Calibration of hip accelerometers for measuring physical activity and sedentary behaviours in adults with Down syndrome.

BACKGROUND: The knowledge base on physical activity and sedentary behaviour in adults with Down syndrome (DS) may advance by accelerometer calibration studies. This study aimed to develop cut-points for sedentary behaviour and moderate-to-vigorous physical activity (MVPA) for adults with DS based on output from accelerometers worn on the dominant and non-dominant hips. METHODS: Sixteen adults with DS (10 men; age 31 ± 15 years) performed 12 tasks including sedentary behaviours and physical activities. We obtained metabolic equivalents (METs) with indirect calorimetry and vector magnitude (VM) output from triaxial accelerometers (wGT3X-BT, ActiGraph) worn on the dominant and non-dominant hips. Receiver operating characteristic curves were used to identify optimal VM cut-points that maximised sensitivity and specificity. RESULTS: Overall classification accuracy was very high (area under the ROC curve: 0.95 and 0.92 for sedentary and MVPA models, respectively). For the non-dominant hip, the optimal VM cut-points were (1) sedentary behaviour ≤236 counts·min-1 and (2) MVPA ≥2167 counts·min-1 . For the dominant hip, optimal cut-points were (1) sedentary behaviour ≤243 counts·min-1 and (2) MVPA ≥2092 counts·min-1 . CONCLUSIONS: The presented VM cut-points for sedentary behaviour and MVPA for adults with DS had high classification accuracy. There were small differences in accelerometer cut-points between the dominant and non-dominant hip.

Accelerometer-based estimation of oxygen uptake in adults with Down syndrome: vector magnitude vs. vertical axis.

BACKGROUND: Triaxial accelerometer output [vector magnitude (VM) counts] may better estimate physical activity intensity as reflected in the rate of oxygen uptake (V̇O2 ) than the traditional vertical axis (VA) counts in adults with Down syndrome (DS). This study examined the accuracy of VM vs. VA counts in estimating V̇O2 in adults with and without DS across different physical activities and sedentary behaviours. METHODS: Sixteen adults with DS (10 men and 6 women; 31 ± 15 years) and 19 adults without DS (10 men and 9 women; 24 ± 5 years) performed 12 tasks. V̇O2 was measured by portable spirometer (K4b2 , Cosmed) and VM and VA with an accelerometer (wGT3X-BT, Actigraph). RESULTS: Vector magnitude and VA were significant predictors of V̇O2 in adults with DS (P < 0.001; R2  = 0.74 and 0.65, respectively) and adults without DS (P < 0.001; P < 0.001; R2  = 0.75 and 0.61, respectively). Absolute error of prediction was significantly smaller for VM than VA for sitting, playing app, drawing, sweeping, standing and basketball (P ≤ 0.005), but smaller for VA than VM for walking at 0.8 m·s-1 (P = 0.005). Bland-Altman plots for adults with and without DS indicated narrower limits of agreement for VM than VA (-5.57 to 5.57 and -6.44 to 6.44 mL·kg-1 ·min-1 ; -6.21 to 6.17 and -7.75 to 7.74 mL·kg-1 ·min-1 , respectively). CONCLUSIONS: Vector magnitude and VA are significant predictors of V̇O2 in adults with and without DS, yet VM more accurately estimated V̇O2 than VA for most tasks. Development of accelerometer-based prediction of physical activity levels in adults with and without DS may improve by utilising VM counts.