Overlap of Subcutaneous Adiposity Across World Health Organization Body Mass Index-for-Age Categories and Anthropometric Correlates of Handgrip Strength in Children Aged 6–12 Years Participating in Organised Club Sport in Buenos Aires, Argentina

Background/Objectives: Body mass index (BMI) is the most widely used screening tool in paediatric nutritional assessment, but it does not measure body fat directly and conveys no information on muscular development. We aimed to (i) compare the sum of six skinfolds (Σ6ST) across World Health Organization (WHO) BMI-for-age categories, accounting for age; (ii) quantify the distributional overlap of Σ6ST between categories; (iii) examine how closely each individual skinfold tracks the remaining subcutaneous adiposity; and (iv) examine the associations of upper-limb girths with handgrip strength before and after adjustment for body size. Methods: This was a cross-sectional study of a convenience sample of 196 children (129 boys, 67 girls) aged 6–12 years engaged in organised club sport in Morón, Buenos Aires, Argentina. International Society for the Advancement of Kinanthropometry (ISAK)-certified anthropometrists measured six skinfolds and relaxed arm, corrected arm and forearm girths; handgrip strength was assessed with a digital dynamometer. Analyses used Kruskal–Wallis and Brunner–Munzel tests, age-adjusted log-linear models with robust standard errors, kernel-density overlap coefficients (OVLs) with bootstrap confidence intervals, and zero-order and partial Spearman correlations. Results: In this sample, 47.3% of boys and 43.3% of girls were classified as having overweight or obesity. Age distributions were similar across BMI categories, and Σ6ST differed across categories after age adjustment (p < 0.001 in both sexes). Adjacent WHO categories nevertheless overlapped substantially (OVL = 0.44–0.57), and the two halves of the normal-weight range overlapped by up to 0.757. Single skinfolds correlated strongly with the sum of the remaining five (ρ = 0.777–0.945) but estimated Σ6ST with a median absolute error of 8.5–14.1%. Handgrip strength correlated most strongly with body size (stature in boys, ρ = 0.723; body mass in girls, ρ = 0.759). After adjustment for age, body mass and stature, it remained associated with corrected arm girth (partial ρ = 0.346 and 0.336) but not with relaxed arm girth. Conclusions: BMI-for-age categories separate groups on average but carry limited information about an individual child’s subcutaneous adiposity. Skinfolds and corrected arm girth are low-cost complements to BMI whose individual-level value requires validation against criterion methods.