics is studied. It shows that oxygen utilization of the contracting muscles is much faster than what was suggested previously. This means that to study, either in vitro or in vivo, initiation and regulation of muscle respiration at the onset of exercise, focus has to be on compounds that are changing rapidly. Future work is necessary to ascertain whether reducing equivalents provided from the TCA cycle are rate limiting forO2. It is also important to obtain information on blood flow heterogeneity in muscle and the impact of
blood flow distribution on O2 and
metabolism, which will add substantially to our understanding of the
regulation of oxidative metabolism.
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ACKNOWLEDGEMENTS |
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The authors thank Merete Vannby, Ingelise Kring, and Winnie Taagerup for technical assistance. They also thank Marcus Novak for performing the magnetic resonance imaging measurements.
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FOOTNOTES |
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The study was supported by a grant from The Danish National Research Foundation (504). In addition, support was obtained from Team Danmark and The Sports Research Council (Idrættens Forskningsråd). J. González-Alonso was supported by a Marie Curie Research Training Grant.
Address for reprint requests and other correspondence: J. Bangsbo, The August Krogh Institute, LHF, Universitetsparken 13, DK-2100 Copenhagen, Denmark (E-mail: JBANGSBO{at}AKI.KU.DK).
The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.
Received 10 January 2000; accepted in final form 10 April 2000.
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TOP
ABSTRACT
INTRODUCTION
SUBJECTS AND METHODS
RESULTS
DISCUSSION
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) and femoral venous (
)
oxygen content (A), thigh oxygen extraction (B),
and thigh oxygen uptake (
)
correction for blood transit times. Means ± SE are given.
