Influence of Accelerometer Processing on Health Relations
PURPOSE: Accelerometry is increasingly being used to assess sedentary time in epidemiological studies, yet the most appropriate means of processing this data remains uncertain. This cross-sectional study examined the influence of selected accelerometer cutpoints and non-wear criteria on associations of sedentary time with adiposity and clustered metabolic risk. METHODS: Data were from the European Youth Heart Study, which included assessment of sedentary time by accelerometer. Sixteen sedentary time variables were constructed based upon combinations of frequently used cut-points (100, 500, 800, 1100 counts-per-minute) and non-wear criteria (10-, 20-, 60-, 100-min consecutive zeros). Adiposity was assessed by sum of four skin-fold thickness measures. A clustered metabolic risk score was calculated as the mean of standardised metabolic syndrome components, including blood pressure, insulin resistance, and inverted fasting HDL-cholesterol. Analyses were conducted using multi-level cross-sectional time-series regression, adjusted for overall physical activity (accelerometer counts per minute). Meta-analysis was used to obtain pooled estimates of the exposure-outcome association over all processing protocols; meta-regression was used to determine the influence of non-wear and cut-point protocol on observed associations. RESULTS: Sedentary time follows a power law with cut-point (exponent=0.27) and zero-string (exponent=0.03), and was positively associated with clustered metabolic risk (β = 0.0051, 95%CI 0.0018, 0.0085). The association was moderated by cut-point, with higher cut-points typically producing stronger associations. No significant association between sedentary time and adiposity was observed. CONCLUSION: Choice of accelerometer cut-point may moderate the association between sedentary time and clustered metabolic risk, suggesting that direct comparisons of associations between studies using different cut-points must be made with caution.
Medicine and Science in Sports and Exercise, 2013, Vol 45, Issue 6