Influence of body composition, hemoglobin concentration, and cardiac size and function of gender differences in maximal oxygen uptake in prepubertal children.

OBJECTIVE To determine the relative contribution of the gender difference in body composition, blood hemoglobin concentration, and cardiac dimension and function at rest and exercise of the gender difference in maximal oxygen uptake (O(2)max) in 10- to 12-year-old children. SUBJECTS Thirty-five healthy children (17 girls and 18 boys; mean +/- SD age, 10.5 +/- 0.4 years). EXPERIMENTAL DESIGN An anthropometric evaluation (body surface area, body fat content, and lean body mass [LBM]), assessment of hemoglobin concentration, echocardiographic evaluation at rest (left ventricular dimensions, and diastolic and systolic indexes at rest), and cardiovascular evaluation during a maximal cycle exercise (stroke volume [SV], total peripheral resistance). RESULTS The boys exhibited a higher mass-relative O(2)max than the girls (47.9 mL/kg/min vs 40.9 mL/kg/min, respectively); but when normalized for LBM (allometric equation), the difference totally disappeared (19.0 mL/kg LBM(1.33)/min vs 18.9 mL/kg LBM(1.33)/min, respectively). No significant gender differences were seen in maximal heart rate and arteriovenous oxygen difference; however, maximal SV (SVmax) was significantly higher in boys than in girls, but when expressed relative to LBM, the difference was no longer significant. CONCLUSIONS These findings demonstrate that contrary to adults, the sole limiting factor of O(2) that distinguished boys from girls was a lower SVmax in the latter; however, this gender difference totally disappeared when normalized for LBM. Consequently, the gender difference in heart size and cardiac function during exercise should be interpreted as only one aspect of the lower LBM in girls and not as reflective of a more basic functional gender difference.