Field Methods to Estimate Fat-Free Mass

The routine evaluation of fat-free mass throughout the season provides useful information for coaches. In fact, fat-free mass is considered the best longitudinal predictor for leg power during late adolescence in young soccer players. However, X-ray absorptiometry (DXA) is costly and cannot be used frequently. Therefore, it is usual to use lower-cost methods, such as bioelectrical impedance analysis (BIA) and anthropometric measurements (circumference, skinfold, and breadth), to estimate fat-free mass. Although different anthropometric equations and BIA are widely used as a practical and non-invasive alternative to DXA to predict fat-free mass changes in young soccer players, no data are currently available about the sensitivity to change of these methods in elite young soccer players during a season.

«Free-fat mass is considered the best longitudinal predictor for leg power during late adolescence in young soccer players»

The aim of this study was to determine the most effective field method for quantifying fat-free mass sensitivity to change in elite young soccer players against DXA, between pre- and mid-season.

A total of 40 elite youth soccer players participated in this study. DXA values provided a criterion measure of fat-free mass. Correlation coefficients, biases, limits of agreement and differences were used as measures of sensitivity to change.


  • All body density, skinfold, and anthropometric equations and BIA devices used to obtain fat-free mass data showed positive and very large correlations (r from 0.70 to 0.89) with DXA (table 3).
  • The magnitudes of the differences were small for DXA, BIA inbody, and all anthropometric equations except those of Faulkner (1966), Durnin and Rahaman (1967), Brook (1971), and Sarría et al. (1998).
  • Only BIA Tanita and one set of equations showed no standardized or substantial differences against DXA and had the lowest bias (Faulkner, 1966; Durnin and Womersley, 1974; Carter, 1982; Slaughter 1988; Reilly et al., 2009; Munguia-Izquierdo et al., 2018).


Table 3. Correlations, biases, limits of agreement, and standardized differences between changes in fat-free mass with DXA and other practical estimates in elite youth soccer players (n = 40).

Significant differences between criterion (DXA) fat-free mass and others practical estimates of fat-free mass using paired t test *(p < 0.05), **(p < 0.01). CI, confidence interval; LoA, level of agreement; CL, confidence level; BIA, bioelectrical impedance analysis.

FSI statements:

  1. Despite the body composition is a key fitness element relevant to soccer players’ performance
    only 5 anthropometric equations showed the best sensitivity in assessing fat-free mass change between pre- and mid-season in elite youth soccer players.
  2. The only one who proposed a «soccer-specific» equation in their study as a valid alternative to DXA to quantify fat-free mass among elite youth male players was our FSI professor Munguia-Izquierzo et al (2018).



To learn more about this topic you can enroll in our FSI Master in Football Strength & Conditioning and learn from our professor Dr. Diego Munguia Izquierdo, a specialist on the topic.