Comparison of Anthropometric Equations to Estimate Body Composition in Chilean Male Adolescent Soccer Players

Background: Anthropometric body-composition assessment in youth soccer commonly relies on equations developed in different source populations and based on distinct body-composition models, potentially altering nutritional and sport-related interpretation. This study compared body-fat estimates from equations intended to report the same outcome, body fat percentage, but based on different operational models, and quantified muscle-mass outputs from anthropometric approaches targeting total-body skeletal, whole-body, and tissue-level muscle compartments in a cohort of Chilean male U-15 and U-16 players. Methods: This secondary cross-sectional method-comparison study analyzed anonymized anthropometric records collected in 2018 from 50 male players. Slaughter, Faulkner, Carter, Deurenberg, and Sloan-Siri estimates of body fat were compared, together with Poortmans, Lee, Doupe, and Kerr muscle-mass approaches. Absolute agreement among adiposity equations was evaluated using a two-way random-effects, absolute-agreement, single-measure ICC [ICC(A,1)] and Bland-Altman analyses. Muscle-mass approaches were compared with repeated-measures analysis; pairwise difference-versus-mean summaries across different muscle constructs were interpreted descriptively rather than as formal agreement tests. Sensitivity analyses excluded Deurenberg and examined a maturation-related Slaughter scenario. Results: Deurenberg yielded the highest body-fat estimate (18.97 ± 3.07%), followed by Slaughter (13.32 ± 3.35%), Faulkner (10.84 ± 1.40%), Sloan-Siri (8.01 ± 2.80%), and Carter (7.91 ± 1.49%). Global differences were large (χ²(4) = 182.96; p < 0.001; Kendall's W = 0.915), and individual absolute agreement was low [ICC(A,1) = 0.157; bootstrap 95% CI 0.102-0.212]. Excluding Deurenberg did not remove the between-method differences (χ²(3) = 124.44; p < 0.001; W = 0.830). For muscle mass, mean estimates ranged from 25.11 ± 3.07 kg with Poortmans to 31.26 ± 4.94 kg with Doupe, with a significant method effect after Greenhouse-Geisser correction (p < 0.001). Conclusions: The adiposity equations yielded substantially different numerical estimates of the same reported outcome, body fat percentage, and showed low individual-level absolute agreement. Muscle-mass approaches also produced large numerical differences, partly expected because they target different compartments. Outputs from different equations or constructs should not be treated as numerically interchangeable. Because no criterion reference method was included, this study quantifies method dependence rather than validity and cannot identify the most accurate equation.