Development of novel calibration model(s) to predict whole-body density in professional football players.

Introduction: Questions continue to be raised about the validity that is in existence to estimate D_b, in professional male footballer players.

Methods: Phase 1: n = 28 anthropometric variables were used on n = 206 footballers, using regression analyses to determine standard error of estimate and R². A cut-off correlation coefficient set at r = 0.950 and 90% R². Phase 2: all variables (z-scores, $\stackrel{-}{x}$ = 0.0, SD = ±1.0) to help reduce heteroscedasticity, β, r, t, significance of t and P-values were calculated. Phase 3: a forced stepwise-backwards regression analysis approach with nine predictors which met the acceptance criteria (r = 0.950, R² = 90% and β weights) was used to develop a 'best fit' and a 'practical' calibration model. Phase 4: cross-validation of the two newly developed calibration method using LoA.

Results: The 'best fit' model SEM (0.115 g ml^-1), the highest R² (6.6%) (P  ≤ 0.005), whereas the 'practical' calibration model SEM (0.115 g ml^-1), R² (4.7%) (P  ≤ 0.005) with r values = 0.271 and 0.596 and R² (%) coefficients = 0.3526 for the 'best fit' and 'practical' calibration models, respectively (P = 0.01).

Conclusions: The two calibration models supported an ecologically and statistically valid contribution and can provide sound judgements about professional footballers' body composition.