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Objective:To compare the phase II oxygen uptake time constant (τV′O2) and V′O2 mean response time (V′O2MRT) in overweight (OW) and non-OW (NO) children during moderate intensity exercise.Design:Between subjects where participants completed a maximal ramp exercise test on an electromagnetically braked cycle ergometer to determine peak V′O2 (V′O2peak) and gas exchange threshold (GET). Gas exchange was measured breath-by-breath using a mass spectrometer. On subsequent visits, 6 square-wave transitions (2 per day) from 0 W to 90% GET were completed. Individual phase II τV′O2 and V′O2MRTs were estimated from time aligned average V′O2 traces.Subjects:Eleven OW (11.8±0.4 years) and 12 NO (11.9±0.4 years) children were recruited to the study. The OW group was significantly heavier (62.9±9.7 vs 39.4±5.8 kg, P<0.001), taller (1.58±0.05 vs 1.47±0.07 m, P<0.001) and had a higher body mass index (25.8±3.4 vs 18.3±1.8 kg m−2, P<0.001).Results:Both τV′O2 (30.2±9.6 vs 22.8±7.1 s, P<0.05) and V′O2MRT (43.5±10.7 vs 36.3±5.3 s, P<0.05) were significantly slower in OW compared with NO children; absolute V′O2peak was higher in the OW compared with NO group (2.23±0.04 vs 1.74±0.04 l min−1, P<0.05); mass relative V′O2peak was lower in OW compared with NO children (35.9±8.3 vs 43.8±6.2 ml kg−1 min−1, P<0.05); allometrically scaled V′O2peak was similar between OW and NO groups whether relative to body mass0.67 (139.8±29.1 vs 147.2±23.9 ml kg−67 min−1) or stature3 (576.0±87.2 vs 544.9±84.9 ml m−3 min−1) (P>0.05); absolute V′O2 at GET was similar between OW and NO groups (0.94±0.24 vs 0.78±0.27 l min−1, P>0.05); GET expressed as percentage of V′O2peak was similar between the groups (42.0±0.1 vs 44.8±0.1%, P>0.05).Conclusion:These findings demonstrate impairment in the factors determining V′O2 kinetics in OW children at a relatively young age. Furthermore, assessment of cardiorespiratory fitness using peak exercise values is likely to be misleading and not useful when designing exercise programmes for OW children. [ABSTRACT FROM AUTHOR]