Bioelectrical impedance analysis (BIA) represents a quick and practical approach for evaluating body composition in sport environments. However, sport-tailored prediction equations are currently unavailable, and applying generic formulas may generate consistent estimation errors. Therefore, this study aimed to: (i) design and validate novel prediction equations for fat-free mass (FFM) components in male elite soccer players; and (ii) determine the validity of previously published predictive models. A sample of 102 male elite soccer players (mean age 24.7 ± 5.7 years), competing in the Italian first division, was assessed during the initial half of the in-season period. Participants were randomly assigned to either a development group or a validation group. Bioimpedance measurements, including resistance (R) and reactance (Xc), were obtained using a foot-to-hand BIA device operating at a single frequency of 50 kHz. Dual-energy X-ray absorptiometry was used as the criterion method to determine FFM, lean soft tissue (LST), and appendicular lean soft tissue (ALST). The newly generated equations were validated through regression analyses, Bland–Altman agreement tests, and evaluation of the area under the curve (AUC) derived from regression receiver operating characteristic (RROC) curves.
The resulting prediction equations were: FFM = − 7.729 + (bodymass × 0.686) + (stature²/R × 0.227) + (Xc × 0.086) + (age × 0.058), R² = 0.97, standard error of estimation (SEE) = 1.0 kg; LST = − 8.929 + (body mass × 0.635) + (stature²/R × 0.244) + (Xc × 0.093) + (age × 0.048), R² = 0.96, SEE = 0.9 kg; ALST = − 24.068 + (body mass × 0.347) + (stature²/R × 0.308) + (Xc × 0.152), R² = 0.88, SEE = 1.4 kg. Train–test validation conducted in the validation sample showed that commonly used athlete equations significantly underestimated all FFM-derived variables (p < 0.01). In contrast, the newly proposed models demonstrated no significant mean bias (p > 0.05), exhibited R² values between 0.83 and 0.91, and showed no proportional bias (p > 0.05). The RROC curve analysis yielded AUC values of 0.92 for FFM, 0.92 for LST, and 0.74 for ALST. Use of generalized prediction equations leads to systematic underestimation of FFM and ALST in elite soccer players. The soccer-specific equations introduced in this study provide accurate estimates of body composition while retaining the advantages of a portable, field-based assessment method.
