Predicting the rate of oxygen uptake from step counts using ActiGraph waist-worn accelerometers in adults with Down syndrome.

BACKGROUND: Step rate predicts ambulatory intensity as reflected in the rate of oxygen uptake (VO2 ) - a measure of energy expenditure. Whether step rate as measured by an accelerometer predicts VO2 in adults with Down syndrome (DS) is unknown. We examined whether step rate predicts VO2 in adults with and without DS. We also developed an equation for predicting VO2 and examined its accuracy. METHOD: Sixteen adults with DS (6 women and 10 men; age 31 ± 15 years) and 19 adults without DS (9 women and 10 men; age 25 ± 6 years) performed standing and walking at their preferred speed, 0.8 and 1.4 m·s-1 . We measured VO2 with a portable spirometer and step rate with a triaxial accelerometer (wGT3X-BT; ActiGraph) on the non-dominant hip, using the low-frequency extension filter. We ran multilevel regression for predicting VO2 from linear and quadratic terms for step rate, group (1 = DS; 0 = non-DS), body mass, height, body mass index (BMI), leg length and sex. We estimated VO2 with the resultant equation and calculated the equation's absolute per cent error, which we compared between groups. RESULTS: VO2 was higher in persons with than without DS only at the fast walking speed (P = 0.018). DS did not predict VO2 . Step rate, step rate squared and BMI were significant predictors of VO2 (P < 0.001; R2  = 0.80). Absolute error across walking speeds was 13.5-18.8% and 11.7-13.4% for adults with and without DS, respectively, and did not differ between groups or speeds. CONCLUSIONS: Step rate, step rate squared and BMI predict VO2 in adults with and without DS. Prediction error does not differ between groups.