Kids are not little adults: what MET threshold captures sedentary behavior in children?

Purpose: The study compares MET-defined cutpoints used to classify sedentary behaviors in children using a simulated free-living design. Methods: A sample of 102 children (54 boys and 48 girls; 7–13 years) completed a set of 12 activities (randomly selected from a pool of 24 activities) in a random order. Activities were predetermined and ranged from sedentary to vigorous intensities. Participant’s energy expenditure was measured using a portable indirect calorimetry system, Oxycon mobile. Measured minute-by-minute VO₂ values (i.e., ml/kg/min) were converted to an adult- or child-MET value using the standard 3.5 ml/kg/min or the estimated child resting metabolic rate, respectively. Classification agreement was examined for both the “standard” (1.5 adult-METs) and an “adjusted” (2.0 adult-METs) MET-derived threshold for classifying sedentary behavior. Alternatively, we also tested the classification accuracy of a 1.5 child-MET threshold. Classification accuracy of sedentary activities was evaluated relative to the predetermined intensity categorization using receiver operator characteristic curves. Results: There were clear improvements in the classification accuracy for sedentary activities when a threshold of 2.0 adult-METs was used instead of 1.5 METs (Se_{1.5 METs} = 4.7 %, Sp_{1.5 METs} = 100.0 %; Se_{2.0 METs} = 36.9 %, Sp_{2.0 METs} = 100.0 %). The use of child-METs while maintaining the 1.5 threshold also resulted in improvements in classification (Se = 45.1 %, Sp = 100.0 %). Conclusion: Adult-MET thresholds are not appropriate for children when classifying sedentary activities. Classification accuracy for identifying sedentary activities was improved when either an adult-MET of 2.0 or a child-MET of 1.5 was used.