Muscularity and the density of the fat-free mass in athletes

The purpose of this study was to use estimates of body composition from a f our-component model to determine whether the density of the fat-free mass ( D-FFM) is affected by muscularity or musculoskeletal development in a heter ogenous group of athletes and nonathletes. Measures of body density by hydr ostatic weighing, body water by deuterium dilution, bone mineral by whole b ody dual-energy X-ray absorptiometry (DXA), total body skeletal muscle esti mated from DXA, and musculoskeletal development as measured by the mesomorp hy rating from the Heath-Carter anthropometric somatotype were obtained in 111 collegiate athletes (67 men and 44 women) and 61 nonathletes (24 men an d 37 women). In the entire group, D-FFM varied from 1.075 to 1.127 g/cm(3) and was strongly related to the water and protein fractions of the fat-free mass (FFM; r = -0.96 and 0.89) and moderately related to the mineral fract ion of the FFM (r = 0.65). Skeletal muscle (%FFM) varied from 40 to 68%, an d mesomorphy varied from 1.6 to 9.6, but neither was significantly related to D-FFM (r = 0.11 and -0.14) or to the difference between percent fat esti mated from the four-component model and from densitometry (r = 0.09 and -0. 16). We conclude that, in a heterogeneous group of young adult athletes and nonathletes, D-FFM and the accuracy of estimates of body composition from body density using the Siri equation are not related to muscularity or musc uloskeletal development. Athletes in selected sports may have systematic de viations in D-FFM from the value of 1.1 g/cm(3) assumed in the Siri equatio n, resulting in group mean errors in estimation of percent fat from densito metry of 2-5% body mass, but the cause of these deviations is complex and n ot simply a reflection of differences in muscularity or musculoskeletal dev elopment.