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Laboratoire Interuniversitaire de Biologie des Activités Physiques et Sportives, F-63001 Clermont-Ferrand, France
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ABSTRACT |
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 TOP
 ABSTRACT
 INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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The aim of this
study was to investigate the acid-base balance during repeated cycling
sprints in children and adults. Eleven boys (9.6 ± 0.7 yr) and ten men (20.4 ± 0.8 yr) performed ten 10-s sprints on a
cycle ergometer separated by 30-s passive recovery intervals. To
measure the time course of lactate ([La]), hydrogen ions
([H+]), bicarbonate ions ([HCO
ventilatory regulation; acidosis
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INTRODUCTION |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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IT IS WELL
ACCEPTED THAT ANAEROBIC glycolysis is lower in prepubertal
children than in young adults during high-intensity exercise. In fact,
some earlier studies showed that postexercise blood and muscle lactate
concentration ([La]) was markedly lower in children compared with
adults (10, 11, 16, 27). For instance, Hebestreit
et al. (16) indicated that, after a 30-s "all-out"
cycling task, postexercise blood [La] was 5.7 and 14.2 mmol/l in
prepubertal boys and men, respectively. This was confirmed by some
studies that showed that blood and muscle pH decreased slightly after
exercise in children compared with adults' values. Using
phosphorus-nuclear magnetic resonance spectroscopy, after a graded
exercise to exhaustion, it has been shown that intramuscular pH in the
calf muscle of prepubertal children only decreased from 0.11 to 0.23 units, whereas the fall in pH represented 0.36-0.38 units in
adults (33, 34). Furthermore, old studies showed that the
decrease in blood pH was slightly smaller in 11- to 12-yr-old children
(not lower than 7.34) than in adults (lower than 7.19) after maximal
exercise (6, 15, 20). More recently, it has also been
observed that, after a 30-s supramaximal exercise, venous blood pH only
reached 7.32 in 10-yr-old children compared with 7.18 in 25-yr-old
adults (16). Despite a lesser reliance on glycolytic
energy pathways in children, these previous studies showed that blood
pH was slightly modified, whereas [La] may increase highly in
children (9). Therefore, according to these results, the
relationship between [La] and pH may be different between children
and young adults. We hypothesized that this smaller decrease in blood
pH compared with the increase in [La] in children may result from a
higher hydrogen ions (H+) buffering capacity by bicarbonate
ions (HCO
E). To test this hypothesis, a
repeated sprint exercise protocol, separated by short recovery intervals, was chosen because such protocol induces a high increase in
[La] in children (19), as well as in adults
(13), and children are often engaged in this type of
activity (3).
Therefore, the aim of the present study was to investigate the acid-base balance during several repeated bouts of short-term, high-intensity cycling exercise separated by short recovery intervals in boys as well as in men.
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METHODS |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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Subjects
Eleven 8- to 10-yr-old prepubertal boys and ten 19- to 21-yr-old men volunteered to participate in the study. All of the subjects were involved in different physical activities, such as ice hockey or swimming. Written informed consent was signed by each subject or his parents. The protocol was approved by the ethics committee of the Auvergne University.Experimental Protocol
Design.
Each subject attended the laboratory for two sessions. The time
interval between the sessions was 
48 h and 
2 wk. The first visit
was used to gather subjects' physical characteristics and to habituate
them to the subsequent testing procedure. During the second session,
the subjects performed 10 short-term cycling sprints separated by 30-s
recovery intervals. Each subject was instructed to refrain from intense
physical exercise 48 h before the second visit.
Session I.
During the first session, body mass (BM) and standing height were
measured. Body fat (%) was estimated from tricipital and subscapular
skinfold thicknesses using the equations of Slaughter et al.
(31). Pubertal stage was determined according to pubic hair and gonadal development (32). The saddle height of
the friction-loaded cycle ergometer (Ergomeca Sorem, Toulon, France) was adjusted to give optimal comfort to each subject and remained unchanged for the second session. The cycle ergometer was previously described in detail by Doré et al. (8). The
subjects' feet were strapped to the pedals to prevent them from
slipping. The subjects performed a warming up that consisted of a 5-min
submaximal cycling followed by two brief sprints (5 s) against a
low-braking load. After 5-min rest, the subjects performed three
sprints on the cycle ergometer against frictional forces of 0.245, 0.491, and 0.736 N/kg BM (corresponding applied loads: 25, 50, and 75 g/kg BM, respectively). The latter were applied in a randomized order.
Each sprint was separated by at least 5 min of rest. These sprints
allowed the researchers to familiarize the subjects with the cycle
ergometer and to calculate the friction load against which the subjects
had to perform during the second session. In fact, velocity, force, and
power values (averaged per half-pedal revolution) recorded during the
acceleration phase of the three sprints were used to draw the
individual force- and power-velocity relationships (2).
Optimal values of friction force and velocity at which the highest
power output is performed were determined from these relationships.
After a 15-min rest, maximal oxygen consumption
(O2 max) was determined by direct
method (CPX Medical Graphics, St. Paul, MN) using a graded cycling
test. The initial power was 30 and 60-75 W for the boys and the
men, respectively. The power was incremented by 15 W every 1 min for the boys and by 30 W every 2 min for the men. The pedaling rate was
maintained at 60 rpm throughout the test. The exercise intensity was
increased until exhaustion of the subject.
Session II.
The subjects performed a 6-min warm-up on the cycle ergometer at a
power output leading heart rate to ~140-150 and 120-130 beats/min in the boys and the men, respectively. After a 5-min rest,
the subjects performed 10 consecutive 10-s sprints separated by 30-s
recovery intervals against a friction load corresponding to 50%
optimal value of friction force for each subject. Before each sprint,
the start position was standardized with the crank of the left leg
located 45° forward of the top dead center. At the signal, the
subjects were told to remain on the saddle and to pedal as fast as
possible to reach maximal pedaling rate. Each subject was verbally
encouraged throughout each sprint. During the resting periods, after
each sprint, the subjects had to remain seated quietly on the cycle
ergometer. Peak power (
peak) was calculated at each
sprint, according to the method described by Doré et al.
(8).
Blood Sampling
Capillary arterialized blood samples (150 µl) were drawn from the earlobe at rest and before the first and after the second, fourth, sixth, eighth, and tenth sprints to determine the time course of H+ ([H+]), HCO
PaCO2),
where Ka is the effective dissociation constant
for plasma weak acids and is the solubility coefficient of
CO2. At each sample, [BE] values were calculated
from [HCO
![[BE]<IT>=</IT>{1<IT>−</IT>(0.014<IT>·</IT>[Hb])}](/content/vol92/issue2/fulltext/479/img002.gif)
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![<IT>·</IT>{([HCO<SUP>−</SUP><SUB>3</SUB>]<IT>−</IT>24<IT>+</IT>(1.43<IT>·</IT>[Hb])<IT>+</IT>7.7}<IT>·</IT>(pH<IT>−</IT>7.4)](/content/vol92/issue2/fulltext/479/img003.gif)
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20°C. Blood [La] values were
measured by an Analox GM7 GB analyzer (Analox Instruments, London, UK)
using L-lactate O2 oxide reductase, which
catalyzes oxidation of L-lactate to pyruvate and hydrogen
peroxide. Given that [H+] and [La] sampling times were
different, a [La] linear interpolation was performed between each
sprint to correspond each [La] value with each [H+].
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Gas Exchange
E and carbon dioxide output
(CO2) were continuously measured breath
by breath during the 10 sprint exercises using a CPX analyzer (Medical
Graphics, St. Paul, MN). The breath-by-breath data were time
interpolated, so that there was a data point every 1 s
(1).
Statistical Analysis
All of the results are expressed as means ± SD. Differences between the two groups (the boys and the men) for anthropometric variables andO2 max
were tested using an unpaired Student t-test. Differences
between the boys and the men for [La] and [H+] values
over the 10 sprints were tested by a two-way ANOVA for repeated
measures (interfactor: age; intrafactor: sprint). When the ANOVA
F ratios were significant, the means between the boys and
the men were compared using an unpaired Student's t-test, and the means between the sprints were compared using a paired Student's t-test. Correlation coefficients between the
different physiological variables were calculated. Using analysis of
covariance (ANCOVA), regression standards (from the regression line
between, e.g., [La] and [HCO| |
RESULTS |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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Physical characteristics and
O2 max of the subjects are described in
Table 1.
O2 max
(ml · min
1 · kg body mass1)
was not significantly different in the boys compared with the men.
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peak
peak remained unchanged during the
10 repeated sprints (sprint 1: 284 ± 50 W;
sprint 10: 285 ± 46 W). In the men,
peak decreased significantly by 28.5% from the
first sprint (1,122 ± 197 W) to the tenth sprint (798 ± 132 W) (P < 0.001).
Blood [La]
The time course of [La] during the 10 sprint exercises in the boys and the men is presented in Fig. 2. In the boys, [La] increased approximately fourfold from 1.9 ± 0.3 mmol/l at rest to 8.1 ± 2.3 mmol/l after the seventh sprint (P < 0.001) and then remained unchanged until the tenth sprint (8.5 ± 2.1 mmol/l). In the men, [La] progressively increased 11-fold from 1.3 ± 0.5 mmol/l at rest to 15.4 ± 2.0 mmol/l after the last sprint (P < 0.001). Blood [La] was slightly higher in the boys than in the men after the first sprint (P < 0.05) but became significantly lower in the boys from the third sprint to the end of the test. After the 10th sprint, [La] was twofold lower in the boys than in the men (P < 0.001).
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Blood [H+]
The time course of blood pH during the 10 sprint exercises in the boys and the men is shown in Fig. 3. In the boys, blood [H+] significantly increased 1.2-fold from 37.9 ± 2.2 nmol/l (pH 7.42 ± 0.02) at rest to 44.3 ± 2.6 nmol/l (pH 7.35 ± 0.02) after the sixth sprint (P < 0.01) and remained unchanged until the tenth sprint (43.8 ± 1.3 nmol/l, pH 7.36 ± 0.01). In the men, blood [H+] progressively increased 1.7-fold from 38.6 ± 1.8 nmol/l (pH 7.41 ± 0.02) at rest to 66.9 ± 9.9 nmol/l (pH 7.18 ± 0.06) after the 10th sprint (P < 0.001). Blood [H+] was significantly lower in the boys than in the men after the second sprint (P < 0.05) and was 1.5-fold lower in the boys after the tenth sprint (P < 0.001).
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Relationships among [La], [H+],
[HCO
0.72; P < 0.001) and the men (r = 0.93; P < 0.001). The ordinate and slope of the
linear regressions were not significantly different between the two
groups [ANCOVA, not significant (NS)] (Fig.
5). Similarly, inverse significant linear
regressions were obtained between [La] and [BE], both in the boys
(r = 0.70; P < 0.001) and the men
(r = 0.96; P < 0.001). The ordinate
and slope of the linear regressions were not significantly different
between the boys and the men (ANCOVA, NS) (Fig.
6). Significant linear regressions were
also found between the decrease in [HCO0.36;
P < 0.05) and the men (r = 0.64;
P < 0.001). The slope of these relationships was
significantly greater in the boys compared with the men (ANCOVA,
P < 0.001) (Fig. 8).
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Gas Exchange
The time course of theE-to-CO2 ratio
(E/CO2) of both the
boys and the men during the 10 sprints is presented in Fig. 9.
E/CO2 was higher in
the boys during the first five rest intervals and was then higher in
the men during the last five sprints.
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DISCUSSION |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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The subjects performed the sprints against a friction load
corresponding to 50% of their optimal force, which allowed them to
reach their optimal velocity and to produce their
peak during the sprints (8). This
friction load represented the same workload relative to the maximal
abilities of the boys and the men. This braking load was 40 and 50 g/kg
BM in the boys and the men, respectively. Furthermore, this friction
load allowed all of the subjects to maintain the ten 10-s cycling
sprints separated by 30-s recovery intervals.
The stressful nature of the present protocol was indicated by the dramatic increase in blood [La], in both the boys and the men. After the 10th sprint, [La] increased >8 and 15 mmol/l in the boys and the men, respectively. In the boys, postexercise [La] was higher than the values reported by Macek et al. (19) after 10 consecutive 10-s cycling sprints separated by 25-s recovery intervals in 13-yr-old boys (5.3 mmol/l). In the men, the increase in [La] was also higher than that reported by Gaitanos et al. (13) after ten 10-s cycling sprints separated by 30-s recovery intervals (12.6 mmol/l). Methodological differences in blood collection, [La] determination, and training status of the subjects may explain the small differences observed with previous studies. In the present study, [La] accumulation was markedly lower in the boys than in the men (Fig. 2). This result is in accordance with previous findings (11, 16, 27), which showed that maximal blood [La] is positively related to age. The underlying mechanisms of this diminished lactate response in children has still to be elucidated, but the pediatric literature suggests a muscle metabolic profile better equipped for oxidative than glycolytic energy turnover (5, 10, 14). The higher [La] in the men were associated with a higher metabolic acidosis in the blood as indicated by their lower blood pH and their higher decrease in BE (i.e., a higher amount of acidic ions appearing in the blood). In the boys, blood pH only decreased by 0.06 units, whereas, in the men, the fall represented 0.23 units (Fig. 3). Furthermore, changes in BE (delta BE from the rest to the last sprint) represented 7.3 and 15.8 mmol/l in the boys and the men, respectively. These results concur with those of previous studies, which showed that, whether determined by blood pH (4, 6, 15-17, 20) or BE (4, 6, 12, 15, 20, 22, 23, 26), the maximal acidosis reached by children is lower than that reached by adults.
In the men, the relationship between blood [H+] and
[La] was curvilinear, which is in agreement with previous studies
(23, 26) (Fig. 4). According to Medbo and Sejersted
(23), the most important explanation for this nonlinearity
is that HCO
The results of the present study highlight that, for a given [La],
[H+] was significantly less during repeated sprint
exercises in the boys than in the men (Fig. 4). Two assumptions may
explain this relatively small increase in blood [H+] in
the boys. First, because it has been shown that intense exercise and
lactic acidosis induce a muscle H+ release independent of
lactate release (22), the release of H+ from
the men's muscles should be higher compared with that from the boys',
whereas the release of lactate might be similar. Second, PaCO2 might be regulated at a lower level by
E in the boys compared with the men.
The first hypothesis is not relevant, because the results of the
present study indicate that, for a given [La], the concentrations of
BE and HCO
On the other hand, the second hypothesis tries to explain the smaller
drop in blood pH in the boys because PaCO2 was lower in the boys than in the men for a given [HCOPaCO2 ratio was
higher in the boys compared with the men. Similarly, for a given blood
[H+], PaCO2 was lower in the boys
compared with the men (Fig. 8). A higher relative E
in the boys would explain their lower PaCO2 during the
first five rest intervals because the results of the present study show
that E/CO2 was higher
in the boys compared with the men. However, the men hyperventilated
more during the last five sprints because their
E/CO2 was higher than
that of the boys (Fig. 9). In the men, the gradual increase in
E/CO2 during the 10 sprints may be explained by the fact that [La] progressively increased (Fig. 2). In the boys,
E/CO2 and [La]
reached a steady state during the last five sprints. These results are
in agreement with previous studies that showed that
E or effective alveolar E to
eliminate a given amount of CO2 was greater in younger
children than in older subjects from rest to supramaximal exercise
(1) and during incremental exercise (7, 21, 24, 25,
28, 29). Little information is available to explain this higher
relative E in children. Nevertheless, it has been suggested that age-related differences in ventilatory responses to
exercise may reflect variations in neural respiratory drive, lung
mechanics, or both (see Ref. 29 for review).
In conclusion, the results of the present study show that, during repeated sprints, children regulate their blood [H+] better than adults do. This finding may be explained by the fact that they ventilate more than adults during the first rest intervals to exhale a given amount of carbon dioxide, which allows them to regulate their PaCO2 to a lower level. In other words, the ventilatory regulation related to the change in acid-base balance induced by lactic acidosis is more important in boys compared with men during the first rest intervals.
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ACKNOWLEDGEMENTS |
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The authors gratefully acknowledge the subjects for their patience, time, and effort.
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FOOTNOTES |
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Address for reprint requests and other correspondence: M. Bedu, Laboratoire Interuniversitaire de Biologie des Activités Physiques et Sportives, UFR Médecine, Université d'Auvergne, BP 38, F-63001 Clermont-Ferrand, France (E-mail: mbedu{at}chu-clermontferrand.fr).
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.
10.1152/japplphysiol.00495.2001
Received 18 May 2001; accepted in final form 24 September 2001.
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TOP
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METHODS
RESULTS
DISCUSSION
REFERENCES
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P < 0.01;
P < 0.001.
