Purpose: The use of oxygen cost (Ȯaero) parameters to predict endurance performance has recently been criticized. Instead, it is suggested that aerobic energy cost (Ėaero) provides greater validity, however a comparison of these quantification methods has not previously been made. Methods: Fifty-six male (n=34) and female (n=22) competitive adolescent (17±1 years) middle-distance runners participated in a sub-maximal and maximal incremental treadmill test. Running economy (RE) was measured at the speed corresponding to lactate turnpoint, and the three speeds prior. Maximal oxygen uptake (V̇O2max), speed at V̇O2max, fraction of V̇O2max utilized across a range of intensities, and speeds from 0.8, 1.5 and 3 km races were also quantified. RE and fractional utilization were calculated in units of Ȯaero and Ėaero. Results: Multiple linear regression models demonstrated no discernible difference in the predictive capability of RE, fractional utilization and V̇O2max when expressed as Ȯaero or Ėaero in both sexes. When plotted as a function of running speed, Ȯaero displayed a stepwise decrease (F=11.59, p<0.001) whereas Ėaero exhibited a curvilinear response (F=4.74, p=0.015). Differences were also evident in the slopes plotted for %V̇O2max and %Ėaeromax against running speed (F=5.38, p=0.021). Conclusions: Quantifying aerobic determinants of performance in units of Ėaero provides no greater validity compared to Ȯaero-based measurement. Although both Ėaero and Ȯaero are sensitive to changes in speed, Ėaero provides the more valid reflection of the underlying metabolic cost of running. Physiologists should also be aware of the potential differences between expression of aerobic running intensity based upon %V̇O2max compared to %Ėaeromax.