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

Purpose The widely accepted definition of sedentary behaviour (SB) refers to i) any waking behaviour characterized by ii) an energy expenditure ≤1.5 metabolic equivalents while iii) in a sitting or reclining posture. At present, there is no single field-based device which accurately measures all three key dimensions of SB. The aim of this cross-sectional study was to develop a novel integrative procedure to combine sleep, activity intensity and posture information from two validated activity monitors to quantify sedentary time. Methods Sixty-three female participants aged 37.1 (±13.6) years with a body mass index (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), low EE (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 (awake and <1.5 metabolic equivalents; METs) and SEDAP (awake and sitting/lying posture) [r(61)=.37, p=.003], followed by SEDSWA and SEDINT (awake, <1.5 METs and sitting/lying posture) [r(61)=.58, p<.001], and the strongest relationship was between SEDAP and SEDINT [r(61)=.91, p<.001]. There was a significant difference between the three measures of sedentary time [F(1.18, 73.15)=104.70, p<.001]. Post-hoc tests revealed all three methods differed significantly [p<.001]. 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). Conclusion It is possible to combine information from two validated activity monitors to obtain a three dimensional 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. Only the combination of these dimensions meets the definition of SB proposed by the Sedentary Behaviour Research Network. The implications of this methodological development for clarifying the impact of SB on health and obesity are yet to be determined.