Diurnal profiles of physical activity and postures derived from wrist-worn accelerometry in UK adults
Background: Wrist-worn accelerometry is the commonest objective method for measuring physical activity in large-scale epidemiological studies. Research-grade devices capture raw triaxial acceleration which, in addition to quantifying movement, facilitates assessment of orientation relative to gravity. No population-based study has yet described the interrelationship and variation of these features by time and personal characteristics. Methods: 2043 UK adults (35-65years) wore an accelerometer on the non-dominant wrist and a chest-mounted combined heart-rate-and-movement sensor for 7days free-living. From raw (60Hz) wrist acceleration, we derived movement (non-gravity acceleration) and pitch and roll (arm) angles relative to gravity. We inferred physical activity energy expenditure (PAEE) from combined sensing and sedentary time from approximate horizontal arm-angle coupled with low movement. Results: Movement differences by time-of-day and day-of-week were associated with arm-angles; more movement in downward arm-positions. Mean(SD) movement was similar between sexes ~31(42)mg, despite higher PAEE in men, 53(22) vs 48(19)J⋅min-1⋅kg-1. Women spent longer with the arm pitched >0° (53% vs 36%) and less time at <0° (37% vs 53%). Diurnal pitch was 2.5-5° above and 0-7.5° below horizontal during night and daytime, respectively; corresponding roll angles were ~0° and ~20° (thumb-up). Differences were more pronounced in younger participants. All diurnal profiles indicated later wake-times on weekends. Daytime pitch was closer to horizontal on weekdays; roll was similar. Sedentary time was higher (17 vs 15hours/day) in obese vs normal-weight individuals. Conclusions: More movement occurred in arm positions below horizontal, commensurate with activities including walking. Findings suggest time-specific population differences in behaviours by age, sex, and BMI.
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