***TASK*** the task is to summarize an input biomedical literature in six sentences 
    ***INPUT*** the input is a biomedical literature 
    ***OUTPUT*** the output is the summary of an input biomedical literature in six sentences 
    ***DOCUMENTATION*** 
    ***EXAMPLES*** 
    Input: This study compared the traditional two-compartment ( fat mass or FM ; fat free mass or FFM ) hydrodensitometric method of body composition measurement , which is based on body density , with three ( FM , total body water or TBW , fat free dry mass)- and four ( FM , TBW , bone mineral mass or BMM , residual)-compartment models in highly trained men ( n = 12 ) , sedentary men ( n = 12 ) , highly trained women ( n = 12 ) , and sedentary women ( n = 12 ) . The means and variances for the relative body fat ( % BF ) differences between the two- and three-compartment models [ 2.2 + /- 1.6 ( SD ) % BF ; n = 48 ] were significantly greater ( P < /= 0.02 ) than those between the three- and four-compartment models ( 0.2 + /- 0.3 % BF ; n = 48 ) for all four groups . The three-compartment model is more valid than the two-compartment hydrodensitometric model because it controls for biological variability in TBW , but additional control for interindividual variability in BMM via the four-compartment model achieves little extra accuracy . The combined group ( n = 48 ) exhibited greater ( P < 0.001 ) FFM densities ( 1.1075 + /- 0.0049 g/cm3 ) than the hydrodensitometric assumption of 1.1000 g/cm3 , which is based on analyses of three male cadavers aged 25 , 35 , and 46 yr . This was primarily because their FFM hydration ( 72.4 + /- 1.1 % ; n = 48 ) was lower ( P < /= 0.001 ) than the hydrodensitometric assumption of 73.72 % 1 . Body composition was assessed in 28 healthy subjects ( body mass index 20 - 28 kg/m2 ) by  dual-energy X-ray absorptiometry , deuterium dilution , densitometry , 40 K counting and four prediction methods ( skinfold thickness , bioelectrical impedance , near-i.r . interactance and body mass index ) . Three- and four-component models of body composition were constructed from combinations of the reference methods . The results of all methods were compared . Precision was evaluated by analysis of propagation of errors . The density and hydration fraction of the fat-free mass were determined . 2 . From the precision of the basic measurements , the propagation of errors for the estimation of fat ( + /- SD ) by the four-component model was found to be + /- 0.54 kg , by the three-component model , + /- 0.49 kg , by deuterium dilution , + /- 0.62 kg , and by densitometry , + /- 0.78 kg . Precision for the measurement of the density and hydration fraction of fat-free mass was + /- 0.0020 kg/l and + /- 0.0066 , respectively . 3 . The agreement between reference methods was generally better than between reference and alternative methods . Dual-energy X-ray absorptiometry predicted three- and four-component model body composition slightly less well than densitometry or deuterium dilution ( both of which greatly influence these multi-component models ) . 4 . The hydration fraction of fat-free mass was calculated to be 0.7382 + /- 0.0213 ( range 0.6941 - 0.7837 ) and the density of fat-free mass was 1.1015 + /- 0.0073 kg/l ( range 1.0795 - 1.1110 kg/l ) , with no significant difference between men and women for either . 5 . The results suggest that the three- and four-component models are not compromised by errors arising from individual techniques . ( ABSTRACT TRUNCATED AT 250 WORDS INTRODUCTION Few studies report on validation of body composition changes using the four-compartment model ( 4C ) , and no such studies are available in strength training . Here we present such a validation study for the determination of body fat and fat-free mass changes in bodybuilders , who used exercise and and rogenic-anabolic steroids . METHODS The study was carried out with 27 male bodybuilders in a cross-sectional study . Fifteen of these subjects also participated in an intervention program where body composition changes were measured . The 4C model served as the gold st and ard . The alternative mechanistic methods were underwater weighing ( uww ) , deuterium dilution ( dil ) , three-compartment model incorporating total body water ( 3Cw ) , three-compartment model incorporating bone mineral content ( 3Cb ) , and descriptive methods , namely dual-energy x-ray absorptiometry ( DXA ) , prediction equations based on body mass index ( BMI ) , skinfold measurement , and bioimpedance analyses . RESULTS From the cross-sectional study , it appeared that biases and errors of most mechanistic methods were small ( maximal 0.5 % BF and 3.4%BF , respectively ; exception 3Cb model ) . The 3Cw model had the lowest error ( 0.9%BF ) . The descriptive methods had small biases ( exception BMI ) but relatively large errors ( range : 5.5 - 8 % ) . Results on body composition changes ( intervention study ) were comparable with the results from the cross-sectional study . CONCLUSIONS Using the 4C model as the st and ard for determination of body fat and fat-free mass , this study revealed that apart from the prediction equation based on BMI and the 3Cb model , all methods gave acceptable group mean values . When accurate measurements on body composition and /or body composition changes on an individual level are needed , only the 3Cw model could serve as an alternative for the 4C method 
    Output:
    
The sum of two DXA scans and adopting a knee-bent position are alternative procedures proposed for evaluating individuals using pencil and fan-beam Hologic instruments , who are taller than the scan area , whereas a half-body scan is recommended for overcoming the limitations of whole-body measurements in subjects who are wider than the scan area width of Lunar densitometers .
Conclusions : Overall , the proposed procedures are useful and valid , in particular the half-body scan alternative , which avoids unnecessary radiation exposure and scanning time .