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The purpose of this study was to evaluate the validity of the CSA activity monitor as a measure of children's physical activity using energy expenditure(EE) as a criterion measure.Thirty subjects aged 10 to 14 performed three 5-min treadmill bouts at 3, 4, and 6 mph, respectively. While on the treadmill, subjects wore CSA (WAM 7164) activity monitors on the right and left hips. ˙VO2 was monitored continuously by an automated system. EE was determined by multiplying the average ˙VO2 by the caloric equivalent of the mean respiratory exchange ratio.Repeated measures ANOVA indicated that both CSA monitors were sensitive to changes in treadmill speed. Mean activity counts from each CSA unit were not significantly different and the intraclass reliability coefficient for the two CSA units across all speeds was 0.87. Activity counts from both CSA units were strongly correlated with EE (r = 0.86 and 0.87, P < 0.001). An EE prediction equation was developed from 20 randomly selected subjects and cross-validated on the remaining 10. The equation predicted mean EE within 0.01 kcal·min-1. The correlation between actual and predicted values was 0.93 (P < 0.01) and the SEE was 0.93 kcal·min-1.These data indicate that the CSA monitor is a valid and reliable tool for quantifying treadmill walking and running in children.