Critical power in adolescent boys and girls — anexploratory study

Craig A. Williams, Jeanne Dekerle, Kerry McGawley, Serge Berthoin, andHelen Carter

Abstract: The purpose of the study was to identify critical power (CP) in boys and girls and to examine the physiological re-sponses to exercise at and 10% above CP (CP+10%) in a sub-group of boys. Nine boys and 9 girls (mean age 12.3 (0.5) y per-formed 3 constant-load tests to derive CP. Eight of the boys then exercised, in random order, at CP and CP+10% until volitionalexhaustion. CP was 123 (28) and 91 (26) W for boys and girls, respectively (p < 0.02), which was equivalent to 75 (6) and 72(10) % of peak oxygen uptake, respectively (p > 0.47). Boys’ time to exhaustion at CP was 18 min 37 s (4 min 13 s), whichwas significantly longer (p < 0.007) than that at CP+10% (9 min 42 s (2 min 31 s)). End-exercise values for blood lactate con-centration (B[La]) and maximal oxygen uptake were higher in the CP+10% trial (5.0 (2.4) mmol�L–1 and 2.15 (0.4) L�min–1, re-spectively) than in the CP trial, (B[La], 4.7 (2.1) mmol�L–1; maximal oxygen uptake, 2.05 (0.35) L�min–1; p > 0.13). Peakoxygen uptake (expressed as a percentage of the peak value) was not attained at the end of the trials (94 (12) and 98 (14) %for CP and CP+10%, respectively). These results provide information about the boundary between the heavy and severe exer-cise intensity domains in children, and have demonstrated that CP in a group of boys does not represent a sustainable steady-state intensity of exercise.

Key words: exercise tolerance, time to exhaustion, exercise intensities, cycle ergometry.

Resume : Le but de cette etude est de determiner la puissance critique (CP) des filles et des garcons et d’analyser chez unsous-groupe de garcons les ajustements physiologiques a cette intensite de travail et a 10 % au-dessus de cette valeur(CP+10 %). Neuf filles et neuf garcons ages de 12,3 (0,5) ans participent a trois epreuves de charge constante concues pourl’evaluation de la CP. Ensuite, huit garcons participent de facon aleatoire a un effort d’intensite equivalant a la CP et a laCP+10 % jusqu’a l’epuisement volontaire. La CP des garcons est de 123 (28) W et celle des filles est de 91 (26) W (p <0,02), ce qui equivaut respectivement a 75 (6) % et 72 (10) % du consommation d’oxygene de pointe (p > 0,47). Le tempsde performance jusqu’a l’epuisement a une intensite correspondant a la CP est de 18 min 37 s (4 min 13 s), ce qui est si-gnificativement plus long (p < 0,007) que le temps de performance (9 min 42 s (2 min 31 s)) a une intensite correspondanta la CP+10 %. Les valeurs de la concentration de lactate sanguin (B[la]) et du consommation d’oxygene sont superieures ala fin de l’effort realise a la CP+10 % (B[la], 5,0 (2,4) mmol�L–1 et consommation d’oxygene, 2,15 (0,4) L�min–1) qu’al’effort realise a la CP (B[la], 4,7 (2,1) mmol�L–1 et consommation d’oxygene, 2,05 (0,35) L�min–1, p > 0,13). A la fin desepreuves, les sujets n’atteignent pas le consommation d’oxygene de pointe (exprime en pourcentage de la valeur depointe), soit 94 (12) % et 98 (14) % aux puissances respectives de CP et de CP+10 %. Ces observations constituent des bali-ses entre l’effort intense et l’effort excessif chez les enfants. De plus, un effort mene a la CP chez un groupe de garconsne correspond pas a une intensite d’exercice qui puisse etre maintenue en regime stable.

Mots-cles : tolerance a l’effort, temps de performance jusqu’a l’epuisement, intensites d’exercice, bicyclette ergometrique.

[Traduit par la Redaction]

IntroductionInterest in investigating the physiological responses to ex-

ercise between the lactate threshold (LT) and maximal oxy-gen uptake ( _VO2 max) has increased since the late 1990sbecause they are quantitatively and qualitatively different tothose found below LT. These differences depend on the in-tensity domain (i.e., moderate, heavy, and severe) in whichthe participant exercises. Within these domains, criticalpower (CP) provides an upper boundary to the heavy-intensity exercise domain and has been established using avariety of methodologies in adults (Hill 1993). CP is theslope of the time–distance or asymptote of the time–powerrelationship, and it has been suggested that it theoreticallyrepresents the intensity that can be maintained indefinitely

Received 10 January 2008. Accepted 6 August 2008. Publishedon the NRC Research Press Web site at apnm.nrc.ca on5 November 2008.

C.A. Williams.1 Children’s Health and Exercise ResearchCentre, School of Sport & Health Sciences, University of Exeter,Exeter, UK.J. Dekerle. Chelsea Research Centre, Chelsea School,University of Brighton, Eastbourne, UK; Laboratoire d’Etudesde la Motricite Humaine, University of Lille, Lille, France.K. McGawley and H. Carter. Chelsea Research Centre,Chelsea School, University of Brighton, Eastbourne, UK.S. Berthoin. Laboratoire d’Etudes de la Motricite Humaine,University of Lille, Lille, France.

1Corresponding author (e-mail: c.a.williams@exeter.ac.uk).

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without fatigue (Moritani et al. 1981). When continuousexercise is performed in the heavy-intensity domain (i.e.,between LT and CP), metabolic, respiratory, and cardio-vascular functions are able to stabilize (Carter et al. 2002;Poole et al. 1988). In adults exercising above CP, a pro-gressive drift of physiological parameters leads to theachievement of _VO2 max (Hill et al. 2002), thereby dramati-cally accelerating an individual’s time to exhaustion (TTE)(Brickley et al. 2002; Pringle and Jones 2002). In adults,TTE above CP is shorter than at CP and results in_VO2 max being achieved (Brickley et al. 2002). Thus, CPhas been defined as the maximal intensity that can be main-tained without attaining _VO2 max (Hill and Smith 1999; Hill1993).

To the best of our knowledge, only 1 study has focusedon CP in children (Fawkner and Armstrong 2002). This issurprising, as the correct identification and interpretation ofthe _VO2 responses to children’s exercise in specific do-mains is essential, because of the smaller absolute differ-ence between the commonly measured ‘‘anaerobicthreshold’’ and the peak oxygen uptake ( _VO2 peak). Hence,the incorrect demarcation of exercising relative to a per-centage of _VO2 peak could result in some children workingin higher or lower exercise domains (severe vs. heavy)than other children. To date, no research has been con-ducted investigating the underlying physiological mecha-nisms regulating the upper boundary of the heavy-intensitydomain in children.

This study was undertaken to determine CP and examineits relationship to both _VO2 peak and ventilatory threshold(VT) in young boys and girls. It was also designed to exam-ine the physiological responses at and just above the CP in-tensity for a sub-group of boys. We hypothesized that byexercising just above the CP intensity, boys will attain_VO2 peak.

Materials and methods

SubjectsNine boys (12.7 (0.3) y, 153 (11) cm, and 43.6

(11.8) kg) and 9 girls (12.0 (0.5) y, 148 (9) cm, and 40.9(9.0) kg) volunteered to participate in the study. A parentor guardian of each participant signed written informedconsent. The institutional ethics committee granted ethicalapproval. Prior to participation in the study, all volunteerscompleted a PAR-Q form. Child participants were excludedif they were taking any medication, such as beta-blockers,or had asthma that could confound their ability to partici-pate fully.

All exercise tests were conducted on a cycle ergometer(Lode Corival, Groningen, the Netherlands) over a 10-dayperiod. For each subject, tests took place at approximatelythe same time of day (within 2 h) to minimize the effects ofdiurnal biological variation on the results. The study was or-ganised in 2 parts. Part 1 comprised a preliminary testingstage and the protocol stage to determine CP in both boysand girls. During part 1 each child visited the laboratory on2 occasions. On day 1, the determination of _VO2 peak and theVT were performed. On day 2, a series of trials to determineCP were conducted.

Part 1: preliminary-testing stage, day 1

Incremental test to determine VT and _VO2 peak

The preliminary test included familiarization with the lab-oratory environment and VT and _VO2 peak were determinedusing a ramp test to voluntary exhaustion using breath-by-breath gas analysis. Pulmonary gas exchange was deter-mined using standard algorithms, allowing for the timedelay between gas concentration and volume signals. Indi-viduals breathed through a low-deadspace (90 mL), low-resistance (0.65 mm H2O�L–1�s–1 at 8 L�s–1) mouthpiece andturbine assembly. Gases were continuously drawn from themouthpiece through a 2 m capillary line of small bore(0.5 mm) at a rate of 60 mL�min–1 and analysed for O2, CO2,and N2 concentrations by a quadrupole mass spectrometer(CaSE QP9000, Gillingham, Kent, UK). The mass spectro-meter was calibrated for each test according to the manufac-turer’s instructions. Expiratory volumes were determinedusing a turbine volume transducer (Interface Associates Inc.,Laguna Niguel, Calif.). The volume and concentration sig-nals were integrated by computer after analog-to-digital con-version. Respiratory gas exchange variables ( _VO2, carbondioxide output ( _VCO2), pulmonary ventilation ( _VE)) werecalculated and displayed for every breath. In all tests, pulmo-nary gas exchange values was measured breath by breath andsubsequently interpolated to 1 s intervals.

Following a 3 min warm-up of unloaded pedalling, the re-sistance increased by 15 W for the girls and 20 W for theboys every minute until voluntary exhaustion. These ramprates were selected to bring the 2 groups to exhaustion inapproximately the same time frame. Verbal encouragementwas provided until the cessation of the test. Throughout theexercise tests, heart rate was recorded telemetrically (PolarElectro Oy, Kempele, Finland). Participants were encour-aged to maintain a cadence of around 70 r�min–1 during thisfirst test session and were required to maintain this cadence(±5 r�min–1) for all subsequent testing. Subjective criteria forthe attainment of maximal effort included excessive hyper-pnea, facial flushing, sweating, and discomfort; objectivecriteria included reaching an RER value > 1.1, children’s ef-fort rating table (CERT) > 7, and a heart rate >195 beats�min–1. All participants satisfied these criteria. The_VO2 peak was determined as the highest recorded 10 s sta-tionary average value during the maximal ramp test. VTwas defined as the _VO2 at which a non-linear increase in_VCO2 and an increase in _VE and in _VE/ _VO2 with noincrease in _VE/ _VCO2 were evident (Wasserman et al.1973). Three independent investigators blindly reviewed theplots of each index and made individual determinations ofVT. To calculate individually the power output correspond-ing to _VO2 peak (P _VO2 peak), regression analysis was carriedout on the second-by-second _VO2 data to determine the yintercept (439 (111) mL�min–1 and 386 (105) mL�min–1 forthe boys and girls, respectively) and the slope (9.9 (0.7)mL�min–1�W–1 and 10.8 (0.9) mL�min–1�W–1 for the boysand girls, respectively) of the P _VO2 relationship forexercise < VT (adjusted R2 = 0.85 (0.06) and 0.75 (0.12);SEE = 145 (21) mL�min–1 and 165 (37) mL�min–1 for theboys and girls, respectively). Maximum minute power(MMP) was derived from the SRM data logger as the high-

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est averaged power over 60 s. Fingertip capillary blood sam-ples (~25 mL) were collected in capillary tubes 3 min afterthe completion of the ramp test and subsequently analyzedfor lactate concentration ([La]) using an automated analyzer(YSI 2300, Yellow Springs, Ohio).

Preliminary-testing, day 2

Determination of the critical powerCritical power was determined using 3 trials to exhaus-

tion, all of which were performed in a single day, as thishas proven an effective and valid procedure in both adults(Brickley et al. 2002) and children (Fawkner and Armstrong2002). The first intensity was assigned as P _VO2 peak. Subse-quent intensities were then randomly performed using inten-sities around the P _VO2 peak, e.g., 95% and 105%, with atleast 2 h between tests. These selected intensities ensuredthe shortest and longest test differed by more than 5 minand that the tests were between 2 and 15 min in duration(Housh et al. 1990).

The children were continuously encouraged to cycle toexhaustion during each trial. The test was ended when theparticipant could no longer maintain a minimum pedal ca-dence of 50 r�min–1 for more than 5 s and the exact time tothe nearest second was recorded. No indication was given tothe participants as to the elapsed time or physiologicalmeasures being recorded. The test required all subjects tobe fitted with a mouthpiece held in place by a headpiece, anose clip, and a Polar heart rate monitor, which were wornthroughout the test period.

Regression analysesThe power vs. time–1 relationship (Hill 2004) was then

plotted and the y intercept, representing CP, was estimatedby least-squares linear regression analysis (Power = CP + W ’(time to failure)–1), where W ’ is an estimate of anaerobicwork capacity and the goodness of fit was determined as R2

and the standard error of estimate (SEE).

Part 2: protocol stage, days 3 and 4In part 2, 8 of the boys volunteered to participate in fur-

ther exercise tests. The preliminary test results of the boysduring part 1 were used to calculate the power output atCP. The boys performed 2 tests to voluntary exhaustion at 2different intensities: at the derived CP and at 10% above CP(CP+10%). The boys were required to wear the same equip-ment throughout the tests as they had in part 1 and all in-struments were used identically. All boys performed CP andthe CP+10% trials on separate days in a random order with atleast 24 h of recovery between tests. Gas analyses were con-tinuously monitored and averaged over 30 s and the _VO2slow component (SC) was calculated as the end-exercise_VO2 minus the 3 min _VO2 value. Additionally, the _VO2corresponding to CP was expressed as a percentage of thedifference between _VO2 peak and VT (%D) according to theequation ð _VO2 peak � _VO2 CPÞ=ð _VO2 peak � _VO2 VTÞ. TheCERT and fingertip blood capillary samples for lactatewere collected every 5 min. When the participant could nolonger maintain the pedal cadence, a final CERT and bloodlactate sample was collected 3 min after completion of thetest.

Statistical analysesResults are presented as means (standard deviation (SD)).

Pearson product–moment correlation coefficients were usedto investigate the relationship between the _VO2 peak, _VO2 atVT, and _VO2 at CP. Additional correlations were calculatedto explore the relationship between _VO2 peak, TTE, and theSC. Once the data were checked for normal distribution andhomogeneity of variation, independent t tests were used toestablish differences between boys and girls and paired sam-ple t tests were used to examine differences in dependentvariables between boys during CP and CP+10%. Significancewas set at the p < 0.05 level. Analyses were performed us-ing SPSS (version 11.0).

Results

The mean physiological data for the subject groups isshown in Table 1. In response to the ramp test, the mean_VO2 peak for boys and girls was 2.21 (0.55) L�min–1 and1.87 (0.40) L�min–1, respectively (p > 0.16). A significantdifference was found for maximal minute power (MMP) be-tween boys and girls, 174 (40) and 134 (31) W, respectively(p < 0.03). The power output at VT was also found to besignificantly different between boys and girls 86 (18) and48 (19) W, respectively (p < 0.01). The percentage of_VO2 peak at VT was 59 (6) for boys and 58 (6) % for girls(p > 0.87).

Adjusted R2 and SEE plots of the power output againsttime–1 from the 3 constant-load exhaustion tests were 0.85(0.22) and 5 (5) W and 0.93 (0.1) and 3 (2) W for boys andgirls, respectively. The boys’ TTE values were 635 (169),312 (87), and 196 (67) s at 136 (32), 153 (41), and 168(49) W, respectively. TTE for girls were 554 (142), 296(77), and 141 (49) s at 103 (28), 115 (30), and 133 (31) W,respectively. The mean CP for the boys and girls was 123(28) and 91 (26) W, respectively (p < 0.02). CP expressedas a percentage of _VO2 peak was found to be 75 (6) and 72(10) % for boys and girls, respectively (p > 0.47). The _VO2values measured during the CP and CP+10% trials, expressedas a %D, were 38 (11) and 53 (11) % during the CP+10%,respectively.

For boys, strong and significant correlations were foundbetween _VO2 peak and _VO2 at VT (r = 0.89; p < 0.001),_VO2 peak and _VO2 at CP (r = 0.96; p < 0.001), and _VO2 atCP and _VO2 at VT (r = 0.86; p < 0.003). For girls, similarlystrong and significant correlations were found between_VO2 peak and _VO2 at VT (r = 0.92; p < 0.001), _VO2 peak and_VO2 at CP (r = 0.84; p < 0.004), and _VO2 at CP and _VO2 atVT (r = 0.86; p < 0.003).

Table 2 shows the physiological responses at CP andCP+10%. The TTE was significantly different between theCP trial (18 min 37 s (4 min 13 s)) and the CP+10% trial(9 min 42 s (2 min 31 s), p < 0.007), but no further signifi-cant differences between variables were found (Table 2).Figure 1 represents the continuous measurement of thephysiological responses for the _VO2, _VE, and heart rate dur-ing the CP and CP+10% trials. Figure 2 shows the responsesof the [La] and CERT at the start, after 5 min, and at endexercise during the CP and CP+10% trials. Figure 3 represents

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a profile of 1 boy for the _VO2 response for the CP andCP+10% trials.

Positive but non-significant correlations were found be-tween the TTE and _VO2 SC during the CP and CP+10% trials(r = 0.43, p > 0.28 and r = 0.01, p > 0.97, respectively). Amoderate but non-significant correlation was found between_VO2 peak and TTE (r = 0.67; p > 0.07) during the CP trial,whereas a significant correlation was found between_VO2 peak and TTE (r = 0.77; p < 0.03) during the CP+10%trial. Moderate but non-significant correlations were alsofound between CP and TTE during the CP trial (r = 0.62;p > 0.10) and during the CP+10% trial (r = 0.69; p > 0.06).

DiscussionThe findings of the present study have shown that there is

a significant difference between the boys and girls for theabsolute power output at CP. CP was found to approximate74% of _VO2 peak for both sexes. The present study has pro-vided preliminary data on boys related to the physiologicalresponses at and above CP. In the boys exercising at CP, aphysiological non-steady-state was observed, in which _VO2

approached _VO2 peak. It was also found that exercising 10%

Table 1. Physiological characteristics from the peak oxygen uptakeand critical power (CP) tests.

Boys (n = 9) Girls (n = 9)

_VO2 peak_VO2 peak (L�min–1) 2.20 (0.55) 1.87 (0.40)Max. minute power (W) 174 (40) 134 (31)*Power at _VO2 peak (W) 166 (14) 127 (10)Max. heart rate (beats�min–1) 196 (4) 194 (10)VT (W) 86 (18) 48 (19)*_VO2 at VT (L�min–1) 1.32 (0.27) 1.09 (0.28)VT (% of _VO2 peak) 59 (6) 58 (6)Peak blood lactate (mmol�L–1) 5.8 (1.4) 6.0 (1.4)

Critical powerCP (W) 123 (28) 91 (26)*CP (% of _VO2 peak) 75 (6) 72 (10)

Note: Values are means ± SD. _V 2 peak, peak oxygen uptake; VT,ventilatory threshold.*Significantly different, p < 0.03.

Table 2. The physiological responses to exercise at critical power(CP) and 10% above CP (CP+10%).

CP(123 (28) W)

CP+10%(133 (11) W)

TTE (min:s) 18:37 (4:13) 9:42 (2:31)*EE [La] (mmol�L–1) 4.7 (2.1) 5.0 (2.4)EE _VO2 (L�min–1) 2.05 (0.35) 2.15 (0.40)EE _VO2 (% of _VO2 peak) 94 (12) 98 (14)EE heart rate (beats�min–1) 189 (11) 189 (9)EE CERT 9 (1) 8 (1)Slow component (mL�min–1) 297 (40) 289 (48)

Note: Values are means ± SD. TTE, time to exhaustion; EE, end exer-cise; [La], lactate concentration; _V 2 peak, peak oxygen uptake; CERT,children’s effort rating table; slow component = EE – 3 min.*Significantly different, p < 0.007.

Fig. 1. Physiological variables for critical power (CP) and 10%above CP (CP+10%) trials in relation to percent completion of timeto exhaustion (TTE) (mean ± SD). _VO2, oxygen uptake; _VE, pul-monary ventilation.

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above CP resulted in a halving of the average TTE. Despitethe shorter TTE in the CP+10% than in the CP trial, similarchanges in the physiological responses were observed. Theassociated link between the _VO2 SC and exercise tolerancein adults and the findings of this study that have shownmoderate but non-significant correlations between the _VO2SC, CP, and TTE in boys remains to be elucidated.

The purpose of this study was to determine criticalpower — a marker used to demarcate the boundary betweenthe heavy and severe domains of exercise intensity for agroup of boys and girls. The _VO2 at CP was found to occurat a similar % _VO2 peak compared with adults (Brickley et al.2002; Gaesser and Wilson 1988). In the only other child-

ren’s study to derive CP, Fawkner and Armstrong (2002) as-sessed the reliability of CP, but only estimated _VO2 at CPfrom data extrapolated from a previous _VO2 peak test.

Fawkner and Armstrong (2002) used 3 tests in 1 day toestimate a CP of 86.2 (18.1) W in a group of 8 boys and9 girls aged 10.3 (0.4) years, but did not differentiate be-tween sexes. In a subsequent publication investigating the_VO2 SC response to heavy exercise in boys and girls, theestimates for boys’ and girls’ CP was found to be signifi-cantly different, 99 (17) and 76 (12) W, respectively(Fawkner and Armstrong (2003b)). This significant differ-ence in CP in absolute terms between sexes was also con-firmed in this study with slightly older children and hasalso been found in a study on adults (Bulbulian et al. 1996).Unlike the study of Fawkner and Armstrong (2003b), thisstudy found no significant sex differences when CP was ex-pressed as a percentage of _VO2 peak (75 (6)% and 72 (10)%for boys and girls, respectively). The age and training statusof the participants is the most likely explanation for thesedifferences. The study of Fawkner and Armstrong (2003b)involved 10-year-old boys and girls who were not engagedin regular training or sport; however, the boys were signifi-cantly fitter according to the average absolute and relative_VO2 peak values. In contrast, both boys and girls in thepresent study were classified as recreationally active; it isalso possible that since girls mature up to 2 years earlierthan boys, this reduced the physiological differences oftendemonstrated during pre- and post-pubertal growth spurts.Unfortunately, owing to the prevailing sociological climateregarding such screening as Tanner indices for sexual ma-turational assessment and since maturational differenceswere not the key purpose of this study, no direct measure-ment was made. Adult studies reporting CP as a percentageof _VO2 max have varied from 80% (Hill and Smith 1999), to60%–90% (Housh et al. 1990), to 69%–79% (Vandewalle etal. 1997). Although there is an element of protocol depend-ence for whether or not the CP is sustainable and steady-state, our results are in accord with the published adultliterature.

In the sub-group of boys and consistent with adult data,

Fig. 2. Blood lactate concentration (B[La]) and children’s effortrating table responses at the start, after 5 min, and at the end of ex-ercise during the critical power (CP) and 10% above CP (CP+10%)trial (mean ± SD). RE, rating of effort.

Fig. 3. Oxygen uptake ( _VO2) response for 1 boy during the criticalpower (CP) and 10% above CP (CP+10%) trials.

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CP did not represent an intensity that can be maintained‘‘for a very long time without fatigue’’. Instead, exercise atCP resulted in an exercise time of ~19 min. End exercise_VO2 at CP (94% of _VO2 peak) and CP+10% (98% of_VO2 peak) were seen to rise to near-maximal values at ex-haustion, but were not significantly different between tests(p > 0.05). Our hypothesis regarding the attainment of_VO2 peak during trials above CP was therefore not confirmed;however, 3 boys did attain _VO2 peak values in both trials.

The TTE in adults at CP has typically been reported to bebetween 20 and 45 min (Brickley et al. 2002). In boys, _VO2was shown to rise over the course of the cycling bout at CP,finally attaining a value of ~94% _VO2 peak. The average in-crease of ~12 W in the CP+10% trial resulted in a shortenedTTE by 9 min compared with the TTE at CP. Interestingly,the _VO2 at 3 min was very similar between the 2 exerciseintensities. Furthermore, the increase in _VO2 over time wassimilar in both trials, despite a shorter exercise time atCP+10%. The higher intensity of exercise at CP+10% led to in-creased absolute values of _VO2 and [La], yet CERT waslower than and heart rate was the same as in the CP trial. Itis possible that the faster rate at which the SC developed inthe CP+10% trial projected _VO2 towards _VO2 peak more rap-idly and exhausted the finite anaerobic capacity of the boys.At CP, the development of the SC was not as rapid and thisenabled the limited anaerobic capacity to be utilized for alonger period of time, resulting in an increased TTE. Therate at which _VO2 rises at exercise onset and the develop-ment of the _VO2 SC has important conceptual and practicalsignificance, as both are known to affect exercise tolerance(Barstow et al. 1996; Jones et al. 2003). Interestingly, therewas an observable _VO2 SC in each trial, contrasting withour previous work (Williams et al. 2001). This is probablyrelated to the mode of exercise, since it is accepted that thesize of this index is greater in cycling exercise than in tread-mill ergometry (Carter et al. 2002; Fawkner and Armstrong2003a). In adult studies, the attainment of _VO2 max at CPdoes not occur and exercise is terminated due to other fac-tors before the _VO2 max is reached. For 5 of the boys in thecurrent study this was also the case; however, individual dif-ferences were noted, as 3 of the boys did attain _VO2 peak.

Factors that determine CP include the balance betweenthe accumulation and removal of fatiguing metabolites (in-organic phospate (Pi), H+), the depletion of high-energyphosphate stores (PCr), and alterations in motor unit recruit-ment. There were no significant differences in [La] andheart rate between trials, but the increasing _VO2 over theduration of the trial might be indicative of changing musclerecruitment patterns (Barstow et al. 1996). At present, thereis too little published work in relation to high-energy phos-phates in children, beyond the speculation that children mayhave an inferior muscle glycolytic activity during high-intensity exercise (Ratel et al. 2006; Zanconato et al. 1993).

Despite the same three boys also attaining _VO2 peak duringthe CP+10% trial, the mean percentage of _VO2 peak increasedduring the CP+10% trial compared with the CP trial. For theremaining 5 boys, the non-attainment of _VO2 peak when exer-cising at CP or CP+10% meant exercise was terminated, pre-sumably due to other factors, before _VO2 peak was reached. Itcould be argued that motivation was a deciding factor in ter-

minating the exercise, but we do not believe this to be thecase. The mirroring of the increasing physiological variablesover time in both trials, despite the shorter TTE during theCP+10% trial, suggests that the boys were well motivated toperform. For all of the boys the equalization of the intensitydomains relative to CP rather than setting the exercise inten-sities using a percentage of _VO2 peak or VT should haveresulted in the boys exercising within the same severe-intensity exercise domain (Fawkner and Armstrong, 2003a).Compared with adults, children’s smaller absolute range ofwork rates from VT to _VO2 peak means this identification ismore difficult and if incorrect would result in earlier exhaus-tion and termination of exercise. The _VO2 corresponding toCP expressed as a %D was found to be ~40% in the CPtrial, although a coefficient of variation of 28% demon-strates the inter-subject variability and the difficulty of en-suring parity of intensity domains between children. Thevariability around the mean D value of ~50% in the CP+10%trial confirms this observation. However, further work is re-quired on the reproducibility of these measures.

In conclusion, a significant difference for the absolutepower output at CP between boys and girls is apparent inthis study. However CP, as a percentage of _VO2 peak, wasfound to be similar for both sexes. When the boys exercisedat CP (equivalent to ~74% _VO2 peak) a physiological non-steady-state was observed, eventually leading to near attain-ment of _VO2 peak. Exercising just above CP (CP+10%)reduced exercise tolerance. Considerable inter-subject vari-ability was found in the TTE and this observation requiresfurther research. However, we have rejected our originalhypothesis that boys exercising just above the CP intensitywill result in the attainment of _VO2 peak. Exercise in thesedomains appears to be well tolerated by motivated boys andis a prerequisite for future investigations, including studieswith girls. The application of the CP concept for childrenenables a more accurate description of the physiological re-sponses to exercise, rather than assuming the boundaries ofthe heavy and severe exercise intensity domains. This wouldallow paediatric physiologists to fully understand children’sresponses to exercise intensity domains and the ensuing con-sequences of fatigue and tolerance to exercise.

AcknowledgementsThis study was part of the ‘‘InterEx’’ project, supported

by a grant from the EU under the Interreg IIIa grant scheme.

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