<b>Should we predict maximum oxygen uptake using fat-free mass or body mass in a non-athletic population</b>

<b>Background</b>: Our purpose was to assess whether fat-free mass (FFM) or body mass (M) is the more appropriate body-size variable to predict maximum oxygen uptake (VO_2max) (L⋅min⁻¹) in a non-athletic population.<b>Methods</b>: We adopted the well-known allometric/power function-model VO_2max=a·X^b⋅e to predict VO_2max (L⋅min⁻¹) using either FFM or M as the predictor variable. These allometric models can be linearised with a log-transformation, and linear regression (or ANCOVA) can then be used to estimate the unknown parameters.<b>Results</b>: Initially, ignoring the effect of age and sex, FFM was the strongest predictor of VO_2max. When we predicted Ln(VO_2max) using only Ln(FFM) adjusting for the effects of age and sex, the explained variance was R²=0.718 (AIC=-1882.5), with the FFM exponent b=0.658. However, when we predicted Ln(VO_2max) allowing body mass AND bodyfat% as separate covariates also adjusting for age and sex, the explained variance increased further to R²=0.733 (AIC=-2077.4), with the M exponent b=0.636. The difference in the explained variances, but more importantly, the difference in the AIC’s was 194.88, confirming the superiority of predicting VO_2max using separate M and bodyfat% covariates, i.e., adopting FFM alone was less successful compared to using M and bodyfat% separately. Our analyses identified a greater negatively weighted bodyfat% term that improved the prediction of VO_2max, found by our subsequent/final prediction models, to be due to excess central adiposity (waist circumference). Note that subsequent/final M and FFM exponents were both estimated to be b=0.67, confirming that VO_2max should be normalized using VO_2max (mL·M^−2/3·min⁻¹) or VO_2max (mL·FFM^−2/3·min⁻¹) rather than VO_2max (mL·FFM⁻¹·min⁻¹) as recommended by previous research.<b>Conclusion</b>: Incorporating FFM into equations to predict VO_2max fails to adequately explain the negative effect of excess central adiposity. However, by incorporating body mass (M) and BF% separately into the equations, a greater negatively weighted BF% coefficient is capable of compensating for this apparent omission.