Allometric Association between Six-Minute Walk Distance and Both Body Size and Shape in Young Obese Girls

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The 6-min walk test (6MWT) provides information regarding functional capacity, response to therapy and prognosis in a variety of chronic cardiovascular disorders. Variability in body size and composition, particularly in obese people, confounds the six-minute covered distance (6MWD). The aim of the present study was to adopt allometric models to identify the most appropriate body size/shape; i.e., body mass (BM), body height (BH), body mass index (BMI) and estimated fat-free mass (FFM); associated with the 6MWD in 190 young girls with obesity. Nonlinear allometric modeling was used to calculate common body size exponents for BM, BH, BMI and FFM. In a validation sample of 35 age-matched obese girls, these allometric exponents were used prospectively. The point estimates for the size exponents (95% confidence interval) from the separate allometric models were: BM 0.23 (0.19-0.27), BH 0.91 (0.78-1.03), BMI 0.33 (0.23-0.44) and FFM 0.28 (0.24-0.33). The presence of significant residual size correlations for 6MWD/BH-0.91 indicates that the influence of body size was not correctly partitioned out. In the validation group, the correlations between 6MWD BM-b and BM, 6MWD BMI-b and BMI, and 6MWD FFM-b and FFM using the established exponents were not statistically different from zero (r = 0.01), implying that participants in the allometric investigation were not penalized based on their BM, BMI, or FFM. We conclude that BM, BMI, BH and FFM, as indicators of body size/shape, are the most valid allometric denominators for the scaling of 6MWD in a group of young girls with obesity.