Objectives: Ventilation ( V ' E ) as a function of CO2 output, and oxygen uptake ( V ' O 2 ) as a function of log 10 V ' E , define cardio-respiratory efficiency, although few data compare efficiency with maximum oxygen uptake ( V ' O 2 max ), or consider reproducibility. Currently there are no data for trained juveniles. Design: Twenty-five trained juvenile cyclists (mean age 14.7 years), performed maximal exercise testing on two occasions, separated by 16 weeks. Methods: V ' E vs. V ' CO 2 slope, oxygen uptake efficiency slope, and V ' O 2 max were measured during cycle ergometer exercise to volitional exhaustion on two occasions, 16 weeks apart. Results: Mean (SD) V ' E vs. V ' CO 2 slope, oxygen uptake efficiency slope, and V ' O 2 max were 28.14 (3.89), 4.16 (0.73), and 75.4 (8.9) ml kg-1 min-1 on visit 1, and 27.92 (4.63), 4.22 (0.76), and 73.6 (9.3) ml kg-1 min-1 on visit 2. Good reproducibility (differences =2.4%), but poor correlations (r = 0.29) between efficiency and V ' O 2 max were recorded. Conclusions: Reproducibility of efficiency measures was comparable to V ' O 2 max , however, poor associations between efficiency and V ' O 2 max suggested independence. Efficient ventilation may be of limited importance in determining the V ' O 2 max in a trained juvenile cyclist.
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