Background Triaxial accelerometer output [vector magnitude (VM) counts] may better estimate physical activity intensity as reflected in the rate of oxygen uptake (V ˙O2 ) 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 V ˙O2 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. V ˙O2 was measured by portable spirometer (K4b 2 , Cosmed) and VM and VA with an accelerometer (wGT3X-BT, Actigraph). Results Vector magnitude and VA were significant predictors of V ˙O2 in adults with DS (P < 0.001; R 2 = 0.74 and 0.65, respectively) and adults without DS (P < 0.001; P < 0.001; R 2 = 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 À1 (P = 0.005). Bland-Altman plots for adults with and without DS indicated narrower limits of agreement for VM than VA (À5.57 to 5.57 and À6.44 to 6.44 mL•kg À1 •min À1 ; À6.21 to 6.17 and À7.75 to 7.74 mL•kg À1 •min À1 , respectively). Conclusions Vector magnitude and VA are significant predictors of V ˙O2 in adults with and without DS, yet VM more accurately estimated V ˙O2 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.

No abstract