Introduction
The maximal oxygen uptake (V̇o2 max) is generally considered as the main determinant of aerobic performance. In children, studies have demonstrated a significant link between V̇o2 max and several running performances such as 1-mile (25 21 28 2 max. In a recent review, Midgley et al. (23 2 max might place maximal stress on the physiological processes and structures that limit V̇o2 max, providing an optimal stimulus for adaptation. However, they concluded that further studies are needed to support this assumption. Nevertheless, the question on how long a subject can spend time at a high percentage of V̇o2 max remains, because it depends on factors that differ between subjects or populations. For example, intensity from 95 to 100% of V̇o2 max seems to be necessary to improve V̇o2 max in aerobically trained subjects (23 3 2 max” exercises will be associated with intensities higher than 90% of V̇o2 max (ts90%) in this paper.
The range of exercise intensities allowing a high percentage of V̇o2 max to be reached could be observed in the heavy intensity domain (above the lactate threshold), where a slow component of the V̇o2 kinetics is superimposed upon the rapid V̇o2 response (primary phase) resulting in a delayed submaximal steady state. In the severe intensity domain, where no steady state of V̇o2 is observed, V̇o2 max could be reached if exercise is of sufficient duration (18 18
During continuous exercise, ts90% depends on the difference between time to achieve 90% of V̇o2 max value (ta90%) and exercise time to exhaustion (TTE) (17 2 value (i.e., 90% V̇o2 max), which is linked to fast V̇o2 kinetics at the onset of exercise. This high percentage of V̇o2 max is reached for a wide range of exercise intensities, even lower than 90% of V̇o2 max, depending on apparition, and the amplitude, of a V̇o2 slow component. When high percentage, or even V̇o2 max is reached, the ability to sustain exercise intensity is related to anaerobic capacity. The relationship between TTE and anaerobic capacity could simply be described as (16 P the exercise power. This model indicates that the higher the anaerobic work capacity, the longer the TTE.
Children present specific physiological responses to exercise that may influence the different parameters determining ts90%. Although in the latter population a faster primary phase of V̇o2 at the onset of exercise (9 2 slow component (35 2
To our knowledge, there are no data available in the pediatric literature related to children' V̇o2 response during TTE at high-percentages of V̇o2 max, nor to time spent at a high percentage of V̇o2 max. However, it has been reported that children would present lower V̇o2 max trainability than adults. So we hypothesized that children were not able to spend as much time as adults at 90% of V̇o2 max during TTE tests, because of a lower slow component amplitude and anaerobic capacity.
Methods
Experimental Approach to the Problem
During a medical examination, the children's sexual maturity was evaluated by the Tanner maturation stages method (30
During all tests, the subjects were instructed to maintain a cycling cadence of 70 rpm, to grip the handlebars and to not lift off the saddle while cycling. Seat and handlebar heights were kept constant for each participant during each session. The subjects' feet were firmly strapped to the pedals, and the saddle height was adjusted to allow slight flexion of the knee at the lowest level of the pedal cycle. For the children, pediatric cranks were used. Strong verbal encouragements were provided to all subjects during each session.
Subjects
Fifteen prepubertal boys and 15 men volunteered to participate in the study. They all were recreationally active but not involved in any systematic training. They practiced less than 4 h·wk−1 of structured physical activity. Mean age, height, and body mass values were 10.3 ± 0.9 years, 141.8 ± 7.1 cm and 36.9 ± 9.1 kg in children and 23.5 ± 3.6 years, 177.9 ± 6.2 cm, and 72.4 ± 9.5 kg in adults, respectively. Adults, children, and their parents, gave written informed consent after the experimental procedures; the associated risks, and the benefits of participation were explained. This study approval from the local ethics committee and all procedures were performed in accordance with the ethical standards of the Helsinki Declaration of 1975 as revised in 1983.
Cardiorespiratory Measurements
During all tests, to measure respiratory gas exchange, adults and children breathed through adult or pediatric masks adapted to their faces. Respiratory gas exchanges were measured breath by breath using an automated gas-analysis system (Ergocard, Medisoft, Dinant, Belgium) to determine ventilation (V̇E ), oxygen uptake (V̇o2 ), and carbon dioxide production (V̇co2 ). Before each test, the O2 and CO2 analysis systems were calibrated using ambient air and a gas mixture of known O2 and CO2 concentrations (16.01% O2 and 4.01% CO2 ). The pneumotachograph was calibrated before each test with a 3-L calibration syringe for low and high flows. Heart rate was monitored and continuously recorded (Polar Electro, Kempele, Finland). For data analysis, V̇o2 , V̇E , V̇co2 , and HR values were averaged every 15 seconds.
Maximal Graded Test
The maximal graded test aimed to determine V̇o2 max, maximal aerobic power (MAP) and power at ventilatory threshold (PVTh). Subjects performed a 5-minute warm-up at 20 W for children and 50 W for adults, and then the power was increased every minute by 15 W for children and 25 W for adults. It was judged that subjects had reached V̇o2 max when 3 or more of the following criteria were obtained: an inability to maintain the required frequency, a maximal HR >90% of predicted maximal HR (220-age) (8 2 despite increasing power (V̇o2 change <2.1 ml·kg−1 ·min−1 for adults (8 −1 ·min−1 for children (33 8 31
The V̇o2 max corresponded to the highest V̇o2 value attained in a 15-second period. The MAP was visually determined as the lowest power associated with V̇o2 max attainement. Ventilatory threshold was determined by at least 2 different and isolated examiners. It was determined from the time course curves of minute ventilation (V̇E ), the ventilatory equivalent in O2 (V̇E /V̇o2 ), the ventilatory equivalent in CO2 (V̇E /V̇co2 ) and end tidal partial pressures in O2 and CO2 (32 E and the O2 ventilatory equivalent without any increase in CO2 ventilatory equivalent (32
Constant Load Exhaustive Exercises
Each constant load exhaustive exercise was preceded by 2 6-minute warm-up periods intercepted by 6 minutes of passive recovery. During warm-up periods, intensities were set at 70 (70% VTh) and 90% (90% VTh) of the power associated to VTh. Then, the constant load exercises were carried out at PΔ50, PΔ75, P100, and P110, where PΔ50 and PΔ75 corresponded to power at VTh plus 50 or 75% of difference between MAP and the power associated with VTh, respectively; and P100 and P110 corresponded to 100 and 110% of MAP, respectively. In the present study, the exercise intensities were chosen to allow subjects reaching V̇o2 max. In adults, the attainment of V̇o2 max during exhaustive constant load exercises is typically obtained at an intensity higher than CP (19 11 2 at CP (≈1.1 L·min−1 ) was slightly lower than V̇o2 at PΔ50 (1.2 L·min−1 ). Retrospectively, we have checked that the mean power at PΔ50 was systematically higher than CP for both children and adults involved in the present study, showing that the chosen intensities were within the severe intensity domain.
Critical power was determined from the 3 constant load tests PΔ50, PΔ75, and P100 according to the protocol described by Moritani et al. (24 P -t relationship for each subject was linearized by plotting (1/TTE) against power using the least squares linear regression technique, and the y -intercept of this relationship was taken as CP.
Time to exhaustion were determined to the nearest second as the time between the start of exercise and the time when the subjects were not able to maintain the required cycling cadence.
During the 4 constant load exhaustive exercises, ts90% was calculated as the difference between the TTE and ta90% (20 2 value above 90% of V̇o2 max and was visually determined in each V̇o2 vs. time curve.
Maximal Accumulated O2 Deficit
The maximal accumulated O2 deficit (MAOD) was calculated from V̇o2 values measured during the constant load exercise at P110 and was determined by the subtraction of the measured accumulated O2 uptake from the theoretical total O2 demand during the P110 exercise (22 2 required to exercise at P110 was extrapolated from the linear relationship between 3 mean V̇o2 /power values obtained during the warm-up period of each constant load test: V̇o2 at rest (null power), steady state V̇o2 (mean last minute value) at 70%VTh and 90%VTh. Total O2 demand for the P110 test was calculated as the product of estimated V̇o2 at P110 and associated TTE. The accumulated O2 uptake during the P110 test was determined as the sum of each 15-second averaged V̇o2 value (in ml·min−1 ).
V̇o2 Kinetics Analysis
The primary phase of the V̇o2 kinetic was calculated from V̇o2 data collected during the first 6-minute warm-up (70% VTh) preceding each of the 4 TTE. Breath-by-breath V̇o2 data were first interpolated to give second-by-second values and time aligned to the start of exercise. The 4 rest to exercises transitions were averaged to obtain a single V̇o2 kinetic. Nonlinear regression techniques were used to fit V̇o2 data after the onset of exercise with an exponential function. An iterative process ensured the sum of squared error was minimized. The mathematical model consisted of a single exponential function that started at the onset of exercise: V̇o2(t ) = ΔV̇o2 ss × (1−e−t /MRT ) (34 2(t ) is the V̇o2 at time t , ΔV̇o2 ss the V̇o2 steady state (asymptotic amplitudes for the exponential) minus V̇o2 at baseline (mean 2-minute V̇o2 value preceding the onset of exercise), and MRT the mean response time (time to reach 63% of the ΔV̇o2 ss).
The amplitude of a V̇o2 slow component was examined during the PΔ50 and calculated, based on 15-second averaged values, as the difference between V̇o2 at 6-minute minus V̇o2 at 3 minutes (ΔV̇o2(6−3min) ) (1 2(6−3min) was calculated for PΔ50 for the 15 adults and 14 children. One child was removed from the data process because of a TTE shorter than 200 seconds. Two children who were exhausted within 300 seconds were kept in the ΔV̇o2(6−3min) analysis.
Statistical Analyses
The experimental data are presented as mean ± SD ). Mean value comparisons between children and adults V̇o2 max, VTh, MAP, RER, HRmax, ta90%, MRT, ΔV̇o2(6−3min) (TTE, ts90%, MAOD) were carried out with the unpaired Student's t test (SigmaStat version 2.0). Relationships between TTE (at PΔ50, PΔ75, P100) and MAOD were analyzed with Pearson correlations. These tests were performed after verification for the normal Gaussian distribution of the data using the Shapiro-Wilk's test. If it was not the case, a Mann-Whitney rank sum test was used. In all analyses, the level of significance was set at p ≤ 0.05.
Results
Children and adults groups did not present significant difference in V̇o2 max and VTh (%V̇o2 max) values (Table 1 ). HRmax was significantly higher in children than in adults (p < 0.01). VTh (ml·kg−1 ·min−1 ) (p < 0.01), MAP, and RERmax (p < 0.001) were significantly lower in children than in adults.
Table 1: Cardiorespiratory values during maximal graded test.*†
Results for the constant load exercises are reported in Table 2 . Time to exhaustion were significantly shorter in children compared to adults at P100 (p < 0.01) and P110 (p < 0.001), whereas no significant difference was found for PΔ50 and PΔ75. For the 4 constant load exercises, ta90% values were not significantly different between children and adults. Children's ts90% were significantly shorter than adults at P100 (p < 0.01) and P110 (p < 0.001). No significant differences in ts90% were found between children and adults for PΔ50 and PΔ75. Whereas 14 of 15 adults achieved 90% of V̇o2 maxduring each test, 14, 13, 9, and 8 of 15 children achieved 90% of V̇o2 max during PΔ50, PΔ75, P100, and P110, respectively.
Table 2: Time spent at 90% of (ts90%), time to achieve 90% of (ta90%), and TTE in children and adults for the 4 tests*†
The MAOD in children (34.3 ± 9.4 ml·kg−1 ) was significantly lower than in adults (53.6 ± 11.1 ml·kg−1 ) (p < 0.001). In children, MAOD was only positively correlated to TTE at P100 (r = 0.59, p < 0.05) (Figure 1A ) but not in adults (Figure 1B ).
Figure 1: Correlation between time to exhaustion (TTE) at P100 and maximal accumulated oxygen deficit (MAOD) for the A) children (n = 15) and B) adults (n = 15).
Children demonstrated significantly (p < 0.001) lower MRT (12.2 ± 3.6 seconds) than adults (21.6 ± 5.2). The ΔV̇o2(6−3min) was significantly lower in children (9.8 ± 10.0 ml) than in adults (220.0 ± 15.6 mL).
To verify that constant load exercises were performed in the severe intensity domain, CP was calculated (Table 3 ) for each subject. PΔ50, PΔ75, P100, and P110 were higher than CP for each child and each adult.
Table 3: Critical power and constant load exercise power in children and adults.*
Discussion
The aim of the study was to compare ts90%, ta90%, and TTE between children and adults. The main result is that, for P100 and P110, ts90%, and TTE values were significantly shorter in children when compared to adult values. However, for PΔ50 and PΔ75, children and adults demonstrated similar ts90% and TTE. Moreover, anaerobic capacity assessed by MAOD was significantly lower in children compared to adults and was significantly related to a shorter TTE at P100 in children.
Whatever the intensity, ta90% mean values were not significantly different between children and adults. In the severe intensity domain, ta90% depends on primary phase of the V̇o2 kinetics, and on the amplitude of the V̇o2 slow component. In the literature, the time constant of the V̇o2 primary phase (time to reach 63% of the V̇o2 steady state) reported in children is generally around 10 seconds shorter than in adults in most of the studies (12 2 kinetics. These differences may arise because of children's limited ability to generate adenosine triphosphate (ATP) anaerobically, coupled with their greater ability to meet the energetic demands of exercise through aerobic pathways (35 15
The lack of difference in ta90% between children and adults in the present investigation suggested that the contribution of a faster V̇o2 adjustment at the onset of exercise could be negligible for long lasting exercises. The amplitude of the V̇o2 slow component has been shown to be lower in children than in adults. For running exercises at PΔ50, Williams et al. (35 2 slow component contributed 8% (288.5 ± 39.7 ml·min−1 ) of the total end-exercise V̇o2 in adults compared to 1% (18.6 ± 18.9 ml·min−1 ) in children. This is consistent with our results because children (9.8 ± 10.0 ml) demonstrated significantly lower V̇o2 slow component amplitude than adults (220.0 ± 15.6 ml) during PΔ50. This result could partly explain the lack of difference in ta90% values in the present study between children and adults. Indeed, the lower V̇o2 slow component amplitude in children could compensate for the shorter V̇o2 adjustment. The underpinning mechanisms of the slow component are to date not entirely understood, and although it is usually modeled as a discrete component, it is likely that the slow component is a function of the mechanisms controlling energy turnover from the first onset exercise. It has been reported that O2 cost (V̇o2 in ml·min−1 ·W−1 ) at the end of exercise was equal between children and adults (10
To our knowledge, no previous study has investigated the TTE at several severe intensities between children and adults. For P100 and P110, we found a significantly lower TTE in children. These results agreed with a cross-sectional study by Berthoin et al. (7 2 max (MAV) significantly increased from 6 years of age to puberty in boys and girls. These authors hypothesized that longer TTE were associated with higher anaerobic capacity at puberty. This result is also consistent with previous results by Renoux et al. (26 r = 0.67, p < 0.05) related to anaerobic capacity assessed by MAOD in adults when running at MAV. Although MAOD methodology is still questioned (14 22,29 13 p < 0.001) in children (34.3 ± 9.4 ml·kg−1 ) compared to adults (53.6 ± 11.1 ml·kg−1 ). We found a significant positive relationship between MAOD and TTE at P100 for children (r = 0.59, p < 0.05) (Figure 1A ) but not in adults (r = −0.25, p = 0.36) (Figure 1B ). This suggests that in children, anaerobic capacity would play a decisive role in TTE at P100. This role seems to be lower in adults as the MAOD vs. TTE relationship failed to reach statistical significance. Thus, a limited anaerobic capacity could partly explain shorter P100 TTE in children. However, the anaerobic capacity is not likely to be the only limiting factor in children. Indeed, the duration of this type of exercises could also be limited by muscle structure, energy metabolism, or neuromuscular activation (4
Conversely to P100 test, during PΔ50 and PΔ75, no significant difference in TTE was found between children and adults. For both populations, no relationship was found between TTE and MAOD, suggesting that anaerobic capacity is not the main determinant of performance at these intensities. We could hypothesize that the similar PΔ50 and PΔ75 TTE between children and adults are linked to a higher relative contribution of the oxidative metabolism in children-that is, a lower anaerobic metabolism contribution-at these exercise intensities below MAP than at P100 and P110 (5,6,15
The present values of ts90% in adults are consistent with previous studies in adults which reported ts90% at P100 (144 ± 60 vs. 155 ± 34 seconds) and P110 (96 ± 30 vs. 75 ± 26 seconds) during cycling exercises (18 2 max values were similar in the 2 groups presently investigated. Secondly, the shorter ts90% at P100 and P110 were associated with shorter TTE at these 2 intensities. Thus, the duration of the exercise was the major factor of the ts90% at maximal and supramaximal intensity between children and adults. As previously mentioned, TTE is itself influenced by anaerobic capacity.
In addition, for 6 children at P100 and 7 at P110, ts90% was equal to 0. This suggests that the duration of the exercise was not sufficiently long to achieve 90% of V̇o2 max. As argued above, we can hypothesize that contrary to adults, the lower anaerobic capacity (i.e., lower MAOD) observed in children does not allow them to exercise as long as necessary to reach 90% of V̇o2 max. Thus, it could be suggested that the lower anaerobic capacity (i.e., lower MAOD) observed in children does not allow them to exercise for a sufficient duration to reach high percentage of V̇o2 max, compared to adults.
In perspective, the lower capacity to sustain high percentage of V̇o2 max in children during maximal and supramaximal intensity exercises could be an explicative factor of the lower increase of V̇o2 max in children than in adults after training programs (3
At submaximal intensities (PΔ50 and PΔ75), ts90% were not different with age. The results are consistent with the similar ta90% and TTE found and discussed above in the 2 groups. This result suggests that at these intensities, the children are as able as adults to elicit and maintain a high percentage of V̇o2 max. Thus within the framework of a training program, if ts90% is considered as the main determinant of V̇o2 max development, no difference should be observed between children and adults in training V̇o2 max increase. To date, although certain methodological explanation about training protocols have been proposed, it seems more likely that a biologic mechanism is responsible for lower V̇o2 max increase with training. Among the factors that contribute to improvements in V̇o2 max with endurance training, increased plasma volume and cellular aerobic capacity are 2 reasonable candidates to explain maturity-related differences (27
In conclusion, this study is the first to compare TTE and time spent at a high percentage of V̇o2 max in children for different exercise intensities. This study showed that for severe exercise intensities below MAP, ts90% were not different between children and adults although it was lower for exercises at or above MAP. These differences in children and adults could be explained by a lower anaerobic capacity in children which did not permit them to sustain exercise as long as adults.
Practical Applications
The results seem particularly interesting for trainers and can help them to adapt the training intensities in children. Indeed, exercising at different relative intensities in children and adults will not induce the same adaptations. Even if the expected V̇o2 max improvement with endurance training in prepubertal children is low, this training should not be overlooked. Indeed, children with a higher level of physical activity, or who have been trained during childhood, show a higher level of physical activity and aerobic fitness in young adulthood.
We demonstrated that to enhance V̇o2 max in children, generally considered as the main determinant of aerobic performance, it seemed necessary to train below MAP. Considering this fact, the present study allows one to give a defined intensity range to reach this objective. This intensity range that we have expressed relatively to CP can be easily calculated by the use of time trials, without any oxygen consumption measure, and so could be used as reference intensities.
Moreover, as highlighted by the present study, to magnify training effects in children, it could also be interesting to consider both anaerobic and aerobic fitness. It could stimulate trainers to include anaerobic (sprint repetition) and aerobic sessions in training program. Anaerobic training is of particular interest because children are characterized by a faster recovery after high-intensity anaerobic exercise. Moreover, the effects of such anaerobic training could be evaluated by using the CP model based on time trial performances.
Acknowledgments
This study was part of the “interEx” project, supported by a grant from the EU under the Interreg IIIa grant Scheme.
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