Accelerometer Cut-Point Calibration

Accelerometer cut-point calibration solves the central translation problem of objective physical-activity measurement: a wearable accelerometer outputs dimensionless 'counts,' but researchers and health guidelines speak in intensities — sedentary, light, moderate, vigorous. Calibration establishes the count thresholds that map the device's output onto those intensity categories. Patty Freedson, Edward Melanson, and John Sirard's 1998 study of the CSA (later ActiGraph) accelerometer set the template, regressing measured energy expenditure in METs on accelerometer counts during treadmill walking and running and solving the regression for the counts corresponding to moderate (3 METs) and vigorous (6 METs) activity. Later work, exemplified by Evenson and colleagues' 2008 calibration for children, increasingly used receiver-operating-characteristic (ROC) analysis to find the cut-point that best discriminates intensity categories. The result in both cases is a small set of count thresholds that turn raw accelerometer data into minutes of activity at each intensity.