Fletcher, Esau & MacIntosh 2009, Economy of running: beyond the measurement of oxygen uptake

J Appl Physiol argument that running economy should be expressed as energy cost (kcal or J per kg per km), accounting for the substrate mix via the respiratory exchange ratio, rather than as oxygen cost alone, because the same oxygen uptake yields different energy depending on fuel burned. Typical energy cost is around 1.0-1.1 kcal/kg/km. Strong.