Sex difference in peak oxygen uptake in prepubertal children

doi:10.1016/j.jsams.2008.05.006

Journal of Science and Medicine in Sport

Volume 12, Issue 6, November 2009, Pages 647-651

https://doi.org/10.1016/j.jsams.2008.05.006 Get rights and content

Abstract

Prepubertal boys’ greater aerobic fitness (peak $\dot{V} O_{2}$ ) has been attributed to their larger lean body mass (LBM); this bestowing a greater heart size and consequent larger maximum cardiac output. No difference in peak arterio-venous (A-VO₂) difference is thought to exist. However other work indicates that boys’ aerobic fitness remains 5% higher even after controlling for differences in LBM. Consequently the purpose of this study was to investigate whether peak $\dot{V} O_{2}$ , heart size, peak cardiac output and peak A-VO₂ difference would be comparable between a group of boys and girls with a similar LBM. A group of 9 prepubertal boys and 9 prepubertal girls with a similar mean LBM (27.0 ± 1.4 boys vs. 27.0 ± 2.0 kg girls) were selected. Left ventricular mass (LVM) and end diastolic volume (LVEDV) were measured using cardiac magnetic resonance imaging. Peak $\dot{V} O_{2}$ was determined on a cycle ergometer following an incremental exercise protocol to exhaustion, and cardiac output was recorded using thoracic bioimpedance. Boys’ peak $\dot{V} O_{2}$ (1.41 ± 0.18 L min⁻¹ vs. 1.23 ± 0.08 L min⁻¹) and A-VO₂ difference (14.8 ± 2.1 mL 100 mL⁻¹ vs. 12.6 ± 1.6 mL 100 mL⁻¹) were significantly (p < 0.05) higher than girls’ values, but there were no significant sex differences in peak cardiac output (10.0 ± 1.4 L min⁻¹ vs. 9.9 ± 1.40 L min⁻¹), LVM (97 ± 13 g vs. 93 ± 20 g) or LVEDV (77 ± 8 mL vs. 70 ± 13 mL). Central factors of heart size and peak cardiac output are proportional to the LBM of the individual and sex independent. Sex differences in peripheral factors such as muscle fibre type profile, may affect A-VO₂ difference and underlie prepubertal boys’ higher peak $\dot{V} O_{2}$ .

Introduction

Prepubertal boys’ peak aerobic fitness (peak $\dot{V} O_{2}$ ) is 10–15% higher than girls, and is thought to be explained by sex differences in lean body mass (LBM). Boys have a small, but significantly greater LBM than girls at this stage of development¹ and when peak $\dot{V} O_{2}$ is expressed in relation to LBM the sex difference becomes non-significant.²

The proponents of the LBM explanation claim that its influence on peak $\dot{V} O_{2}$ is exerted, according to the Fick principle, through central rather than peripheral factors. Whilst accepting the methodological caveats with assessing maximal cardiac output and arterio-venous (A-VO₂) difference during exercise in children,³ peak A-VO₂ difference is thought to be similar,⁴ but maximum cardiac output larger in boys. The latter arising from a larger absolute stroke volume (SV) combined with a similar maximum heart rate to that of girls.2, 5 Boys’ larger SV reflects their greater LBM; when maximum SV is expressed in ratio with LBM (mL kg LBM⁻¹) the sex difference is removed.1, 2

If the LBM explanation is correct, peak $\dot{V} O_{2}$ should be similar in boys and girls with a comparable LBM. The data however challenge this, with peak $\dot{V} O_{2}$ reportedly 4–6% higher in boys than girls with a similar LBM.5, 6 Indeed other studies, where differences in LBM were statistically controlled for (mL kg LBM⁻¹ min⁻¹), boys peak $\dot{V} O_{2}$ remains 4–8% greater.⁷ Clearly uncertainty remains around this question and warrants further investigation.

Previous studies have typically measured cardiac size and volume using echocardiography. Echocardiographic derived values are however dependent on geometric assumptions about the shape of the ventricle and use prediction equations derived from adults.⁸ Cardiac magnetic resonance imaging (CMRI) is proposed to be a more accurate and reproducible technique to quantify cardiac dimensionality, as it avoids these aforementioned problems.⁸

Consequently the purpose of this study was to test the hypothesis that peak $\dot{V} O_{2}$ , heart size determined using CMRI, peak cardiac output and peak A-VO₂ difference would be comparable between a group of boys and girls with a similar LBM.

Section snippets

Methods

Thirty-one (boys: n = 18, girls: n = 13) participants volunteered and gave written informed consent to participate in the study. All were healthy and none were taking prescription medications. The study received prior ethical approval from the institutional ethics committee.

Stature was measured to the nearest 0.01 m using a stadiometer (Holtain, Crymych, UK) and body mass to the nearest 0.1 kg using beam balance scales (Avery, Birmingham, UK). Body surface area was subsequently calculated (Haycock

Results

There were no significant (p > 0.05) group sex differences in LBM, body mass or stature (Table 1). Similarly no differences in age [10.1 ± 0.5 years (boys) vs. 10.2 ± 0.3 years (girls)], body fat percentage [13 ± 5% (boys) vs. 16 ± 8% (girls)], body surface area [1.12 ± 0.05 m² (boys) vs. 1.14 ± 0.07 m² (girls)] or Hb concentration [13.5 ± 0.6 g dL⁻¹ (boys) vs. 13.5 ± 0.8 g dL⁻¹ (girls)]. All children were more than 1 year before predicted age at PHV [−3.1 ± 0.2 years (boys) vs. −1.8 ± 0.3 years (girls)], thus considered

Discussion

These data indicate that when comparing boys and girls of a similar LBM, there is no difference in cardiac size and maximal output, yet boys’ have a greater peak A-VO₂ difference than girls. Consequently boys exhibit a significantly higher peak $\dot{V} O_{2}$ .

Rowland⁵ alludes to it as the unexplained 5% – the remaining sex difference in peak $\dot{V} O_{2}$ that cannot be explained by body composition, haemoglobin concentration or cardiac factors. Intriguingly, adult data reveal similar findings. Adult females’

Practical implications

•
Prepubertal boys have a greater aerobic fitness than girls. Understanding this may help reconcile differences seen in endurance performance between young boys and girls.
•
The smaller heart size and lower maximum cardiac output of young girls should not be interpreted negatively. Heart size and maximum cardiac output are largely determined by the lean body mass of the individual and not their sex.
•
Matching young boys and girls of a similar lean body mass for performance tests or for team selection

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Original paperSex difference in peak oxygen uptake in prepubertal children

Abstract

Introduction

Section snippets

Methods

Results

Discussion

Practical implications

Am J Cardiol

Chest

Chest

Mech Ageing Dev

Children's exercise physiology

Cardiovascular responses to endurance training in children: effect of gender

Eur J Clin Invest

Relationship of body size, physique, and composition to physical performance in young boys and girls

Int J Sports Med

Gender differences and determinants of aerobic fitness in children aged 8–11 years

Eur J Appl Physiol

Normal range of human left ventricular volumes and mass using steady state free precession MRI in the radial long axis orientation

Magn Reson Mater Phys

An assessment of maturity from anthropometric measurements

Med Sci Sports Exerc

Assessment of body composition in children

Pediatr Exerc Sci

Original paper
Sex difference in peak oxygen uptake in prepubertal children