Copenhagen Muscle Research Centre, The August Krogh Institute, University of Copenhagen, DK-2100 Copenhagen, Denmark
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ABSTRACT |
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The aim of the present
study was to examine whether ATP production increases and mechanical
efficiency decreases during intense exercise and to evaluate how
previous exercise affects ATP turnover during intense exercise. Six
subjects performed two (EX1 and EX2) 3-min one-legged knee-extensor
exercise bouts [66.2 ± 3.9 and 66.1 ± 3.9 (±SE) W]
separated by a 6-min rest period. Anaerobic ATP production, estimated
from net changes in and release of metabolites from the active muscle,
was 3.5 ± 1.2, 2.4 ± 0.6, and 1.4 ± 0.2 mmol
ATP · kg dry wt1 · s
1
during the first 5, next 10, and remaining 165 s of EX1,
respectively. The corresponding aerobic ATP production, determined from
muscle oxygen uptake, was 0.7 ± 0.1, 1.4 ± 0.2, and
4.7 ± 0.4 mmol ATP · kg dry
wt
1 · s
1, respectively. The mean
rate of ATP production during the first 5 s and next 10 s was
lower (P < 0.05) than during the rest of the exercise
(4.2 ± 1.2 and 3.8 ± 0.7 vs. 6.1 ± 0.3 mmol
ATP · kg dry wt
1 · s
1).
Thus mechanical efficiency, expressed as work per ATP produced, was
lowered (P < 0.05) in the last phase of exercise
(39.6 ± 6.1 and 40.7 ± 5.8 vs. 25.0 ± 1.3 J/mmol
ATP). The anaerobic ATP production was lower (P < 0.05) in EX2 than in EX1, but the aerobic ATP turnover was higher
(P < 0.05) in EX2 than in EX1, resulting in the same muscle ATP production in EX1 and EX2. The present data suggest that the
rate of ATP turnover increases during intense exercise at a constant
work rate. Thus mechanical efficiency declines as intense exercise is
continued. Furthermore, when intense exercise is repeated, there is a
shift toward greater aerobic energy contribution, but the total ATP
turnover is not significantly altered.
blood flow; aerobic exercise; anaerobic ATP production; lactate; creatine phosphate
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INTRODUCTION |
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MUCH KNOWLEDGE
EXISTS about energy production during submaximal exercise, i.e.,
a work intensity lower than that eliciting maximum oxygen uptake
(O2 max) (36).
Less information has been obtained about ATP turnover and mechanical
efficiency during supramaximal exercise. This may be due to the
difficulties in quantifying the anaerobic energy production and
determining oxygen utilization of the exercising muscles during this
type of exercise.
To estimate the anaerobic energy production, muscle biopsies have been obtained before and after intense dynamic exercise. On the basis of the decrease in muscle ATP and creatine phosphate (CP), as well as accumulation of metabolites like pyruvate and lactate, the anaerobic energy production of the biopsied muscle has been estimated in several studies that employ cycle exercise (13, 35, 37). It is, however, difficult from measurements on muscle biopsy material to determine the anaerobic energy turnover during whole body exercise such as cycling, because the mass and the activity of the muscles involved are unknown. Furthermore, the metabolic response of the biopsied muscle may not be representative of all of the muscles included in the exercise. Another problem is that the release of metabolites into the blood from the exercising muscles is often not taken into account when energy turnover is calculated, although this may represent a substantial contribution to the total energy production when the exercise lasts more than a few seconds (4).
In a few studies, the aerobic contribution to the energy turnover of
the exercising muscles during intense and maximal cycle exercise has
been determined through measurements of the pulmonary oxygen
uptake (O2) (35,
37). However, such estimations require that the mass of the
active muscles be known. Furthermore, it is assumed that the pulmonary
O2 represents the
O2 of the exercising muscle, but this
assumption is not valid in the initial phase of intense exercise, as
there is a time delay in the transport of blood from the exercising
muscle to the lungs (9). The problems in determining the
aerobic and anaerobic ATP and energy yield are minimized by using a
knee-extensor exercise model in which the exercise is confined to the
quadriceps muscle, allowing a rather precise determination of the mass
of the active muscle (3, 4). Furthermore, by insertion of
catheters in the femoral blood vessels, arterial as well as venous
blood draining the exercising muscle can be collected, and blood flow
to the exercising muscle can be measured. The model has been used to
quantify the aerobic and anaerobic energy contribution during an
intense exercise bout (6). However, little information
exists about the muscle metabolic changes and
O2 at the onset of exercise. In a recent
study it was observed that heat production increased during intense
knee-extensor exercise (24), which in part may be related
to differences in the efficiency of the metabolic reactions involved in
the exercise (46). However, it is not clear to what extent
the various metabolic pathways are contributing to ATP turnover
throughout the exercise and whether part of the increase in heat
production is due to an increase in the rate of ATP production during
intense exercise.
Various studies have focused on energy turnover during repeated maximal
exercise, and it has been suggested that the energy turnover per work
unit is lowered when intense exercise is repeated (22,
41). However, in these studies the aerobic energy turnover was
not determined, and the work produced decreased when the exercise was
repeated, which makes it difficult to relate the metabolic changes to
the development of force. In studies using repeated knee-extensor
exercise with a constant exercise intensity, it has been observed that,
when a second exercise bout was performed after both a 2.5-min and a
60-min rest period, the anaerobic energy production per work unit was
significantly reduced without a change in leg
O2 (7, 8). However, in
these studies, the muscle
O2 in the
initial phase of exercise could not be determined accurately because of
the limited number of measurements.
Thus the aim of the present study was to examine whether ATP turnover increases and mechanical efficiency decreases during supramaximal concentric exercise and to evaluate whether previous exercise altered the ATP production and mechanical efficiency. Subjects performed a 3-min intense knee-extensor exercise bout at a supramaximal work rate, which was repeated after a 6-min rest period. Arterial and femoral venous blood samples were collected, and leg blood flow was measured frequently during the exercise. In addition, a muscle biopsy was taken before and immediately after the exercise and on a separate occasion also twice in the initial phase of exercise.
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SUBJECTS AND METHODS |
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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. The subjects were fully informed of any risks and discomforts associated with the experiments before giving their informed consent to participate. The study was approved by the Frederiksberg, Copenhagen Ethics Committee.Methods
During the experiment, subjects performed one-legged knee-extensor exercise in the supine position on an ergometer that permitted the exercise to be confined to the quadriceps muscle (2). Before the experiment, the subjects had practiced the exercise on more than three separate occasions.The subjects had a light breakfast ~3 h before an experiment, and they reported to the laboratory ~2 h before the experiment. With the subject 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 ~1 h of rest, the subject performed two 3-min knee-extensor
exercise bouts (EX1 and EX2) with the experimental leg [66.2 ± 3.9 and 66.1 ± 3.9 (±SE) W; kicking frequency: 60.7 ± 0.2 and 60.6 ± 0.2 kicks/min] separated by a 6-min rest
period (Fig. 1). Before each of the
exercise bouts, the leg was passively moved for 5 s to accelerate
the flywheel to obtain a constant power output from onset of exercise.
After 60 min of rest, the entire exercise protocol was repeated with
the same leg to allow for early measurements of thigh blood flow (EX3
and EX4).
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Blood was drawn from the femoral artery ~10 and 5 s before as well as 2, 5, 10, 15, 30, 45, 60, 90, 115, 150, and 170 s during EX1 and EX2. Femoral venous blood was collected ~15 and 5 s before as well as 2, 6, 9, 14, 29, 59, 89, 113, 145, and 167 s during EX1 and EX2. Femoral venous blood flow was measured by the thermodilution technique (3) approximately every 30 s after ~1 min of EX1 and EX2, as well as 3 s before and 3, 7, 29, 34, 61, 66, 92, and 162 s during EX3 and EX4 performed after 1 h of rest.
Values of blood flow obtained at the same time during the first two exercise bouts (EX1 and EX2) agreed with the corresponding values of the two bouts after 60 min of rest (EX3 and EX4) [100 s: 4.83 ± 0.46 (EX1) vs. 5.13 ± 0.44 (EX3) l/min; 150 s: 5.63 ± 0.70 (EX1) vs. 5.39 ± 0.58 l/min (EX3); 100 s: 5.19 ± 0.64 (EX2) vs. 5.38 ± 0.59 (EX4) l/min; 150 s: 5.51 ± 0.63 (EX2) vs. 5.71 ± 0.61 (EX4) l/min]. Thus blood flow values after the 60-min rest period were used in the calculations. An occlusion cuff placed just below the knee was inflated (220 mmHg) during the exercises to avoid contribution of blood from the lower leg. Before and immediately after each of the exercise bouts, a biopsy was obtained from the m. vastus lateralis. On a separate occasion, the subjects performed the same knee-extensor exercise as in the main experiment for 5 and 15 s, i.e., 5 and 15 kicks, separated by a 45-min rest period, and a muscle biopsy was obtained at rest and after each of the exercise bouts.
Blood analysis.
Oxygen saturation of blood and hemoglobin concentration were determined
spectrophotometrically (Radiometer OSM-3 hemoximeter). The hemoximeter
was calibrated spectrophotometrically by the cyanomethemoglobin method
(18). Hematocrit (Hct) determinations were made in
triplicate by use of microcentrifugation. A part of the blood sample
(100 µl) was hemolyzed within 10 s of sampling by a 1:1 dilution
with a buffer solution (Yellow Spring Instruments, Yellow Springs, OH)
to which were added 20 g/l Triton X-100 for analysis of lactate with a
lactate analyzer (model 23, Yellow Spring Instruments) (21). Another part of the blood sample was immediately
placed in ice-cold water and centrifuged rapidly for 30 s. Then
the plasma was collected and stored at 80°C until analyzed for
pyruvate using a fluorometric assay (34).
Muscle sampling and analysis.
Muscle biopsies were immediately frozen in liquid N2 and
stored at 80°C. The frozen muscle samples were weighed before and after freeze-drying to determine water content. The freeze-dried sample
was dissected free of blood and connective tissue and extracted in a
solution of 0.6 M perchloric acid (PCA) and 1 mM EDTA, neutralized to
pH 7.0 with 2.2 M KHCO3, and stored at
80°C until
analyzed for CP and lactate by fluorometric assays (34).
Muscle mass. The mass of quadriceps femoris muscles was estimated by use of magnetic resonance imaging. Briefly, for each subject, 30-33 parallel axial T1-weighed images (sections) of the right thigh were obtained with a multi-slice spin-echo FLASH sequence (repetition time = 500 ms, echo time = 15 ms) by 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 using 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.
Calculations.
O2 and lactate and pyruvate release by
the thigh were calculated by multiplying the blood flow or, for
pyruvate, the plasma flow, by the difference between the femoral venous
and arterial (v-adiff) concentrations, with blood transit
time taken into account (see the next paragraph). A continuous blood
flow curve was constructed for each subject by linear connection of the
consecutive data points to obtain time-matched values of blood flow
with the blood variables.
Statistics
Two-way ANOVA with repeated measures was used for evaluation of changes during the exercises as well as between EX1 and EX2. If a significant F value was observed, then the Newman-Keuls post hoc tests were used to locate the differences. A significance level of 0.05 was chosen. Standard error of the mean (±SE) is given in the text only when this value cannot be obtained from data in Figs. 1-7.
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RESULTS |
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Anaerobic Energy Production
Muscle CP of 79.8 ± 3.8 mmol/kg dry wt at rest decreased (P < 0.05) by 14 and 23% during the first 5 and 15 s, respectively, reaching 23.6 ± 11.3 mmol/kg dry wt at the end of EX1. The rate of CP degradation was higher (P < 0.05) during the first 5 and after 10 s than during the remaining 165 s (Fig. 2A). Muscle CP had returned to resting level before EX2, and it decreased to the same level as during EX1 (24.2 ± 2.3 mmol/kg dry wt). Muscle ATP decreased (P < 0.05) from 23.8 ± 0.6 to 20.7 ± 0.6 mmol/kg dry wt during EX1.Muscle lactate was 4.7 ± 0.7 mmol/kg dry wt at rest, and it
increased (P < 0.05) in the first 5 and 15 s of
EX1 to 9.2 ± 2.5 and 13.3 ± 4.1 mmol/kg dry wt,
respectively. At the end of EX1, muscle lactate was 84.1 ± 12.8 mmol/kg dry wt. The rate of muscle lactate accumulation was 0.9 ± 0.5 mmol · kg dry wt1 · s
1
during the first 5 s, which was not significantly different from the remaining part of EX1 (Fig. 2B). Muscle lactate at the
end of EX2 (77.5 ± 8.0 mmol/kg dry wt) was the same as at the end of EX1, but it was higher (P < 0.05) before EX2
(27.1 ± 4.3 mmol/kg dry wt) than before EX1. Thus muscle lactate
accumulation was lower (P < 0.05) in EX2 than in EX1
(Fig. 2B).
Thigh blood flow was 1.71 ± 0.09 l/min immediately before EX1, and it increased rapidly during exercise, reaching 3.85 ± 0.35 l/min after 29 s and 5.39 ± 0.82 l/min at the end of exercise. Before EX2, thigh blood flow (3.13 ± 0.33 l/min) was higher (P < 0.05) than before EX1, and it remained higher (P < 0.05) until 165 s of exercise.
Estimated net release of lactate (blood flow × v-adiff lactate) was significant after 14 s (1.7 mmol/min), and it increased (P < 0.05) to 12.7 mmol/min during exercise (Fig. 3). The mean rate of lactate efflux was lower (P < 0.05) during the first 5 s and next 10 s than during the remaining 165 s of exercise (Fig. 2B). In EX2 the release of lactate was higher (P < 0.05) during the first 9 s compared with EX1, whereas after 60 s and toward the end of exercise, lactate release was higher (P < 0.05) in EX1 than in EX2 (Fig. 3). The total release of lactate during EX1 was higher (P < 0.05) than in EX2 (Fig. 2B).
The rate of lactate production by the quadriceps muscle, determined as the sum of the rates of lactate release and lactate accumulation, was not different in the various phases of exercise (Fig. 2B). The mean rate of lactate production was less (P < 0.05) during EX2 than during EX1 (Fig. 2B).
A significant (P < 0.05) net release of pyruvate was
observed after 14 s of EX1 (45 ± 13 µmol/min), and it
peaked after 59 s at 183 ± 35 µmol/min. The mean rate of
pyruvate release was lower (P < 0.05) during the first
5 s and after 10 s than during the rest of EX1 (6 ± 8 and 24 ± 10 vs. 141 ± 26 µmol/min). In EX2, a release
(P < 0.05) was observed before and during the first 6 s, but after 14 s and throughout the rest of exercise,
pyruvate release was less in EX2 than in EX1. The total release of
pyruvate was less (P < 0.05) in EX2 than in EX1
(0.39 ± 0.07 vs. 0.17 ± 0.05 mmol).
The rate of CP utilization during the first 5 s, next 10 s,
and remaining 165 s corresponded to rates of ATP production of 1.5 ± 0.6, 1.1 ± 0.4, and 0.2 ± 0.1 mmol
ATP · kg dry wt1 · s
1,
respectively (Fig. 4). For the same time
periods, lactate production corresponded to rates of ATP production of
1.4 ± 0.7, 0.7 ± 0.3, and 1.0 ± 0.1 mmol
ATP · kg dry wt
1 · s
1,
respectively (Fig. 4). By adding ATP production associated with pyruvate release and other sources (see METHODS), the rate
of total anaerobic ATP production was estimated. It was higher
(P < 0.05) during 0-5 and 5-15 s than during
the rest of EX1 (Fig. 4). The mean rate of anaerobic ATP production was
less (P < 0.05) in EX2 than in EX1 (1.2 ± 0.1 vs. 1.6 ± 0.1 mmol ATP · kg dry wt
1 · s
1).
Aerobic Energy Production
ThighATP Production and Mechanical Efficiency
The mean rate of ATP production (determined as the sum of the rates of anaerobic and aerobic ATP production) was 4.2 ± 1.2 and 3.8 ± 0.7 mmol ATP · kg dry wtThe mean rate of ATP production was the same in EX1 and EX2 (6.0 ± 0.4 vs. 6.3 ± 0.5 mmol ATP · kg dry
wt1 · s
1), but a greater
(P < 0.05) fraction was provided from anaerobic sources during EX1 than during EX2 (27 ± 3 vs. 20 ± 1%;
Fig. 5). Thus the average work output per ATP production was not
different between EX1 and EX2 (Fig. 6).
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DISCUSSION |
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The present data show that the rate of ATP turnover increases during intense exercise performed at a constant work rate. Thus mechanical efficiency declines as intense exercise is continued. Furthermore, when intense exercise is repeated within some minutes, there is a shift toward greater aerobic energy contribution, but the total ATP production and mechanical efficiency are not significantly altered.
ATP Production During Intense Exercise
It was observed that ATP utilization during a constant load exercise was ~55% higher during the last phase of exercise compared with the first 15 s, which resulted in a decrease in the mechanical efficiency from 39 to 25 J/mmol ATP. It should, however, be considered how reasonable the estimations of ATP turnover are. The most critical assumption in the calculations is the P/O, which on the basis of in vitro studies on rat liver mitochondria has been suggested to be 2.5 (28). It should be noted that, if the in vivo P/O had been as unrealistically low as 0.9, there would not have been a difference between the first 15 s of exercise and the remainder of exercise. Likewise, a fall in P/O from 2.5 at the start to 1.2 at the end of exercise would be required to make the work performed per ATP produced constant throughout the exercise. However, even though there are indications from in vitro studies that mitochondrial respiration can be partly uncoupled at an increased intercellular Ca2+ concentration (25), it is unlikely that P/O changes that much. This notion is supported by the observation that P/O determined in mitochondria isolated from a human muscle biopsy obtained immediately after intense exhaustive exercise was the same as at rest (U. F. Rasmussen, P. Krustrup, J. Bangsbo, and H. N. Rasmussen, unpublished observation). The contribution of oxygen released from Mb is also uncertain, but because the assumed 50% reduction in MbO2 used in the present study may be in the high range (43), the difference in ATP turnover between the first and later phases of exercise would be even greater if the MbO2 utilized was actually smaller. Together, these considerations suggest that it is reasonable to conclude that the ATP production per work unit is lower in the first period of exercise. It should also be noted that the value for the last phase of exercise is of the same magnitude as the value found in animal studies using isolated muscle [for examples, 13-24 J/mmol ATP (17); 27 J/mmol ATP (33)].The question is what causes the change in ATP production per work unit during concentric exercise. On the basis of in vitro studies on isolated muscles, it has been suggested that factors such as lowered pH, as well as elevated temperature and increased Pi levels, lower mechanical efficiency of contracting muscles through an increase in both Ca2+-ATPase and myosin-ATPase activity (12, 15, 16, 47).
In the present study, muscle lactate increased progressively during the
intense exercise, leading to an estimated decrease in pH from 7.1 to
~6.7 (29), which may have affected mechanical efficiency. In several human studies, it has been observed that O2 progressively increases when
submaximal exercise is continued, but only at intensities leading to a
significant accumulation of lactate in the blood (23), and
it has been suggested that lowered muscle pH elevates
O2 and thus decreases mechanical efficiency. However, the finding of the same ATP turnover during EX1
and EX2, even though muscle pH probably was lower in the initial phase
of EX2 (7), does indicate that any effect of lowered muscle pH is small. In accordance, in a previous study, the rate of ATP
turnover was found to be the same whether intense knee-extensor exercise was preceded without or with intense arm exercise, with the
latter condition leading to a further reduction of muscle pH
(10). However, in that study and during EX2 in the present study, muscle pH may only during a minor fraction of the exercise have
been lower than in EX1; therefore, it cannot be excluded that pH has an
effect on ATP turnover and mechanical efficiency during intense exercise.
In separate experiments, it was observed that the muscle temperature
increased from ~34.5 to 36.0°C during intense exercise at an
intensity similar to that used in the present study. It is possible
that the increase in temperature affected the mechanical efficiency,
because, among other effects, it has been shown in vitro to decrease
P/O (45), causing an increase in
O2 solely to maintain the rate of ATP
production. During submaximal cycle exercise at a frequency of 60 rpm,
it has been observed that
O2 was higher
when muscle temperature was elevated by prior passive heating
(20). However, in that study the difference was only ~5%, suggesting that any effect of temperature was small, which is
supported by the fact that total ATP turnover and mechanical efficiency
were the same during EX1 and EX2 in the present study, although the
mean temperature was ~0.5°C higher in EX2 (data not shown). An
increase in ATP turnover during the exercise may also be related to a
lowering of molar free energy in ATP hydrolysis (
G) caused by
increases in metabolites like ADP and Pi, as well as
lowering of pH and increasing temperature. It is, however, unclear
whether a lowering of
G will change the work done per ATP split
(46).
An increase in the total muscle ATPase activity as exercise progressed may have been due to recruitment of more fibers or more ATP-consuming fibers, i.e., fibers that have a high ATP utilization per force produced. However, it is not established whether there is a difference in efficiency between the various fiber types during concentric contractions in vivo. In in vitro studies, with use of either isolated muscles or single muscle fibers, it is observed that the efficiency of the different fiber types is closely related to the velocity, pattern, and type of contraction (11, 12, 14, 17). At low contraction velocities, the efficiency of slow-twitch (ST) fibers is higher than for fast-twitch (FT) fibers, whereas it appears to be the opposite at high speeds (11). In the present study, the quadriceps muscle was contracting at an average speed of ~135°/s, corresponding to ~20% of maximal velocity (1), which has been suggested to be within the optimum speed of ST fibers and nonoptimal for FT fibers (11, 17). Thus the higher ATP utilization during the last phase of exercise may be explained by a greater recruitment of FT fibers. However, further studies are needed to examine fiber type recruitment and efficiency of the different fibers during intense exercise.
ATP Production During Repeated Intense Exercise
When the intense exercise was repeated, muscleEnergy Production During Intense Exercise
On the basis of net change in reactant levels of the metabolic reactions, it is possible to estimate the total energy production by use of average values of energy produced in each of the reactions determined in vitro. A molar enthalpy change (Since Krogh and Lindhard (32) in the early part of the
last century determined the oxygen deficit as the difference between energy demand and O2, oxygen deficit has
been frequently used as a measure of the anaerobic energy production
(30, 35). The calculations have been based on the
assumptions that the relation between energy turnover and power output
is linear from moderate submaximal to supramaximal exercise, i.e., a
work output higher than the intensity eliciting
O2 max, and that the energy turnover is
constant during intense exercise. The first assumption has been
questioned (4), and the present study demonstrates that
the latter assumption is not valid. Apparently, in the initial phase of
intense exercise, the mechanical efficiency of the exercising muscle is
higher than during steady-state submaximal exercise, whereas as the
intense exercise progresses, it may be lower compared with submaximal
exercise. This means that the oxygen deficit determined in the
traditional way overestimates the anaerobic energy turnover if the
exercise time is short (<2 min). Thus, for exhaustive intense exercise, the magnitude of the true oxygen deficit depends on the
duration and intensity of the exercise. This can also explain the
observation of an agreement between the oxygen deficit and the
anaerobic energy production determined from muscle and blood metabolic
measurements for an exhaustive knee-extensor exercise bout lasting 3.2 min (6). It may be that the magnitude of overestimation of
the true anaerobic energy turnover by the oxygen deficit method in the
first phase of the exercise corresponded to the underestimation in the
last phase of exercise.
Summary
The present data show that ATP production increases and mechanical efficiency decreases during intense exercise at a constant intensity, which may in part be due to a change in fiber type recruitment, an elevated temperature, and a lowered pH. However, further studies are needed to clarify the cause of the increase in ATP production per work unit during intense exercise and its functional importance. When intense exercise is repeated, the total ATP turnover is not changed if the exercise duration is the same. ![]() |
ACKNOWLEDGEMENTS |
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We thank Merete Vannby, Ingelise Kring, and Winnie Tagerup for excellent technical assistance.
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FOOTNOTES |
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This study was supported by a grant from The Danish National Research Foundation (504-14). In addition, support was obtained from Team Danmark and The Sports Research Council (Idrættens Forskningsråd).
Address for reprint requests and other correspondence: J. Bangsbo, The August Krogh Institute, LHF, Universitetsparken 13, DK-2100 Copenhagen Ø, 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. Section 1734 solely to indicate this fact.
Received 20 June 2000; accepted in final form 23 January 2001.
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