Accelerometer-measured physical activity: Importance of total volume per day and standardized measures

Purpose: Use of accelerometers in research studies has increased over the past 25 years. The first accelerometer, the Caltrac, was worn on the waist and estimated physical activity EE (kcal). Subsequently, the emphasis shifted to measuring minutes of moderate-to-vigorous PA (MVPA) done in 10-min bouts. However, light PA and intermittent MVPA also have important health benefits. Thus, it is plausible that, for health benefits, the most important variable is the total volume of PA performed. Total activity counts per day (TAC) is a proxy for the total volume of PA that is more highly related to disease biomarkers than minutes of MVPA per day. Wolff et al. (2013) have developed TAC percentiles for age and gender-specific groups, based on NHANES ActiGraph data (2003-06). TAC is a more direct expression of what the accelerometer actually measures, rather than a derived variable based on regression equations or cut-points. TAC could harmonize PA outcomes across studies, while still allowing researchers to compute other metrics.