A novel procedure for integrating three objectively measured dimensions of free-living sedentary time

Introduction The widely accepted definition of sedentary behaviour (SB) refers to any waking behaviour characterized by an energy expenditure ≤1.5 metabolic equivalents (METs) while in a sitting or reclining posture. At present, there is no single field-based device which accurately measures sleep, activity intensity and posture. The aim of this study was to develop a novel integrative procedure to combine sleep, activity intensity and posture information from two validated activity monitors to quantify free-living sedentary time. Methods Sixty-three female participants aged 37.1 (±13.6) years with a BMI of 29.6 (±4.7) kg/m2 were continuously monitored for 5-7days to track free-living SB with the SenseWear Armband (for sleep and activity intensity) and the activPAL (for posture). A set of data merging operations controlled via a simple graphical user interface were developed. After accounting for sleep time, differences in sedentary time according to sitting/reclining (SEDAP), activity intensity ≤1.5 METs (SEDSWA) and the integration of these dimensions (SEDINT) were compared. Results The three SB measures were positively inter-correlated with the weakest relationship between SEDSWA and SEDAP, followed by SEDSWA and SEDINT, and the strongest relationship was between SEDAP and SEDINT. There was a significant difference between the three measures of sedentary time. Post-hoc tests revealed all three methods differed significantly from each other. SEDSWA resulted in the most sedentary time (11.74±1.60hours/day), followed by SEDAP (10.16±1.75hours/day), and SEDINT (9.10±1.67hours/day). Sedentary time accumulated on weekdays compared with weekend days did not differ for any of the measurement methods. More sedentary time was accumulated in the longest bout category (>40minutes) when measured with SEDSWA. Conclusion It is possible to combine information from two validated activity monitors to obtain a measure of free-living SB based on posture and activity intensity during waking hours. The positive correlation suggests that both activity intensity and posture are related aspects of the same phenomenon (sedentary behaviour). However, sedentary time according to posture and sedentary time according to activity intensity are conceptually different. The implications of this methodological distinction for understanding the impact of SB on markers of health and obesity are yet to be determined.