Cut-point-free accelerometer metrics to assess children's physical activity: An example using the school day

The aims were to (a) investigate associations between a novel accelerometer metric: the minimum acceleration value above which the most active 30-minutes were accumulated during the school day (M30_ACC ), and health indicators, and (b) demonstrate that applying an equivalent cut-point to the M30_ACC metric gives comparable prevalence results as a moderate-to-vigorous physical activity (MVPA) cut-point approach. Two hundred and ninety-six children (age 9-10-years) wore wrist-mounted accelerometers for 7-days. School day MVPA and M30_ACC were calculated. Body mass index (BMI), waist-to-height ratio (WHtR), and cardiorespiratory fitness (CRF) were also measured. Mixed linear models investigated associations between M30_ACC and health indicators. Agreement between ranked MVPA and M30_ACC values was assessed using percent agreement, kappa, sensitivity, and specificity statistics. M30_ACC thresholds associated with health indicators were 213 mg (BMI), 206 mg (WHtR), and 269 mg (CRF) for girls. The equivalent values for boys were 234 mg (BMI), 230 mg (WHtR), and 327 mg (CRF). Less than half of girls and 75% of boys accumulated 30 minutes of school day MVPA. Just <50% of girls and >80% of boys had M30_ACC values ≥200 mg, which is equivalent to brisk walking. Agreement between MVPA and M30_ACC tertiles was high, reflected by the sensitivity and specificity values of >90%. Results demonstrate the utility of M30_ACC as a PA metric that is not heavily influenced by researcher decisions. M30_ACC has potential as an accelerometer-specific metric for generating PA guidelines related to health indicators and easily understood forms of activity such as brisk walking.