Calibration of an accelerometer during free-living activities in children

Objective. The aims of this study were to develop an equation to predict energy expenditure and to derive cut-points for moderate and vigorous physical activity intensity from the Actigraph accelerometer output in children aged 12 years. Methods. The children performed a series of activities (lying, sitting, slow walking, fast walking, hopscotch and jogging) while wearing an Actigraph and a portable metabolic unit. The sample was divided into a developmental and a validation group. Random intercepts models were used to develop a prediction equation in the developmental group. The equation was assessed in the validation group by calculating limits of agreement (actual minus predicted energy expenditure). Thresholds for moderate and vigorous activity were derived by refitting the energy expenditure model with VO2 as the outcome. Results. The developmental group comprised 163 children, while the validation group comprised 83 children. The equation, adjusted for age and gender, adequately predicted energy expenditure from accelerometer counts. Physical activity intensity cut-points were derived from resting VO2. The lower threshold for moderate intensity (four METs), adjusted for age and gender, was 3581 counts per minute. The lower threshold for vigorous activity (six METs) was 6130 counts per minute. Conclusion. The prediction equation and the derived cut-points will help to better interpret the output of the Actigraph in children aged 12 years. The cut-point for moderate to vigorous physical activity is higher than that reported previously.