Examining reactivity to the measurement of physical activity and sedentary behavior among women in midlife with elevated risk for cardiovascular disease

Objectives: To estimate the extent of reactivity to measurement of physical activity (PA) and sedentary behavior among women in midlife with elevated risk for cardiovascular disease (CVD). Design: Secondary analysis of a 10-day observational study of PA and sedentary behavior. Main Outcome Measures: PA (steps, minutes of light PA, total minutes of moderate-to- vigorous PA [MVPA]) and percent time in sedentary behavior per day were assessed using ActiGraph GT3X tri-axial accelerometers in 75 women in midlife with elevated CVD risk (e.g. hypertension; M_Age = 51.61, M_BMI = 34.02 kg/m²). Two-level multilevel models were used to test for evidence of reactivity, with the addition of random effects to test for evidence of individual differences in observed trends. Results: All outcomes showed linear trends across days (ps < 0.001), though this masked what appeared to be meaningful dropoff after Day 1 or Day 2 (with little difference between subsequent days; srs ranging from 0.15 to 0.32). The random effect was significant only for percent time in sedentary behavior (χ ²[1] = 10.40, p = 0.02). Conclusions: Consistent small to medium effects were found for all PA and sedentary behavior outcomes, underscoring the importance of considering measurement reactivity in populations with elevated CVD risk.