LEARNING OUTCOME: To learn that the Caltrac accelerometer was not a significant predictor of physical activity related energy expenditure as measured by doubly labeled water in a sample of 31 free-living children.

Obesity is on the rise among U.S. children. Physical activity is known to play an important role in the maintenance of body weight in children. In order to effectively assess the success of weight control intervention programs, it is crucial to have a simple, inexpensive tool that can be used to monitor physical activity levels among free-living children. The objective of this study was to determine whether the Caltrac accelerometer was a meaningful predictor of physical activity related energy expenditure (AEE) in pre-adolescent children. The sample consisted of 31 children (14 girls, 17 boys; 22 Caucasian, 9 Mohawk) with a mean age of 8.3 (± 2.0) years. AEE was measured by subtracting post-prandial resting metabolic rate, measured via indirect calorimetry, from total daily energy expenditure (TDEE), derived from the doubly labeled water method. Average daily “activity counts” were measured using a Caltrac accelerometer, which was worn for three days, two weekdays and one weekend day, within the DLW dosing period. AEE was significantly related to sex (r=0.42, p=.02), fat mass (r=.32, p=.07), and fat free mass (r=.32, p=.07) but not to “activity counts” (r=−.09, p=.63). After adjusting for sex, race, fat mass, and fat free mass using multiple correlation regression analysis (R=.53), “activity counts” did not significantly increase the amount of variation explained in AEE. An estimate of calories expended in physical activity was derived from “activity counts” using a formula developed by Sallis et al. This estimate was significantly higher than AEE (956kcal/day vs. 469kcal/day respectively, t=5.9, p .001). In conclusion, the Caltrac was not a meaningful predictor of AEE in our sample. In addition, caloric estimates of AEE derived from the Caltrac “activity counts” greatly overestimated AEE in free-living children.