| Summary: | <p>Abstract</p> <p>Background</p> <p>Sex comparisons between girls and boys in response to exercise in trained adolescents are missing and we investigated similarities and differences as a basis for clinical interpretation and guidance.</p> <p>Methods</p> <p>A total of 24 adolescent females and 27 adolescent males aged 13–19 years underwent a maximal bicycle exercise stress test with measurement of cardiovascular variables, cardiac output, lung volumes, metabolic factors/lactate concentrations and breath-by-breath monitoring of ventilation, and determination of peak VO<sub>2</sub>.</p> <p>Results</p> <p>Maximum heart rate was similar in females (191 ± 9 bpm) and males (194 ± 7 bpm), cardiac index at maximum exercise was lower in females (7.0 ± 1.0 l/min/m<sup>2</sup>) than in males (8.3 ± 1.4 l/min/m<sup>2</sup>, P < 0.05). Metabolic responses and RQ at maximum exercise were similar (females: 1.04 ± 0.06 vs. males: 1.05 ± 0.05). Peak VO<sub>2</sub> was lower in females (2.37 ± 0.34 l/min) than in males (3.38 ± 0.49 l/min, P < 0.05). When peak VO<sub>2</sub> was normalized to leg muscle mass sex differences disappeared (females: 161 ± 21 ml/min/kg vs. males: 170 ± 23 ml/min/kg). The increase in cardiac index during exercise is the key factor responsible for the greater peak VO<sub>2</sub> in adolescent boys compared to girls.</p> <p>Conclusions</p> <p>Differences in peak VO<sub>2</sub> in adolescent boys and girls disappear when peak VO<sub>2</sub> is normalized to estimated leg muscle mass and therefore provide a tool to conduct individual and intersex comparisons of fitness when evaluating adolescent athletes in aerobic sports.</p>
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