Original paperSex difference in peak oxygen uptake in prepubertal children
Introduction
Prepubertal boys’ peak aerobic fitness (peak ) 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 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 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 should be similar in boys and girls with a comparable LBM. The data however challenge this, with peak 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 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 , 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 .
Rowland5 alludes to it as the unexplained 5% – the remaining sex difference in peak that cannot be explained by body composition, haemoglobin concentration or cardiac factors. Intriguingly, adult data reveal similar findings. Adult females’
Practical implications
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Prepubertal boys have a greater aerobic fitness than girls. Understanding this may help reconcile differences seen in endurance performance between young boys and girls.
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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.
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Matching young boys and girls of a similar lean body mass for performance tests or for team selection
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