Predictive Validity of an Age-Specific MET Equation Among Youth of Varying Body Size

Abstract

A number of equations have been developed to calibrate activity monitors with youth but a variable that has not been adequately addressed is the potential impact of body size on these equations. PURPOSE: The purpose of this study was to cross-validate an age-specific MET regression equation among children and adolescents of varying body size. METHODS: A total of 67 children (41 boys and 26 girls) between 6 and 13 yrs of age completed five 3-minute walking trials (1, 2, 2.5, 3, and 4 mph) on a motorized treadmill. During each trial, participants wore an Actigraph accelerometer while oxygen consumption was assessed by indirect calorimetry. Using the Actigraph activity counts, the predicted MET values were determined with an age-specific MET equation and were compared to the measured MET value using dependent t-tests. Participants were divided into body mass tertiles (small: ≤ 26.5 kg; medium: 26.5 < body mass ≤ 35.9; large: > 35.9 kg) to determine if body mass Influenced the difference between measured and predicted MET values. RESULTS: The measured MET value was similar to the predicted MET value at 1 mph (2.1 vs 2.2 METS); however, the predicted MET value was significantly greater than the measured MET value at the remaining speeds (2.7 vs 3.4 METS (2 mph); 3.1 vs 4.5 METS (2.5 mph); 3.7 vs 5.5 METS (3 mph); 5.1 vs 6.8 METS (4 mph)) (p<0.05). The mean difference between the measured and predicted MET values was significantly greater at each treadmill speed in the two lighter tertiles compared to the heaviest tertile. CONCLUSION: The results suggest that the age-specific MET equation, which is widely used in pediatric physical activity research, significantly overestimates energy expenditure at slow walking speeds (2-4 mph). This effect was primary observed among smaller children.