Variability of objectively measured sedentary behavior

Introduction The primary purpose of this study was to evaluate variability of sedentary behavior (SB) throughout a 7-d measurement period and to determine if <7 d of SB measurement would be comparable with the typical 7-d measurement period. Methods Retrospective data from Ball State University's Clinical Exercise Physiology Laboratory on 293 participants (99 men, 55 ± 14 yr, body mass index = 29 ± 5 kg·m^-2; 194 women, 51 ± 12 yr, body mass index = 27 ± 7 kg·m^-2) with seven consecutive days of data collected with ActiGraph accelerometers were analyzed (ActiGraph, Fort Walton Beach, FL). Time spent in SB (either <100 counts per minute or <150 counts per minute) and breaks in SB were compared between days and by sex using a two-way repeated-measures ANOVA. Stepwise regression was performed to determine if <7 d of SB measurement were comparable with the 7-d method, using an adjusted R² of ≥0.9 as a criterion for equivalence. Results There were no differences in daily time spent in SB between the 7 d for all participants. However, there was a significant interaction between sex and days, with women spending less time in SB on both Saturdays and Sundays than men when using the 100 counts per minute cut-point. Stepwise regression showed using any 4 d would be comparable with a 7-d measurement (R² > 0.90). Conclusions When assessed over a 7-d measurement period, SB appears to be very stable from day to day, although there may be some small differences in time spent in SB and breaks in SB between men and women, particularly on weekend days. The stepwise regression analysis suggests that a measurement period as short as 4 d could provide comparable data (91% of variance) with a 1-wk assessment. Shorter assessment periods would reduce both researcher and subject burden in data collection.