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Background:This study sought to explore the dose-response rate/association between aerobic fitness (VO2max) and self-reported physical activ-ity (PA) and to assess whether this association varies by sex, age, and weight status. Methods:VO2max was assessed using the 20-m shuttle-run test. PA was assessed using the Physical Activity Questionnaire (PAQ) for Adoles-cents (aged>11 years, PAQ-A) or for Children (aged≤11 years, PAQ-C). The associations between VO2max and PAQ were analyzed using anal-ysis of covariance (ANCOVA), adopting PAQ and PAQ2 as covariates but allowing the intercepts and slope parameters of PAQ and PAQ2 to vary with the categorical variables sex, age group, and weight status. Results:ANCOVA identified a curvilinear association between VO2max and PAQ, with positive linear PAQ terms that varied for both sex and weight status but with a negative PAQ2 term of-0.39 (95％confidence interval (CI):-0.57 to-0.21) that was common for all groups in regard to age, sex, and weight status. These curvilinear (inverted U) associations suggest that the benefits of increasing PA (same dose) on VO2max is greater when children report lower levels of PA compared to children who report higher levels of PA. These dose-response rates were also steeper for boys and were steeper for lean children compared to overweight/obese children. Conclusion:Health practitioners should be aware that encouraging greater PA (same dose) in inactive and underweight children will result in greater gains in VO2max (response) compared with their active and overweight/obese counterparts.