BACKGROUND: Accelerometer cut-points specific to adults with Down syndrome (DS) may improve the measurement of physical activity and sedentary behavior in this population. The purpose of the study was to generate cut-points for sedentary behavior and moderate and vigorous PA for adults with DS based on raw acceleration data from a GENEactiv hip accelerometer. METHODS: Thirty-five adults with DS (age 35 ± 8 years; 19 men) performed 17 tasks each lasting 6 min: sitting; watching a movie; playing app on tablet; drawing; standing; folding clothes; vacuuming; sweeping; moving a box; basketball; soccer; fitness circuit; dancing; walking at the preferred speed and at 0.8 and 1.4 m.s-1; and running. Oxygen uptake was measured with portable calorimetry (K5, Cosmed). One metabolic equivalent (MET) was operationalized as VO2 during sitting allowing for determination of METs during each task. A GENEactiv accelerometer worn on the non-dominant hip was used to determine the Vector Magnitude of raw acceleration data. Vector Magnitudecut-points for sedentary behavior (≤1.5 METs while sitting) and for moderate (3.0­-5.9 METs) and vigorous (≥6 METs) physical activity were determined with Receiver Operating Characteristic (ROC) curves. Classification performance was evaluated with the area under the ROC curve. Youden’s index was used to identify optimal cut-points maximizing sensitivity and specificity. RESULTS: Area under the ROC curve was: (a) sedentary behavior (0.94; 95% CI: 0.92 - 0.96); (b) moderate physical activity (0.85; 95% CI: 0.82 - 0.87); and (c) vigorous physical activity (0.79; 95% CI: 0.71 - 0.87). Vector Magnitude cut-points were: (a) sedentary behavior ≤160 g∙min-1 (sensitivity 1.00; specificity 0.81; Youden’s index 0.81); (b) moderate physical activity ≥320 g∙min-1 (sensitivity 0.96; specificity 0.65; Youden’s index 0.61); and (c) vigorous physical activity ≥1211 g∙min-1 (sensitivity 0.58; specificity 0.87; Youden’s index 0.45). CONCLUSIONS: ROC models for sedentary behavior and moderate and vigorous physical activity intensity based on raw acceleration data for a hip-worn GENEactiv accelerometer have acceptable classification accuracy in adults with DS.Funding: NIH Grant R15HD098660

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