Purpose: Simple body composition models, such as two-compartment models, use assumptions which may be violated in specific athletic populations (e.g., the constancy of fat-free mass density [DFFM] and hydration [TBW:FFM]). The present analysis examined FFM characteristics of muscular physique athletes.
Methods: Twenty-six athletes (16 males: 94.5 ± 9.9 kg, 12.2% ± 4.2% fat; 10 females: 63.8 ± 5.7 kg, 19.7% ± 4.9% fat) completed duplicate assessments of dual-energy x-ray absorptiometry (DXA), bioimpedance spectroscopy (BIS), and single-frequency and multifrequency bioelectrical impedance analysis (SFBIA; MFBIA). Fat-free mass was calculated via five-compartment (5C) model, and FFM characteristics (i.e.
, dffm, tbw: FFM, FFM mineral, FFM protein) were compared between sexes and between the sample and reference values (RV) from cadaver analysis.
Tbwbis: FFM5C was designated as the reference TBW:FFM model, and alternate models were produced using BIS, MFBIA, SFBIA, impedance-based equations, and DXA output.
Results: Males had lower TBW:FFM and FFM mineral, but higher FFM protein than females (P < 0.05). DFFM was significantly lower than RV in males (1.096 ± 0.006 g·cm, RV: 1.103 ± 0.013 g·cm; P < 0.001) and appeared higher in females (1.093 ± 0.012 g·cm, RV: 1.087 ± 0.021 g·cm; P = 0.17). TBW:FFM did not differ from RV in females (75.3% ± 4.0%; RV, 75.7% ± 4.4%; P = 0.74) or males (72.7% ± 1.9%; RV, 72.7% ± 3.1%; P = 0.97). Fat-free mass mineral was lower than RV in males (5.5% ± 0.4%; RV, 6.8% ± 0.8%; P < 0.001) and higher in females (6.5% ± 0.5%; RV, 6.1% ± 0.8%; P = 0.04). Fat-free mass protein was significantly greater than RV in males (21.9% ± 1.8%; RV, 20.6% ± 2.6%; P = 0.014), but the difference from RV in females was not statistically significant (18.3% ± 4.1%; RV, 17.1% ± 4.5%; P = 0.39). Alternate TBW:FFM models varied substantially in their agreement with RV.
Conclusions: The FFM characteristics were shown to differ between male and female physique athletes. These results may have implications for optimal body composition assessment methods when atypical physique characteristics are present.