The utility of shorter epochs in direct motion monitoring.

This cross-sectional study using direct motion monitoring evaluated whether short epochs increased estimates of moderate or vigorous physical activity (MPA or VPA) and enhanced differences in daily VPA comparing overweight (OW) and nonoverweight (NOW) children. Seventy-seven children (ages 8-10 years) wore accelerometers for 7 days. We calculated two estimates (mean minutes per day) of MPA and VPA using motion counts based on a 15-s epoch and a calculated 60-s epoch produced by totaling each consecutive group of four 15-s motion counts. We compared estimates as a function of mean motion count x min(-1) for sex, age, and status as OW or NOW. The results showed that a 15-s epoch produced higher estimates of VPA (mean difference of 7 min per day, p < .001). The average number of VPA minutes added using the 15-s epoch vs. the 60-s epoch was 8.8 for more active children compared with 5.8 for less active children (p < .001). There was no difference in VPA minutes between OW and NOW children. These findings suggest modestly increased sensitivity to VPA using shorter epochs; this was particularly true for the most active children. Shorter epochs, however might not be useful in clarifying the relationship between VPA and obesity in children.