The graph shows the metabolic demand and oxygen uptake (in steady state). The oxygen demand (also named VO2tot – dark blue curve) increases with the intensity (speed or power). The oxygen demand is similar to the energy demand needed at a certain intensity. However it is converted into ml/min/kg of oxygen instead of using kJ or a similar unit of energy. Therefore the increment of oxygen demand in relation to the intensity shows the efficiency.
The light blue curve shows the actual oxygen uptake (VO2) in steady state conditions. The unit is ml/min/kg – oxygen normalized to the body weight. As can be seen, at lower intensities, the actual oxygen uptake almost matches the oxygen demand, thus the needed amount of energy is almost completely covered by aerobic metabolism.
At higher intensities however, a gap is opening up and the oxygen uptake cannot match the demand. This gap is shown as the light blue area, and shows the amount of energy (or more precisely oxygen) which needs to be covered by glycolytic metabolism.