Critical power in adolescent boys and girls - NRC Research Press

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Critical power in adolescent boys and girls — an exploratory study Craig A. Williams, Jeanne Dekerle, Kerry McGawley, Serge Berthoin, and Helen Carter

Abstract: The purpose of the study was to identify critical power (CP) in boys and girls and to examine the physiological responses 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 performed 3 constant-load tests to derive CP. Eight of the boys then exercised, in random order, at CP and CP+10% until volitional exhaustion. 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), which was significantly longer (p < 0.007) than that at CP+10% (9 min 42 s (2 min 31 s)). End-exercise values for blood lactate concentration (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, respectively) 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). Peak oxygen 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 exercise intensity domains in children, and have demonstrated that CP in a group of boys does not represent a sustainable steadystate intensity of exercise. Key words: exercise tolerance, time to exhaustion, exercise intensities, cycle ergometry. Re´sume´ : Le but de cette e´tude est de de´terminer la puissance critique (CP) des filles et des garc¸ons et d’analyser chez un sous-groupe de garc¸ons les ajustements physiologiques a` cette intensite´ de travail et a` 10 % au-dessus de cette valeur (CP+10 %). Neuf filles et neuf garc¸ons aˆge´s de 12,3 (0,5) ans participent a` trois e´preuves de charge constante conc¸ues pour l’e´valuation de la CP. Ensuite, huit garc¸ons participent de fac¸on ale´atoire a` un effort d’intensite´ e´quivalant a` la CP et a` la CP+10 % jusqu’a` l’e´puisement volontaire. La CP des garc¸ons est de 123 (28) W et celle des filles est de 91 (26) W (p < 0,02), ce qui e´quivaut respectivement a` 75 (6) % et 72 (10) % du consommation d’oxygene de pointe (p > 0,47). Le temps de performance jusqu’a` l’e´puisement a` une intensite´ correspondant a` la CP est de 18 min 37 s (4 min 13 s), ce qui est significativement plus long (p < 0,007) que le temps de performance (9 min 42 s (2 min 31 s)) a` une intensite´ correspondant a` la CP+10 %. Les valeurs de la concentration de lactate sanguin (B[la]) et du consommation d’oxygene sont supe´rieures a` la fin de l’effort re´alise´ 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’a` ` la fin des l’effort re´alise´ 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 e´preuves, les sujets n’atteignent pas le consommation d’oxygene de pointe (exprime´ en pourcentage de la valeur de pointe), soit 94 (12) % et 98 (14) % aux puissances respectives de CP et de CP+10 %. Ces observations constituent des balises entre l’effort intense et l’effort excessif chez les enfants. De plus, un effort mene´ a` la CP chez un groupe de garc¸ons ne correspond pas a` une intensite´ d’exercice qui puisse eˆtre maintenue en re´gime stable. Mots-cle´s : tole´rance a` l’effort, temps de performance jusqu’a` l’e´puisement, intensite´s d’exercice, bicyclette ergome´trique. [Traduit par la Re´daction]

Received 10 January 2008. Accepted 6 August 2008. Published on the NRC Research Press Web site at apnm.nrc.ca on 5 November 2008. C.A. Williams.1 Children’s Health and Exercise Research Centre, School of Sport & Health Sciences, University of Exeter, Exeter, UK. J. Dekerle. Chelsea Research Centre, Chelsea School, University of Brighton, Eastbourne, UK; Laboratoire d’Etudes de 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: [email protected]).

Appl. Physiol. Nutr. Metab. 33: 1105–1111 (2008)

Introduction Interest in investigating the physiological responses to exercise between the lactate threshold (LT) and maximal oxygen uptake (V_ O2 max) has increased since the late 1990s because they are quantitatively and qualitatively different to those found below LT. These differences depend on the intensity domain (i.e., moderate, heavy, and severe) in which the participant exercises. Within these domains, critical power (CP) provides an upper boundary to the heavyintensity exercise domain and has been established using a variety of methodologies in adults (Hill 1993). CP is the slope of the time–distance or asymptote of the time–power relationship, and it has been suggested that it theoretically represents the intensity that can be maintained indefinitely

doi:10.1139/H08-096

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without fatigue (Moritani et al. 1981). When continuous exercise is performed in the heavy-intensity domain (i.e., between LT and CP), metabolic, respiratory, and cardiovascular functions are able to stabilize (Carter et al. 2002; Poole et al. 1988). In adults exercising above CP, a progressive drift of physiological parameters leads to the achievement of V_ O2 max (Hill et al. 2002), thereby dramatically 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 V_ O2 max being achieved (Brickley et al. 2002). Thus, CP has been defined as the maximal intensity that can be maintained without attaining V_ O2 max (Hill and Smith 1999; Hill 1993). To the best of our knowledge, only 1 study has focused on CP in children (Fawkner and Armstrong 2002). This is surprising, as the correct identification and interpretation of the V_ O2 responses to children’s exercise in specific domains is essential, because of the smaller absolute difference between the commonly measured ‘‘anaerobic threshold’’ and the peak oxygen uptake (V_ O2 peak). Hence, the incorrect demarcation of exercising relative to a percentage of V_ O2 peak could result in some children working in higher or lower exercise domains (severe vs. heavy) than other children. To date, no research has been conducted investigating the underlying physiological mechanisms regulating the upper boundary of the heavy-intensity domain in children. This study was undertaken to determine CP and examine its relationship to both V_ O2 peak and ventilatory threshold (VT) in young boys and girls. It was also designed to examine the physiological responses at and just above the CP intensity for a sub-group of boys. We hypothesized that by exercising just above the CP intensity, boys will attain V_ O2 peak.

Materials and methods Subjects Nine 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 parent or guardian of each participant signed written informed consent. The institutional ethics committee granted ethical approval. Prior to participation in the study, all volunteers completed a PAR-Q form. Child participants were excluded if they were taking any medication, such as beta-blockers, or had asthma that could confound their ability to participate fully. All exercise tests were conducted on a cycle ergometer (Lode Corival, Groningen, the Netherlands) over a 10-day period. For each subject, tests took place at approximately the same time of day (within 2 h) to minimize the effects of diurnal biological variation on the results. The study was organised in 2 parts. Part 1 comprised a preliminary testing stage and the protocol stage to determine CP in both boys and girls. During part 1 each child visited the laboratory on 2 occasions. On day 1, the determination of V_ O2 peak and the VT were performed. On day 2, a series of trials to determine CP were conducted.

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Part 1: preliminary-testing stage, day 1 _ 2 peak Incremental test to determine VT and VO The preliminary test included familiarization with the laboratory environment and VT and V_ O2 peak were determined using a ramp test to voluntary exhaustion using breath-bybreath gas analysis. Pulmonary gas exchange was determined using standard algorithms, allowing for the time delay between gas concentration and volume signals. Individuals breathed through a low-deadspace (90 mL), lowresistance (0.65 mm H2O
L–1
s–1 at 8 L
s–1) mouthpiece and turbine assembly. Gases were continuously drawn from the mouthpiece 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 spectrometer was calibrated for each test according to the manufacturer’s instructions. Expiratory volumes were determined using a turbine volume transducer (Interface Associates Inc., Laguna Niguel, Calif.). The volume and concentration signals were integrated by computer after analog-to-digital conversion. Respiratory gas exchange variables (V_ O2, carbon dioxide output (V_ CO2), pulmonary ventilation (V_ E)) were calculated and displayed for every breath. In all tests, pulmonary gas exchange values was measured breath by breath and subsequently interpolated to 1 s intervals. Following a 3 min warm-up of unloaded pedalling, the resistance increased by 15 W for the girls and 20 W for the boys every minute until voluntary exhaustion. These ramp rates were selected to bring the 2 groups to exhaustion in approximately the same time frame. Verbal encouragement was provided until the cessation of the test. Throughout the exercise tests, heart rate was recorded telemetrically (Polar Electro Oy, Kempele, Finland). Participants were encouraged to maintain a cadence of around 70 r
min–1 during this first test session and were required to maintain this cadence (±5 r
min–1) for all subsequent testing. Subjective criteria for the attainment of maximal effort included excessive hyperpnea, facial flushing, sweating, and discomfort; objective criteria included reaching an RER value > 1.1, children’s effort rating table (CERT) > 7, and a heart rate > 195 beats
min–1. All participants satisfied these criteria. The V_ O2 peak was determined as the highest recorded 10 s stationary average value during the maximal ramp test. VT was defined as the V_ O2 at which a non-linear increase in V_ CO2 and an increase in V_ E and in V_ E/V_ O2 with no increase in V_ E/V_ CO2 were evident (Wasserman et al. 1973). Three independent investigators blindly reviewed the plots of each index and made individual determinations of VT. To calculate individually the power output corresponding to V_ O2 peak (PV_ O2 peak), regression analysis was carried out on the second-by-second V_ O2 data to determine the y intercept (439 (111) mL
min–1 and 386 (105) mL
min–1 for the 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 boys and girls, respectively) of the PV_ O2 relationship for exercise < 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 the boys 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 samples (~25 mL) were collected in capillary tubes 3 min after the completion of the ramp test and subsequently analyzed for lactate concentration ([La]) using an automated analyzer (YSI 2300, Yellow Springs, Ohio). Preliminary-testing, day 2 Determination of the critical power Critical power was determined using 3 trials to exhaustion, all of which were performed in a single day, as this has proven an effective and valid procedure in both adults (Brickley et al. 2002) and children (Fawkner and Armstrong 2002). The first intensity was assigned as PV_ O2 peak. Subsequent intensities were then randomly performed using intensities around the PV_ O2 peak, e.g., 95% and 105%, with at least 2 h between tests. These selected intensities ensured the shortest and longest test differed by more than 5 min and that the tests were between 2 and 15 min in duration (Housh et al. 1990). The children were continuously encouraged to cycle to exhaustion during each trial. The test was ended when the participant could no longer maintain a minimum pedal cadence of 50 r
min–1 for more than 5 s and the exact time to the nearest second was recorded. No indication was given to the participants as to the elapsed time or physiological measures being recorded. The test required all subjects to be fitted with a mouthpiece held in place by a headpiece, a nose clip, and a Polar heart rate monitor, which were worn throughout the test period. Regression analyses The power vs. time–1 relationship (Hill 2004) was then plotted and the y intercept, representing CP, was estimated by least-squares linear regression analysis (Power = CP + W ’ (time to failure)–1), where W ’ is an estimate of anaerobic work capacity and the goodness of fit was determined as R2 and the standard error of estimate (SEE). Part 2: protocol stage, days 3 and 4 In part 2, 8 of the boys volunteered to participate in further exercise tests. The preliminary test results of the boys during part 1 were used to calculate the power output at CP. The boys performed 2 tests to voluntary exhaustion at 2 different intensities: at the derived CP and at 10% above CP (CP+10%). The boys were required to wear the same equipment throughout the tests as they had in part 1 and all instruments were used identically. All boys performed CP and the CP+10% trials on separate days in a random order with at least 24 h of recovery between tests. Gas analyses were continuously monitored and averaged over 30 s and the V_ O2 slow component (SC) was calculated as the end-exercise V_ O2 minus the 3 min V_ O2 value. Additionally, the V_ O2 corresponding to CP was expressed as a percentage of the difference between V_ O2 peak and VT (%D) according to the equation ðV_ O2 peak  V_ O2 CP Þ=ðV_ O2 peak  V_ O2 VT Þ. The CERT and fingertip blood capillary samples for lactate were collected every 5 min. When the participant could no longer maintain the pedal cadence, a final CERT and blood lactate sample was collected 3 min after completion of the test.

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Statistical analyses Results are presented as means (standard deviation (SD)). Pearson product–moment correlation coefficients were used to investigate the relationship between the V_ O2 peak, V_ O2 at VT, and V_ O2 at CP. Additional correlations were calculated to explore the relationship between V_ O2 peak, TTE, and the SC. Once the data were checked for normal distribution and homogeneity of variation, independent t tests were used to establish differences between boys and girls and paired sample t tests were used to examine differences in dependent variables between boys during CP and CP+10%. Significance was set at the p < 0.05 level. Analyses were performed using SPSS (version 11.0).

Results The mean physiological data for the subject groups is shown in Table 1. In response to the ramp test, the mean V_ O2 peak for boys and girls was 2.21 (0.55) L
min–1 and 1.87 (0.40) L
min–1, respectively (p > 0.16). A significant difference was found for maximal minute power (MMP) between boys and girls, 174 (40) and 134 (31) W, respectively (p < 0.03). The power output at VT was also found to be significantly different between boys and girls 86 (18) and 48 (19) W, respectively (p < 0.01). The percentage of V_ O2 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 against time–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 and girls, 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 expressed as a percentage of V_ O2 peak was found to be 75 (6) and 72 (10) % for boys and girls, respectively (p > 0.47). The V_ O2 values measured during the CP and CP+10% trials, expressed as a %D, were 38 (11) and 53 (11) % during the CP+10%, respectively. For boys, strong and significant correlations were found between V_ O2 peak and V_ O2 at VT (r = 0.89; p < 0.001), V_ O2 peak and V_ O2 at CP (r = 0.96; p < 0.001), and V_ O2 at CP and V_ O2 at VT (r = 0.86; p < 0.003). For girls, similarly strong and significant correlations were found between V_ O2 peak and V_ O2 at VT (r = 0.92; p < 0.001), V_ O2 peak and V_ O2 at CP (r = 0.84; p < 0.004), and V_ O2 at CP and V_ O2 at VT (r = 0.86; p < 0.003). Table 2 shows the physiological responses at CP and CP+10%. The TTE was significantly different between the CP 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 significant differences between variables were found (Table 2). Figure 1 represents the continuous measurement of the physiological responses for the V_ O2, V_ E, and heart rate during the CP and CP+10% trials. Figure 2 shows the responses of the [La] and CERT at the start, after 5 min, and at end exercise during the CP and CP+10% trials. Figure 3 represents #

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Table 1. Physiological characteristics from the peak oxygen uptake and critical power (CP) tests. Boys (n = 9)

Girls (n = 9)

_ 2 peak VO V_ O2 peak (L
min–1) Max. minute power (W) Power at V_ O2 peak (W) Max. heart rate (beats
min–1) VT (W) V_ O2 at VT (L
min–1) VT (% of V_ O2 peak) Peak blood lactate (mmol
L–1)

2.20 (0.55) 174 (40) 166 (14) 196 (4) 86 (18) 1.32 (0.27) 59 (6) 5.8 (1.4)

1.87 (0.40) 134 (31)* 127 (10) 194 (10) 48 (19)* 1.09 (0.28) 58 (6) 6.0 (1.4)

Critical power CP (W) CP (% of V_ O2 peak)

123 (28) 75 (6)

91 (26)* 72 (10)

Fig. 1. Physiological variables for critical power (CP) and 10% above CP (CP+10%) trials in relation to percent completion of time to exhaustion (TTE) (mean ± SD). V_ O2, oxygen uptake; V_ E, pulmonary ventilation.

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%).

TTE (min:s) EE [La] (mmol
L–1) EE V_ O2 (L
min–1) EE V_ O2 (% of V_ O2 peak) EE heart rate (beats
min–1) EE CERT Slow component (mL
min–1)

CP (123 (28) W) 18:37 (4:13) 4.7 (2.1) 2.05 (0.35) 94 (12) 189 (11) 9 (1) 297 (40)

CP+10% (133 (11) W) 9:42 (2:31)* 5.0 (2.4) 2.15 (0.40) 98 (14) 189 (9) 8 (1) 289 (48)

Note: Values are means ± SD. TTE, time to exhaustion; EE, end exercise; [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.

a profile of 1 boy for the V_ O2 response for the CP and CP+10% trials. Positive but non-significant correlations were found between the TTE and V_ O2 SC during the CP and CP+10% trials (r = 0.43, p > 0.28 and r = 0.01, p > 0.97, respectively). A moderate but non-significant correlation was found between V_ O2 peak and TTE (r = 0.67; p > 0.07) during the CP trial, whereas a significant correlation was found between V_ O2 peak and TTE (r = 0.77; p < 0.03) during the CP+10% trial. Moderate but non-significant correlations were also found 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).

Discussion The findings of the present study have shown that there is a significant difference between the boys and girls for the absolute power output at CP. CP was found to approximate 74% of V_ O2 peak for both sexes. The present study has provided preliminary data on boys related to the physiological responses at and above CP. In the boys exercising at CP, a physiological non-steady-state was observed, in which V_ O2 approached V_ O2 peak. It was also found that exercising 10% #

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

above CP resulted in a halving of the average TTE. Despite the shorter TTE in the CP+10% than in the CP trial, similar changes in the physiological responses were observed. The associated link between the V_ O2 SC and exercise tolerance in adults and the findings of this study that have shown moderate but non-significant correlations between the V_ O2 SC, CP, and TTE in boys remains to be elucidated. The purpose of this study was to determine critical power — a marker used to demarcate the boundary between the heavy and severe domains of exercise intensity for a group of boys and girls. The V_ O2 at CP was found to occur at a similar %V_ O2 peak compared with adults (Brickley et al. 2002; Gaesser and Wilson 1988). In the only other child-

1109 Fig. 3. Oxygen uptake (V_ O2) response for 1 boy during the critical power (CP) and 10% above CP (CP+10%) trials.

ren’s study to derive CP, Fawkner and Armstrong (2002) assessed the reliability of CP, but only estimated V_ O2 at CP from data extrapolated from a previous V_ O2 peak test. Fawkner and Armstrong (2002) used 3 tests in 1 day to estimate a CP of 86.2 (18.1) W in a group of 8 boys and 9 girls aged 10.3 (0.4) years, but did not differentiate between sexes. In a subsequent publication investigating the V_ O2 SC response to heavy exercise in boys and girls, the estimates for boys’ and girls’ CP was found to be significantly different, 99 (17) and 76 (12) W, respectively (Fawkner and Armstrong (2003b)). This significant difference in CP in absolute terms between sexes was also confirmed in this study with slightly older children and has also been found in a study on adults (Bulbulian et al. 1996). Unlike the study of Fawkner and Armstrong (2003b), this study found no significant sex differences when CP was expressed as a percentage of V_ O2 peak (75 (6)% and 72 (10)% for boys and girls, respectively). The age and training status of the participants is the most likely explanation for these differences. The study of Fawkner and Armstrong (2003b) involved 10-year-old boys and girls who were not engaged in regular training or sport; however, the boys were significantly fitter according to the average absolute and relative V_ O2 peak values. In contrast, both boys and girls in the present study were classified as recreationally active; it is also possible that since girls mature up to 2 years earlier than boys, this reduced the physiological differences often demonstrated during pre- and post-pubertal growth spurts. Unfortunately, owing to the prevailing sociological climate regarding such screening as Tanner indices for sexual maturational assessment and since maturational differences were not the key purpose of this study, no direct measurement was made. Adult studies reporting CP as a percentage of V_ O2 max have varied from 80% (Hill and Smith 1999), to 60%–90% (Housh et al. 1990), to 69%–79% (Vandewalle et al. 1997). Although there is an element of protocol dependence for whether or not the CP is sustainable and steadystate, our results are in accord with the published adult literature. In the sub-group of boys and consistent with adult data, #

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CP did not represent an intensity that can be maintained ‘‘for a very long time without fatigue’’. Instead, exercise at CP resulted in an exercise time of ~19 min. End exercise V_ O2 at CP (94% of V_ O2 peak) and CP+10% (98% of V_ O2 peak) were seen to rise to near-maximal values at exhaustion, but were not significantly different between tests (p > 0.05). Our hypothesis regarding the attainment of V_ O2 peak during trials above CP was therefore not confirmed; however, 3 boys did attain V_ O2 peak values in both trials. The TTE in adults at CP has typically been reported to be between 20 and 45 min (Brickley et al. 2002). In boys, V_ O2 was shown to rise over the course of the cycling bout at CP, finally attaining a value of ~94% V_ O2 peak. The average increase of ~12 W in the CP+10% trial resulted in a shortened TTE by 9 min compared with the TTE at CP. Interestingly, the V_ O2 at 3 min was very similar between the 2 exercise intensities. Furthermore, the increase in V_ O2 over time was similar in both trials, despite a shorter exercise time at CP+10%. The higher intensity of exercise at CP+10% led to increased absolute values of V_ O2 and [La], yet CERT was lower than and heart rate was the same as in the CP trial. It is possible that the faster rate at which the SC developed in the CP+10% trial projected V_ O2 towards V_ O2 peak more rapidly and exhausted the finite anaerobic capacity of the boys. At CP, the development of the SC was not as rapid and this enabled the limited anaerobic capacity to be utilized for a longer period of time, resulting in an increased TTE. The rate at which V_ O2 rises at exercise onset and the development of the V_ O2 SC has important conceptual and practical significance, as both are known to affect exercise tolerance (Barstow et al. 1996; Jones et al. 2003). Interestingly, there was an observable V_ O2 SC in each trial, contrasting with our previous work (Williams et al. 2001). This is probably related to the mode of exercise, since it is accepted that the size of this index is greater in cycling exercise than in treadmill ergometry (Carter et al. 2002; Fawkner and Armstrong 2003a). In adult studies, the attainment of V_ O2 max at CP does not occur and exercise is terminated due to other factors before the V_ O2 max is reached. For 5 of the boys in the current study this was also the case; however, individual differences were noted, as 3 of the boys did attain V_ O2 peak. Factors that determine CP include the balance between the accumulation and removal of fatiguing metabolites (inorganic phospate (Pi), H+), the depletion of high-energy phosphate stores (PCr), and alterations in motor unit recruitment. There were no significant differences in [La] and heart rate between trials, but the increasing V_ O2 over the duration of the trial might be indicative of changing muscle recruitment patterns (Barstow et al. 1996). At present, there is too little published work in relation to high-energy phosphates in children, beyond the speculation that children may have an inferior muscle glycolytic activity during highintensity exercise (Ratel et al. 2006; Zanconato et al. 1993). Despite the same three boys also attaining V_ O2 peak during the CP+10% trial, the mean percentage of V_ O2 peak increased during the CP+10% trial compared with the CP trial. For the remaining 5 boys, the non-attainment of V_ O2 peak when exercising at CP or CP+10% meant exercise was terminated, presumably due to other factors, before V_ O2 peak was reached. It could be argued that motivation was a deciding factor in ter-

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minating the exercise, but we do not believe this to be the case. The mirroring of the increasing physiological variables over time in both trials, despite the shorter TTE during the CP+10% trial, suggests that the boys were well motivated to perform. For all of the boys the equalization of the intensity domains relative to CP rather than setting the exercise intensities using a percentage of V_ O2 peak or VT should have resulted in the boys exercising within the same severeintensity exercise domain (Fawkner and Armstrong, 2003a). Compared with adults, children’s smaller absolute range of work rates from VT to V_ O2 peak means this identification is more difficult and if incorrect would result in earlier exhaustion and termination of exercise. The V_ O2 corresponding to CP expressed as a %D was found to be ~40% in the CP trial, although a coefficient of variation of 28% demonstrates the inter-subject variability and the difficulty of ensuring parity of intensity domains between children. The variability around the mean D value of ~50% in the CP+10% trial confirms this observation. However, further work is required on the reproducibility of these measures. In conclusion, a significant difference for the absolute power output at CP between boys and girls is apparent in this study. However CP, as a percentage of V_ O2 peak, was found to be similar for both sexes. When the boys exercised at CP (equivalent to ~74% V_ O2 peak) a physiological nonsteady-state was observed, eventually leading to near attainment of V_ O2 peak. Exercising just above CP (CP+10%) reduced exercise tolerance. Considerable inter-subject variability was found in the TTE and this observation requires further research. However, we have rejected our original hypothesis that boys exercising just above the CP intensity will result in the attainment of V_ O2 peak. Exercise in these domains appears to be well tolerated by motivated boys and is a prerequisite for future investigations, including studies with girls. The application of the CP concept for children enables a more accurate description of the physiological responses to exercise, rather than assuming the boundaries of the heavy and severe exercise intensity domains. This would allow paediatric physiologists to fully understand children’s responses to exercise intensity domains and the ensuing consequences of fatigue and tolerance to exercise.

Acknowledgements 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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