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Objective monitors such as the activPAL characterize time when the thigh is horizontal as sedentary time. However, there are physiological differences between lying, bent-legged sitting, and straight-legged sitting. We introduce a three-monitor configuration to assess detailed sedentary postures and demonstrate its use in characterizing such positions in free-living conditions. We explored time spent in each sedentary posture between prolonged (>1 h) versus non-prolonged (<1 h) sedentary bouts. In total, 35 healthy adults (16♀, 24 ± 3 years; 24 h/day for 6.8 ± 1.0 days) wore an activPAL accelerometer on their thigh, torso, and shin. Hip and knee joint flexion angle estimates were determined during sedentary bouts using the dot-product method between the torso–thigh and thigh–shin, respectively. Compared to lying (69 ± 60 min/day) or straight-legged sitting (113 ± 100 min/day), most time was spent in bent-legged sitting (439 ± 101 min/day, p < 0.001). Most of the bent-legged sitting time was accumulated in non-prolonged bouts (328 ± 83 vs. 112 ± 63 min/day, p < 0.001). In contrast, similar time was spent in straight-legged sitting and lying between prolonged/non-prolonged bouts (both, p > 0.26). We document that a considerable amount of waking time is accumulated in lying or straight-legged sitting. This methodological approach equips researchers with a means of characterizing detailed sedentary postures in uncontrolled conditions and may help answer novel research questions on sedentariness.

Objective. Approaches to differentiate sitting and lying are available within the default activPAL software from a single thigh-worn monitor. Dual-monitor methods use multiple monitors positioned on the thigh and torso to characterize sitting versus lying. We evaluated the validity between these two methods to measure waking sitting and lying time in free-living conditions. We also examined if the degree-threshold distinguishing sitting/lying for the dual-monitor (<30 and <45) impacted results. &#xD;Approach. Thirty-five young adults (24±3 years, 16 females) wore an activPAL 24-hrs per day on their thigh and torso during free-living conditions (average: 6.8±1.0 days, 239 total). Data were processed using the default activPAL software (thigh-only) or a custom MATLAB program (dual-monitor). &#xD;Main Results. The single-monitor recorded less lying time (59±99 mins/day) and more sitting time (514±203 mins/day) than the dual-monitor method regardless of 30 (lying: 85±94 mins/day; sitting: 488±166 mins/day) or 45 lying threshold (lying: 170±142, sitting: 403±164 mins/day; all, p<0.001). The single monitor lying time was weakly correlated to the dual-monitor (30: =0.25, 45: =0.21; both, p<0.001), whereas sitting was moderate-strong (30: =0.76, 45: =0.58; both, p<0.001). However, the mean absolute error was 81 mins/day (30) and 132 min/day (45) for both lying and sitting.&#xD;Significance. The method of differentiating sitting/lying from a single thigh-worn activPAL records more sitting time and less lying time compared to a dual-monitor configuration (regardless of degree-threshold) that considered the position of the torso. A further refinement of algorithms or implementation of multiple-monitor methods may be needed for researchers to derive detailed sedentary positions. &#xD;

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