Physical activity and posture profile of a South African cohort of middle-aged men and women as determined by integrated hip and thigh accelerometry

Background: Physical activity and sedentary behaviour are central to public health recommendations and highlight the need for precise measurement. Descriptive studies of objectively measured physical activity behaviours in African populations are rare. We aimed to develop a method of combining the signals from hip and thigh accelerometers to quantify physical behaviours and describe these by socio-demographic factors in a population of middle-aged men and women from urban South Africa. Methods: Physical behaviours were quantified by integrating the signals from two triaxial accelerometers worn simultaneously during free-living, in a subsample of participants from the Middle-aged Soweto Cohort (MASC) (n=794; mean (SD) age: 53.7 (6.3) years). Acceleration time-series from the two accelerometers were combined and movement-related acceleration derived using Euclidean Norm Minus One (ENMO, in milli-g). This was summarised as total movement volume (mean ENMO) and time spent in non-movement ( 85 mg); thigh pitch angle and a sleep diary were used to further divide non-movement time (min/day) spent into sleep, awake sitting/lying, and standing. Socio-demographic factors were self-reported, and weight and height were measured. Results: Mean (SD) wear time for combined thigh-hip accelerometry was 128 (48) hours. Movement volume was 15.0 (6.5) mg for men (n=437; 53.6 (6.2) years) and 12.2 (3.4) mg for women (n=357; 53 (5.8) years). Men spent more time in MVPA and sitting/lying, while women spent more time standing. Age was inversely associated with movement volume, MVPA and LPA in men and women. When compared to their normal weight counterparts, men who were overweight or obese spent less average daily time in MVPA, while women who were overweight or obese spent less time in LPA and more time sitting/lying. Socio-economic status was inversely associated with volume, MVPA and time spent sleeping, and positively associated with time spent sitting/lying, in both men and women. Conclusions: Integrating signals from hip and thigh accelerometers enables characterisation of physical behaviours that can be applied in an African population. Age, female sex, BMI and socio-economic status are inversely associated with physical activity and directly associated with sedentary behaviour.