DOI 10.1378/chest.112.6.1554 1997;112;1554-1560Chest

 WelsmanNeil Armstrong, Brian J. Kirby, Alison M. McManus and Joanne R. Size

BodyExercise With Reference to Sex and Prepubescents' Ventilatory Responses to

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Prepubescents' Ventilatory Responsesto Exercise With Reference to Sex andBody Size*Neil Armstrong, PhD; Brian J. Kirby, MD; Alison M. McManus, PhD;f andJoanne R. Welsman, PhD

Study objectives: To examine the ventilatory responses of prepubescent children to submaximaland peak exercise using appropriate allometric modeling to control for differences in body size.Design: Cross-sectional study of a representative sample of children.Setting: Middle schools (8 to 11 years) in Exeter, UK.Participants: We studied 101 boys and 76 girls aged 11.1 (0.4) years and classified Tanner stage1 for pubic hair (no true pubic hair).Measurements: At rest: stature, mass, sum of skinfolds, hemoglobin concentration, FVC, andFEV!. During treadmill exercise at 7, 8, 9, and 10 km/h, and at peak exercise: oxygen uptake(Vo2), minute ventilation (Ve), tidal volume (Vt), and respiratory frequency (Rf).Results: At peak exercise, boys' Vo2, Ve, and Vt were significantly (p<0.01) higher than girls'values and remained so even when the influence of body size was controlled using allometricprinciples. There were no significant (p>0.05) sex differences in Rf or the ratios Vt/FVC or

Ve/Vo2. When data were compared at the same relative exercise intensity (ie, 70 to 75% or 80 to85% peak Vo2), no significant (p>0.05) sex differences in Rf, Vt/FVC, or Ve/Vo2 were detected.Boys' higher (p< 0.001) Vo2 values were reflected by their higher Ve which remained higher thanvalues for girls at both submaximal levels even when the influence of body size was covaried out.Conclusions: Prepubescent boys demonstrate higher peak Vo2 than girls and this is supported bya higher Ve and Vt, even when the influence of body size is accounted for using allometry. Otherventilatory responses to both peak exercise and exercise at the same relative intensity are

remarkably similar in both boys and girls. (CHEST 1997; 112:1554-60)

Key words: allometric modeling; children; exercise; ventilation

Abbreviations: ANCOVA=analysis of covariance; MW=maximal voluntary ventilation; RER=respiratory exchangeratio; Rf= respiratoiy7 frequency; Vco2=carbon dioxide output; Ve=ventilation; Vo2=oxygen uptake; VT=tidal volume

T n comparison to adults and adolescents, relatively-*¦ little is known about the responses of prepubes¬cent children to exercise. Data are sparse and largelylimited to oxygen uptake (Vo2) and heart rate re¬

sponses, usually during peak exercise conditions.1Consequently, knowledge of prepubescent chil¬dren's ventilatory responses to treadmill exercise, inrelation to sex and body size, is limited.

Ventilatory variables such as minute ventilation(Ve) and tidal volume (Vt) are highly related to

*From the Children's Health and Exercise Research Centre,Institute of Clinical Science, University of Exeter, UK.Currently at the Centre for Physical Education and SportsScience at the University of Hong Kong.Supported by the British Heart Foundation and the HealthyHeart Research Trust.Manuscript received December 6, 1996; revision accepted Mav6, 1997.Reprint requests: Neil Armstrong, PhD, Children s Health and ExerciseResearch Centre, University of Exeter, Heavitree Rd, Exeter, UnitedKingdom EX1 2LU; email: N.Armstrong@exeter.aeuk

anthropometric characteristics, particularly mass andstature.2 Therefore, studies of young people's venti¬

latory responses to exercise have generally examinedsize-related function by dividing the measure bymass (ie, the ratio standard). This is assumed toremove the influence of body size, but it is well-documented that per-body mass ratios do not renderperformance measures independent of body size.35The use of allometric models (y=a#xb) to derive

truly size-independent exercise performance mea¬

sures has been advocated by several authors46 andhas been demonstrated to be theoretically and sta¬

tistically superior to the simple ratio standard in

analyzing size-related function. The ratio methodimposes the mathematical relationship y=a-x10upon the performance (y) and body size (x) measureswhereas allometric modeling allows the body sizeexponent (b) to be derived from the data set underinvestigation. Parameters a and b in the allometric

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relationship may be solved using analysis of covari-ance (ANCOVA) with log y as the dependent vari¬able and log x entered as the covariate. A furtheradvantage of this approach is that more than one

body size variable may be investigated by incorpo¬rating additional covariates.5 Few studies have exam¬

ined young people's ventilatory responses to exercise

using allometry, but evidence from a study of ado¬lescent boys identified mass and stature exponentswhich demonstrated that ratio standards do not

adequately remove the influence of body size frommeasures such as peak Ve2. To our knowledge, dataon prepubescent children are not available.As part of an ongoing longitudinal study, we have

collected resting, submaximal, and peak exerciseventilatory data on a large, well-defined sample ofprepubescent children. The purpose of this article isto use these data to enhance understanding of theventilatory responses of prepubescent children toexercise and specifically to provide a comparativeanalysis of the ventilatory responses of boys and girlsto submaximal and peak exercise using allometricmodeling to control for differences in body size.

Materials and Methods

SubjectsAll of the children in year 6 (age, 11.1 ±0.4 years) of the 15

state schools in the city of Exeter, UK were invited to participatein a longitudinal study of physical activity patterns, aerobic andanaerobic fitness, and body composition. Seven hundred forty-five children (70% of those eligible) volunteered, and writteninformed consent was obtained from both the children and theirparents or guardian. The study received ethical approval from theExeter District Health Authority Ethical Committee. In an

attempt to detect sample bias, the stature and body mass of thevolunteers were compared with the stature and body mass ofthose who declined to participate. No significant difference(p>0.05) was detected in either sex. Twenty-five percent of theeligible children in each school were randomly selected fromthose who volunteered. The data reported herein are thoseobtained from the 101 boys and 76 girls, classified as prepubes¬cent, using the indexes of pubic hair of Tanner,7 who satisfactorilycompleted the procedures described below during the first yearof the study.

Method

Age was computed from date of birth and date of examination.Sexual maturity was visually assessed from pubic hair using theindexes developed by Tanner.7 All observations were made by thesame nurse, during a preexercise examination that screened out

any children from whom maximal exercise was contraindicated,and only subjects classified as Tanner stage 1 (no true pubic hair)were included in this study. Stature, body mass, skinfold thick¬nesses, and dynamic lung volumes were measured according tothe techniques described by Weiner and Lourie.8 Stature was

measured using a stadiometer (Holtain; Crymych; Dyfed, UK)and body mass was measured using a balance beam (Avery;

Birmingham, UK). Skinfold thicknesses were measured over thetriceps and subscapular regions using calipers (Holtain; Cry¬mych) and were added together to give the sum of skinfolds.Blood hemoglobin concentration was determined from duplicatefingertip blood samples. Blood samples were immediately as¬

sayed using a HemoCue Photometer (Clandon Scientific; Farn-borough, UK) that was calibrated against a control cuvet prior toeach measurement. FVC and FEV: were measured using a

spirometer (Vitalograph; Vitalograph Ltd; Maids Moreton, UK)and the ratio FEVj/FVC was computed. Maximal voluntaryventilation (MW) was predicted from the following equation:FEV^S.9Ve, Vo2, and other ventilatory data were determined during an

exercise test that involved running on a motorised treadmill(Woodway; Cranlea Medical Ltd; Birmingham, UK) using a

discontinuous, incremental protocol. Initially the children were

habituated to both the general environment and the specificexperimental procedures and then they warmed up by runningon the treadmill at a speed of 6 km/h (1.67 m/s) for 3 min. Thetest commenced with four 3-min bouts at belt speeds of 7 km/h(1.94 m/s), 8 km/h (2.22 m/s), 9 km/h (2.50 m/s), and 10 km/h(2.78 m/s). For the remainder ofthe test, the belt speed was heldconstant at 2.78 m/s and the gradient elevated by 2.5% at the endof each 3-min period. Each child was allowed a 1-min restbetween stages but continued exercising until the test was

terminated at the point of voluntary exhaustion. If the childshowed signs of intense effort (hyperpnea, facial flushing, un¬

steady gait, sweating) and if his/her heart rate had leveled offprior to the final exercise intensity, or had reached a value near

or above 200 beats/min, or if his/her respiratory exchange ratio(RER) was at least 1.0, peak Vo2 was accepted as a maximalindex.1 All subjects described herein satisfied these criteria. Thechildren returned to the Centre the following morning and, as

one of a further series of tests, they repeated the run at 8 km/h.Throughout the tests, inspired and expired gases were moni¬

tored continuously using an on-line analysis system (OxyconSigma; Cranlea Medical Ltd; Birmingham, UK) that was recali¬brated prior to each test. Heart rate was monitored using an ECG(Rigel; Morden, UK). Duplicate fingertip blood samples were

taken following each exercise stage, and immediately assayed forwhole-blood lactate concentration using an analyzer (YSI 2300Stat Plus; Clandon Scientific; Farnborough, UK). The analyzerself-calibrated with a known concentration of lactate every fivesamples, and the calibration was checked regularly against com¬

mercially prepared samples of verified concentration.

Data AnalysisData were stored and analyzed using a statistical package

(SPSS-PC+ ; SPSS Inc; Chicago). Descriptive statistics were

computed for resting variables, variables at peak exercise, andvariables at each of the four initial exercise stages. Pearsonproduct moment correlation coefficients were computed whereappropriate. To allow7 comparisons between girls and boys atsimilar relative Vo2, descriptive statistics were computed forvariables from children who exercised within the specific ranges70 to 75% and 80 to 85% of peak Vo2.We have discussed in detail elsewhere the theoretical princi¬

ples underlying the use of log-linear models to partition out theeffect of body size from measures of physiologic function duringgrowth.61011 In the present study, ANCOVA on log-transformeddata was used to investigate sex differences in the relationshipsamong body mass, stature, and resting and exercise ventilator)7measures and identify appropriate allometric exponents. For allvariables, ANCOVA was initially computed with both stature andmass as covariates. In cases in which the term for one anthropo¬metric variable was not significant, a second ANCOVA was

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computed with a single covariate. Means, adjusted for thecovariates, and exponents are thus reported from the model thatrepresented the best fit for the ventilator}' measure underinvestigation.

Sex differences in resting data, as well as peak and submaximalexercise data, were examined using analysis of variance. Therepeatability coefficient of the children's ventilation (Ve) at 8km/h was determined from the mean and SD of the differencesbetween the two measures.12

Results

The children's physical characteristics and restingventilatory data are presented in Table 1. There wereno significant differences (p>0.05) in age, stature,mass, sum of skinfolds, or blood hemoglobin concen¬

tration between boys and girls. In both sexes, FVCand FEVX were significantly correlated with mass

and stature (Table 2). Boys demonstrated signifi¬cantly greater (p<0.001) FVC, FEVl9 and predictedMW7 than girls. Girls had significantly higher(p<0.05) FE\yFVC ratios than boys. Means forFVC adjusted for stature and mass using log-linearANCOVA remained significantly higher in boys thangirls (p<0.001) with respective exponents of 1.53(SE, 0.29) and 0.27 (SE, 0.23). Means for FEVXadjusted for stature were also significantly higher in

boys (p<0.001) with a common exponent of 2.04(SE, 0.20).Peak exercise data are displayed in Table 3. Ve

and Vt at peak Vo2 were significantly (p<0.001)correlated with stature and mass in both boys andgirls (Table 2). Respiratory frequency (Rf) at peakVo2 was not significantly (p>0.05) related to eitheranthropometric measure. Boys demonstrated signif¬icantly higher (p<0.01) peak Vo2 than girls whetherexpressed in absolute terms or with the influence ofbody size controlled using log-linear ANCOVA. No

Table 1.Age, Anthropometric, and RestingVentilatory Data in Prepubescent Children by Sex*

Boys (n=101) Girls (n= 76)

Age, yr 11.1(0.4) 11.0(0.4)Stature, m 1.45 (0.06) 1.44 (0.08)Mass, kg 36.3 (6.1) 34.7 (5.2)Sum of skinfolds, mm 19.4 (9.0) 21.1 (7.2)Hemoglobin concentration, g/L 13.5 (0.6) 13.6 (0.7)FVC, L 2.50 (0.37)* 2.19 (0.40)Adjusted FVC, L 2.43f 2.18FEVi, L 2.20 (0.30)f 1.97 (0.35)Adjusted FEVj, L 2.151 1.96FEVj/FVC, % 88.4 (5.6)* 90.1 (6.0)Predicted MW, L 82.38 (11.28)1 73.70 (13.03)*Values are expressed as mean (SD).Significant difference between groups: p<0.001.1 Significant difference between groups: p<0.05.

Table 2.Pearson Correlation Coefficients AmongStature, Mass, and Ventilatory Variables in

Prepubescent Children by Sex*

Boys Girls

Stature Mass Stature Mass

FVC 0.588FEV, 0.576Peak Ve 0.431Peak Vt 0.579Ve at 70-75% of peak Vo2 0.442Vt at 70-75% of peak Vo2 0.458Ve at 80-85% of peak Vo2 0.413Vt at 80-85% of peak Vo2 0.571

0.601 0.6320.517 0.6670.458 0.5350.538 0.5260.613 0.614

0.5160.4950.6230.4360.707

0.406 0.348 NS 0.248 NS0.6330.686 0.300f

0.609 0.468 0.256 NS

*All coefficients significant (p<0.001p>0.05. Sample sizes for resting andand girls n=76; at 70-75% peak Vo280-85% peak Vo2, boys n=58, girls n:

) except fp<0.05 and NSpeak variables, boys n=101

boys n=63, girls n=31; at=42.'

significant sex differences (p>0.05) were detected at

peak Vo2 in RER, Rf, blood lactate, heart rate,ventilatory equivalent for oxygen, or ventilatoryequivalent for carbon dioxide. At peak Vo2, boysexhibited significantly higher (p<0.001) absolute Veand Vt (p<0.01) than girls. No significant sex dif¬ferences (p>0.05) were detected in the ratios Vt/FVC or Ve/MW, but girls demonstrated signifi¬cantly higher (p<0.001) Rf/VT. Means for Ve and Vtat peak Vo2 adjusted for stature and mass remainedsignificantly higher in boys (p<0.001). Allometricexponents relating stature and mass to Ve were 0.69(SE, 0.33) and 0.48 (SE, 0.10), respectively. Corre¬sponding exponents for Vt were 1.71 (SE, 0.36) and0.27 (SE, 0.11).

Table 3.Peak Ventilatory Data of PrepubescentChildren by Sex*

Boys (n=101) Girls (n= 76)

Vt, L 1.14(0.21)* 0.96(0.20)Adjusted Vt, L 1.10f 0.95Vt/FVC, % 46.0 (7.7) 44.7 (9.4)Rf, breaths/min 52.2 (9.7) 53.2 (11.6)RfA'T 48.35 (16.40)f 59.99 (27.26)Ve, L/min 58.42 (10.49)f 49.62 (8.58)Adjusted Ve, L/min 56.601 49.60

VE/predicted MW, % 71.6 (13.2) 68.9 (14.8)Vo2, L/min 1.81 (0.26)* 1.51 (0.22)Adjusted Vo2, L/min 1.76* 1.52Ventilator)7 equivalent for oxygen 29.8(3.2) 29.5(3.1)Ventilatory equivalent for carbon 27.9 (2.4) 28.7 (2.2)

dioxideRER 1.07(0.07) 1.04(0.07)Blood lactate, mmol/L 4.8 (1.4) 5.0 (1.5)Heart rate, beats/min 200 (7) 201 (6)*Values are expressed as mean (SD).Significant difference between groups: p<0.001."Significant difference between groups: p<0.01.

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Descriptive ventilatory responses of boys and girlsrunning at 7, 8, 9, and 10 km/h are presented inTables 4 and 5. Girls demonstrated significantlyhigher (p<0.05) Rf, percent peak Vo2, ventilatoryequivalent for oxygen, ventilatory equivalent forcarbon dioxide, adjusted Ve, and Rf/VT (p<0.001)than boys at each treadmill velocity. Boys exhibitedsignificantly higher (p<0.01) Vt than girls at eachtreadmill velocity regardless of whether Vt was

expressed in absolute terms or with the influence ofbody size controlled using allometric principles.There was no significant sex difference (p>0.05) ineither Vt/FVC or Ve expressed in absolute terms at

any submaximal treadmill velocity.The mean (SD) Ve values running at 8 km/h

during the second visit to the Centre were 33.21(4.93) and 33.61 (7.19) L/min for girls and boys,respectively, and were not significantly different(p>0.05) from the values from the first visit. Therepeatability coefficients were 7.74 L/min and 6.80L/min for girls and boys, respectively.The ventilatory data of the children who exercised

within the specific ranges 70 to 75% and 80 to 85%of peak Vo2 are presented in Tables 6 and 7,respectively. There were no significant sex differ¬ences (p>0.05) in Rf, Vt/FVC, RER, heart rate,blood lactate, percent peak Vo2, ventilatory equiva¬lent for oxygen, or ventilatory equivalent for carbondioxide at either 70 to 75% or 80 to 85% of peak Vo2.Boys' Ve and Vt were significantly higher (p<0.001)than girls' values within the range 80 to 85% of peakVo2. The differences in Ve remained highly signifi¬cant when the influence of body mass was controlledusing log-linear ANCOVA yielding a common mass

exponent of 0.73 (SE, 0.09). For Vt, both stature andmass were significant covariates with exponents of1.65 (SE, 0.54) and 0.37 (SE, 0.16), respectively.Adjusted means for Vt remained significantlygreater (p<0.001) in the boys. In the range 70 to

75% peak Vo2, boys' Ve was significantly higher(p<0.01) than girls' Ve and remained so when theinfluence of body mass was covaried out yielding an

exponent of 0.81 (SE, 0.11). Boys' Vt was signifi¬cantly higher (p<0.05) than girls' Vt, but once

stature had been controlled for, the difference was

no longer significant (p<0.05) with an exponent of2.19 (SE, 0.49).

Discussion

The FEVX and FVC data are consistent withprevious reports on similarly aged children.1314 Thepresent finding that both FEVl and FVC are signif¬icantly higher in boys whereas the ratio of FEV2 toFVC is higher among girls compared to boys ofsimilar body size is in agreement with the extantliterature.1516 The reasons for sex differences in

dynamic lung volumes are not clear but it has beensuggested that prepubescent girls have larger airwaysrelative to lung size when compared to those ofboys15 and that the mechanical properties ofthe lungmay be different between girls and boys.17 Wang etal16 endorse the view that sex differences in lungfunction seem to go beyond dynamic growth ofairways and they indicate that the lungs of femalesubjects may have greater ventilation-perfusion ho¬mogeneity than the lungs of male subjects.The peak Vo2 values reported herein are compa¬

rable to previously published data and the findingthat boys' values are significantly higher than those ofgirls even when body size is controlled appropriatelyis in accord with the literature.110 We have recentlyexamined the assessment and interpretation of pre¬pubescent children's peak Vo2 using data drawnfrom the present population.18 We reported a mass

exponent of 0.66, but since the analysis was de¬scribed in detail in our earlier article, the issue will

Table 4.Ventilatory Responses ofPrepubescent Girls (n=76) Running at Various Treadmill Velocities*

Running Velocity, km/h

10

Vt, L

Adjusted Vt, LVt/FVC, %Rf, breaths/minRf/VTVe, L/min

Adjusted Ve, L/min% peak Vo2Ventilatory equivalent for oxygenVentilatory equivalent for carbon dioxideRER

0.83 (0.18)0.82

38.6 (8.5)38.9(11.4)51.9 (28.4)

30.76 (5.77)30.85

74.6 (7.1)24.2 (2.8)26.2 (2.6)0.93 (0.05)

0.84(0.17)0.84

39.4 (8.5)41.1(10.2)52.4 (22.4)

33.47 (5.77)33.58

80.1 (7.8)24.6(2.1)26.2 (2.1)0.94 (0.04)

0.88(0.17)0.88

40.9 (7.6)45.8 (10.8)55.6 (23.3)

39.17(7.15)39.33

87.1 (7.9)26.5 (2.6)27.0 (2.3)0.98 (0.05)

0.91 (0.18)0.89

42.3 (8.3)50.1 (11.2)59.4 (27.0)

44.41 (7.41)44.66

94.3 (7.2)28.1(2.6)27.8 (2.2)1.01 (0.06)

^Values are expressed as mean (SD).

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Table 5.Ventilatory Responses of Prepubescent Boys (n=101) Running at Various Treadmill Velocities*

Running Velocity, km/h

10

Vt, L

Adjusted Vt, LVt/FVC, %Rf, breaths/minRfA7TVe, L/minAdjusted Ve, L/min% peak Vo2Ventilator)7 equivalent for oxygenVentilatory equivalent for carbon dioxideRER

0.95 (0.23)0.91

38.2 (8.5)34.3(11.5)40.5 (23.9)

30.92(7.11)29.55

67.0 (8.9)23.0 (2.9)25.5 (2.6)0.90 (0.06)

0.97 (0.25)0.93

39.2 (9.0)37.1 (11.5)47.7 (23.4)

34.14(7.27)32.79

72.5 (8.6)23.6 (2.8)25.7 (2.5)0.92 (0.05)

0.98 (0.24)0.94

39.5 (7.8)41.0(10.2)45.6 (19.3)

38.96 (8.69)37.23

78.5 (9.4)24.8 (2.9)26.1 (2.5)0.95 (0.05)

1.02 (0.24)0.98

41.1 (7.5)44.6 (10.7)47.2 (19.5)

44.26 (9.74)42.35

85.1 (8.8)26.1(3.2)26.7 (2.5)0.98 (0.06)

*Values are expressed as mean (SD).

not be addressed further herein. Suffice to note thatit is not readily apparent why prepubertal boys havesignificantly higher values of peak Vo2 than girls,even when differences in body size are appropriatelyaccounted for and no significant differences in he¬moglobin concentration, subcutaneous fat, thighmuscle volume, or habitual physical activity likely toinfluence peak Vo2 are present.18-20A previous study with 11- to 15-year-old boys2

identified allometric exponents for peak Ve of 2.06for stature and 0.68 for mass, values that correspondto those predicted from the theory of geometricsimilarity.21 However, these authors did not combineboth anthropometric measures in a single allometricanalysis. In the present sample, both stature andmass proved to be significant covariates in thelog-linear ANCOVA investigating Ve and Vt at peakVo2 in boys and girls, despite the homogeneity in

Table 6.Ventilatory Data of Prepubescent Childrenby Sex, Exercising at 70 to 75% of Peak Vo2*

Boys (n=63) Girls (n=31)

Vt, L 0.98 (0.24)* 0.88 (0.20)Adjusted Vt, L 0.95 0.87Vt/FVC, % 40.0 (8.1) 40.6 (9.4)Rf, breaths/min 36.2 (11.4) 35.9 (9.6)Ve, L/min 33.81 (6.67)1 30.22 (4.93)Adjusted Ve, L/min 32.781 30.20Vo2, L/min 1.31 (0.20)§ 1.12 (0.17)% peak Vo2 72.7 (1.7) 72.2 (1.7)Ventilatory equivalent for oxygen 23.3 (2.9) 24.1 (2.4)Ventilatory equivalent for carbon 25.3 (2.6) 26.0 (2.3)

dioxideRER 0.92 (0.05) 0.93 (0.06)Blood lactate, mmol/L 1.8 (0.8) 2.2 (0.7)Heart rate, beats/min 170 (12) 170 (12)*Values are expressed as mean (SD)^Significant difference between groups: p<0.05."Significant difference between groups: p<0.01.§ Significant difference between groups: p<0.001.

their physical characteristics. Furthermore, the de¬viation of the exponents from the values assumed byratio scaling confirms that Ve and Vt at peak Vo2 donot increase in direct proportion with increases in

body size. These findings suggest that the conven¬

tional approach of accounting for body size differ¬ences in the interpretation of children's ventilatorydata, ie, by dividing by either body mass or stature, isinappropriate.The boys' significantly higher Ve and Vt at peak

Vo2 reflects their higher peak Vo2 and is consistentwith the literature for children of this age.22-24However, Ve at peak Vo2 is not a well-definedvariable and, unlike peak Vo2, it is dependent on theexercise protocol.25 Direct comparison of ventilatorydata between studies is therefore problematic unlessvery similar exercise protocols have been used. Amore secure approach is to limit discussion on sex

differences to articles that have reported intrastudycomparisons of boys and girls.

Table 7.Ventilatory Data of Prepubescent Childrenby Sex, Exercising at 80 to 85% of Peak Vo2*

Boys (n=58) Girls (n=42)Vt, L

Adjusted Vt, LVt/FVC, %Rf, breaths/minVe, L/min

Adjusted Ve, L/minVo2, L/min% peak Vo2Ventilatory equivalent for oxygenVentilatory equivalent for carbon

dioxideRERBlood lactate, mmol/LHeart rate, beats/min

1.01 (0.23)10.97f

40.6 (6.8)42.3(11.4)

41.02 (7.35)f39.811

1.49(0.22)f82.7 (1.7)25.0 (2.7)26.2 (2.5)

0.95 (0.05)2.2 (0.9)182 (12)

0.85(0.17)0.84

39.5 (8.9)43.2 (11.3)

35.30 (6.19)35.27

1.26 (0.17)82.4 (1.9)24.9 (3.3)26.5 (2.7)

0.94 (0.06)2.4 (0.6)179 (12)

*Values are expressed as mean (SD).Significant difference between groups: p<0.001.

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In accord with the literature, no significant sex

differences were detected at peak Vo2 in Rf, RER,heart rate, blood lactate, ventilatory equivalent forcarbon dioxide, or Ve/MW.1'26-27 Our finding of nosignificant sex difference in ventilatory equivalent foroxygen at peak Vo2 is in agreement with Rutenfranzet al24 but in conflict with others.22'2328 Similarly, our

finding of no significant sex difference in the ratioVt/FVC is in disagreement with earlier studies.2328The conflicting data may be due to differences in

sample size and selection, exercise protocols, or tothe fact that the subjects in the present study were allformally classified as prepubescent, whereas those inearlier studies were of similar age but their maturitywas not assessed. The present data, drawn from a

large, well-defined sample, indicated that althoughboys support their higher peak Vo2 with higher Veand Vt than girls, the relative ventilatory responses(eg, Ve/Vo2, VE/(carbon dioxide output [Vco2], Vt/FVC) to peak exercise of prepubescent boys and girlsare very similar.The repeatability coefficient reported herein sug¬

gests that submaximal Ve may be a less reliablevariable than others have found submaximal Vo2 tobe.29 Nevertheless, Tables 4 and 5 illustrate the risein Ve and its components (Vt and Rf) in response to

increasing running velocity. Similarly, the ratios Vt/FVC, Ve/Vo2, and Ve/Vco2 rise through to peakvalues, illustrating in both sexes the relative increasein depth of breathing and increasing ventilation inrelation to Vo2 and Vco2. It has been suggested thatcompared to adults, children have an exaggeratedventilatory response to a given metabolic rate.27 Thereasons for this are unknown, but hypotheses includedifferences in sensitivity of respiratory centers tocarbon dioxide,30 greater ventilatory neural drive,31and differences in airway dimensions and mechanicalwork of breathing.32 Although this issue cannot beexplored further with the present data, no childexhibited any ventilatory distress even at peak exer¬

cise levels.Some studies have indicated that young people

have different breathing patterns during exercise toadults. Boule et al33 exercised nine boys and nine

girls, aged 6 to 15 years, to maximum on a cycleergometer and reported that although Vt rose lin¬early with exercise intensity, Rf plateaued at 67% ofmaximal exercise and did not increase further. Sim¬ilarly, Rowland and Green34 computed the ratioRF/Vt during a progressive treadmill running pro¬tocol in 15 boys, aged 10 to 14 years, and demon¬strated a rise until high intensity levels when theratio declined. They suggested that their findingsindicated "a blunted breathing rate rise at highworkloads."27 Our data do not support these studies,

as both Vt and Rf increased through to peak exer¬

cise, as did the ratio Rf/VT.To our knowledge, there are no previous studies

with which we can directly compare our submaximalventilatory data, but they clearly indicate a widerange of sex differences in ventilatory responses toexercise at the same absolute intensity. The girls'significantly higher Rf, ventilatory equivalent foroxygen, and Ve controlled for body size at eachtreadmill velocity are worthy of note. Although theboys' demonstrated higher Vt at each stage this wasnot due to a greater utilization of their vital capacity.As the girls were exercising at a significantly higher

percent peak Vo2 than the boys at each of thesubmaximal treadmill velocities, this may account forthe observed differences. We therefore recomputeddata from the children who exercised within thenarrow ranges to 70 to 75% and 80 to 85% peak Vo2to compare the ventilatory responses of boys andgirls. Sample sizes inevitably vary and some childrenappear in both data sets but, as there were no

significant differences in percent peak Vo2, heartrate, RER, or blood lactate, we can assume that thedata allow a comparative analysis of the ventilatoryresponses of prepubescent boys and girls exercisingat the same relative submaximal intensities.

In contrast to Ve at peak Vo2, Ve at both submaxi¬mal intensities was related to body mass alone. Thereappear to be no other data against which to comparethe allometric exponents obtained, but the values of0.81 and 0.73 again confirm the inadequacy of thesimple per-body mass ratio to partition body sizedifferences from the interpretation of children'sventilatory responses. Vt at 70 to 75% peak Vo2 was

allometrically related to stature alone with mass

becoming an additional significant factor at thehigher relative intensity as at peak exercise.

Boys' absolute Vo2 is obviously higher than that ofgirls at each chosen level and this is reflected byboys' higher Ve which is a function of their higherVt. Boys' Ve remained significantly higher than girls'Ve at both relative exercise intensities even when theinfluence of body mass was covaried out. At thelower relative intensity, the sex difference in Vt was

no longer significant once stature had been con¬

trolled for. However, the most striking findings are

that there were no significant sex differences in Rf,Vt/FVC, ventilatory equivalent for oxygen, or venti¬latory equivalent for carbon dioxide at either 70 to75% or 80 to 85%peak Vo2. This suggests that at thesame relative exercise intensity, prepubescent boysand girls exhibit very similar ventilatory responses,with the exception that boys support their higherabsolute Vo2 with a higher Ve.

In summary, prepubescent boys demonstratehigher peak Vo2 than girls and this is supported by a

CHEST/ 112/6 /DECEMBER, 1997 1559

 1997 by the American College of Chest Physicians by guest on July 13, 2011chestjournal.chestpubs.orgDownloaded from

higher Ve and Vt even when the influence of bodysize is accounted for using allometric principles.However, other ventilatory responses to both peakexercise and exercise at the same relative intensityappear to be independent of sex.

ACKNOWLEDGMENTS: We gratefully acknowledge the tech¬nical assistance of Jenny Frost, Alison Husband, Teresa Smith,and Sue Vooght.

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DOI 10.1378/chest.112.6.1554 1997;112; 1554-1560Chest

WelsmanNeil Armstrong, Brian J. Kirby, Alison M. McManus and Joanne R.

to Sex and Body SizePrepubescents' Ventilatory Responses to Exercise With Reference

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