Background: Despite a consensus emerging that affirms that shorter epochs should be used in youth to correctly register physical activity levels in free-living conditions, little is known about its effect on children's physical activity conducted in structured periods of time. This study analyzed the effect that epoch length (1, 2, 3, 5, 10, 15, 30 and 60s) may have on different physical activity intensities in physical education lessons. Methods: A sample of 1912 individual measures of physical education lessons were measured with a GT3X accelerometer. Data were collected from 1227 Swiss Elementary school students recruited in 17 elementary schools. PE lessons lasted from 45 minutes to one and a half hours. Data, originally collected in 1-s epoch, were then reintegrated into 2s, – 3s – 5s – 10s – 15s – 30s –60s epochs...