Effects of Body Mass Index and Tilt Angle on Output of Two Wearable Activity Monitors

Document Type


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Medicine & Science in Sports & Exercise


Volume 43


Issue 5

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FEITO, Y., D. R. BASSETT, B. TYO, and D. L. THOMPSON. Effects of Body Mass Index and Tilt Angle on Output of Two Wearable Activity Monitors. Med. Sci. Sports Exerc., Vol. 43, No. 5, pp. 861-866, 2011. Accelerometer-based activity monitors have been used to provide objective measures of physical activity and energy expenditure (EE) in free-living individuals. However, output from these devices has not been compared among normal, overweight, and obese individuals. Purpose: The purpose of this study was to examine the effects of body mass index (BMI) and device tilt angle on activity counts recorded by wearable monitors in a controlled laboratory setting. A secondary aim was to examine the effects of these variables on estimated EE. Methods: Seventy-one healthy adults wore an Actical and an ActiGraph GTIM on the right and left hip, respectively, while walking at 40, 67, and 94 nrmin-1. EE was measured by indirect calorimetry and compared with estimated values using published equations. Three-way repeated-measures ANOVA were used to examine differences in outcome variables (activity counts and EE) between speeds, BMI, and tilt angle for each device. Results: No significant differences in activity counts were observed among BMI categories for either the Actical or ActiGraph (P > 0.05). For the Actical, however, among those with an absolute tilt angle <10°, the obese group recorded higher activity counts than the normal weight group (P = 0.01). Using the Heil two-regression model, the Actical overestimated EE by up to 35% at the intermediate speed and up to 12% at the fastest speed (P < 0.001). The Freedson METs regression equation yielded closer estimates of EE than the Freedson kilocalorie regression equation. Conclusions: Our findings indicate that the Actical has limitations when comparing individuals with varying BMI and tilt angles in a controlled laboratory environment. The ActiGraph seems to be a more suitable device for making these comparisons. Reprinted by permission of the publisher.

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