Introduction
Different aspects of movement and posture-defined physical behaviour—such as physical activity, sitting and sleep—are vital and modifiable determinants of health.1 2 Traditionally, much of the research into physical behaviours has operated in subdisciplinary silos (eg, physical activity, exercise, sedentary behaviour, sleep) partially owing to variations in methodological paradigms, in particular differences in measurements.3–5 Recent advances in wearable technology, such as accelerometers, provide the potential to concurrently quantify multiple aspects of such behaviours in free-living conditions continuously across a number of days or weeks.6 7 This presents opportunities for a major breakthrough in our ability to understand how all these aspects of physical behaviour synergistically influence health and promote chronic disease prevention.7
One area of vigorous debate regarding the use of accelerometers is where they should be placed, with the aim to maximise feasibility and the breadth and depth of collected data. In the first generation of accelerometer studies, most large-scale studies focused on physical activity used devices worn on a belt around the waist/hip.8–10 This location was initially chosen due to its simplicity (ease of setup and wear) and close proximity to a person’s centre of gravity (minimising the effect of extraneous movement). However, due to it’s interference with clothing (requiring removal of the device when changing, etc) and sleep, waist/hip-worn devices have often been used only for waking hours, or part thereof.
Waist/hip-worn devices are also limited regarding the aspects/constructs of physical behaviour that they can currently identify. For instance, although they have been extensively validated for measuring energy expenditure,11 they have difficulty quantifying postures and distinguishing between different physical behaviours (eg, sitting vs standing, walking on a flat surface vs stair climbing).12 Wrist-worn devices, traditionally favoured in sleep research, have also gained popularity for physical activity assessment. This ‘watch-like’ wrist attachment carries less burden for research participants, resulting in higher compliance, and thus, may be more feasible for complete monitoring of 24 hours daily cycles than waist/hip-worn methods.13 14 However, similar to waist/hip-worn devices, wrist-worn accelerometers currently have difficulty distinguishing between basic aspects of physical behaviour, such as posture and activity type.12 15
An emerging accelerometer placement location is the thigh. Thigh-worn accelerometers are typically taped to the front of the thigh and can be worn under clothing 24 hours a day for multiple days.16–18 In addition to energy expenditure outcomes,19 thigh placement allows detection of the specific physical behaviours (ie, sitting/lying, standing, walking, running, stair climbing, cycling) with excellent accuracy.20 21 As such, an increasing number of major international cohorts have recently adopted such methods to measure thousands of participants, such as the Maastricht Study (n~8000), HUNT4 (n~38 000) and the 1970 British Birth Cohort (n~6000).22 The successful incorporation of thigh-worn accelerometry by these studies demonstrates that thigh-worn accelerometry is feasible for comprehensively quantifying physical behaviour across the 24 hours cycle in large-scale health research.
The Prospective Physical Activity Sitting and Sleep consortium (ProPASS) is a recent research collaboration platform22 of investigators utilising observational studies of thigh-worn accelerometry. ProPASS’s ultimate scientific objective is to produce longitudinal evidence on the associations of physical activity, posture and sleep with long-term health outcomes and longevity. To fulfil these aims, ProPASS will harmonise and integrate thigh-worn accelerometry and corresponding health outcomes data—including linkage to administrative health data such as mortality and cause-specific hospital admissions. Besides its function to harmonise previously collected data, a fundamental aspect of ProPASS is its prospective nature. As such, ProPASS will develop standards to support future population-based studies to collect preharmonised thigh-worn accelerometry data. Meeting these objectives and handling sensitive health-related data is complex and demands long-term planning.
In line with publications describing previous accelerometry consortia,23 this paper had a dual aim:
To identify studies potentially eligible for inclusion in ProPASS via a systematic scoping review to summarise observational studies that collected 24 hours thigh-worn triaxial accelerometery data in population or community-based adult samples.
To guide the development of ProPASS by compiling and sumarising key discussions and decisions arising from the initial ProPASS collaborators meeting (held in October 2018 in Copenhagen, Denmark) into an expert collaborator statement.