Body mass-to-waist ratio strongly correlates with skeletal muscle volume in children

PURPOSE: We hypothesized that body mass-to-waist ratio is strongly associated with the total-body skeletal muscle volume (SMV) in children. The purpose of the present study was to examine this hypothesis. METHODS: By using magnetic resonance imaging, total-body SMV (SMV(MRI)) was determined in 70 boys and 53 girls aged 6 to 12 years. Waist was measured at each of the level of umbilicus (Wumb) and the minimum circumference (Wmin), and the ratio of body mass to each of the two measured values was calculated (BM/Wumb and BM/Wmin, respectively). A single regression analysis was used to examine the relationships between SMV(MRI) and either BM/Wumb or BM/Wmin. On the basis of the obtained regression equations, SMV(MRI) was estimated and referred to as SMV(BM/Wumb) or SMV(BM/Wmin). RESULTS: In both boys and girls, SMV(MRI) was highly correlated to BM/Wumb (r = 0.937 for boys and r = 0.939 for girls, P < 0.0001) and BM/Wmin (r = 0.915 and 0.942, P < 0.0001). R(2) and the standard error of estimate for SMV(BM/Wumb) were 0.878 and 706.2 cm(3), respectively, in boys and 0.882 and 825.3 cm(3), respectively, in girls, and those for SMV(BM/Wmin) were 0.837 and 814.0 cm(3), respectively, in boys and 0.888 and 804.1 cm(3), respectively, in girls. In both boys and girls, there were no significant differences between SMV(MRI) and either SMV(BM/Wumb) or SMV(BM/Wmin,) without systematic errors in Band-Altman plots. There was no significant effect of model on the absolute values of the residuals in both boys and girls. CONCLUSION: The current results indicate that body mass-to-waist ratio can be a convenient outcome measure for assessing the total-body skeletal muscle volume in children.