Empirically derived cut-points for sedentary behaviour during working and non-working hours: how important is the context in which we sit?

Introduction: Sedentary behaviour (SB) is associated with a number of health outcomes, independent of physical activity. Studies that have used accelerometers to define SB tend to use a <100 counts per minute (cpm) threshold to define SB across all domains; however, this cut-point was not empirically derived. It is not known whether accelerometer cut-points for SB differ depending on the context in which it occurs. The aim of this study was to empirically derive accelerometer cut-points for working and non-working time.

Methods: A convenience sample of 30 university employees (10 males, 30 females; age 40.47±10.95 years; BMI 23.93±2.46 kg/sq.metre) wore the ActiGraph GT3X+ and activPAL devices simultaneously for seven days. Data were downloaded in one minute epochs and non-wear time (derived from an activity diary) was removed. Generalised estimating equations (allowing for the correlation of cpm with adjacent minutes) were used to make minute by minute comparisons of sedentary time from the two devices, using sitting from the activPAL as the criterion measure.

Results: After data reduction, 29 participants provided 42,847 worktime and 19,649 non-worktime minutes. Derived cpm for sedentary time during worktime was significantly lower compared to sedentary time during non-worktime (52 [95%CI 50-54] vs. 93 [84-104]). Compared to the 100cpm threshold, the empirically derived cut-points performed better; higher area-under-the-curve and lower mean differences were found for both working and non-working times.

Conclusion: Cut-points for SB depended on domain: SB at work was less active than SB at home. Thus, the nature of sitting does depend on the context.