In the top figure, oxygen deficit (pink) represents the anaerobic energy expenditure component to exercise: rapid glycolytic ATP re-synthesis and the use of stored ATP/PC. In this bout of long duration, low to moderate intensity, steady state exercise, the rapid glycolytic component does not make a significant contribution to total energy expenditure. The bottom left figure reveals oxygen uptake measurements for brief, non-steady state, heavy to severe exercise (e.g., a single weight lifting exercise or a quick sprint up a steep hill); vertical lines mark the start and finish to the exercise. The question marks indicate that it is not possible to determine the rapid glycolytic ATP re-synthesis from oxygen-only measurements. The bottom right figure includes a (theoretical) estimate of rapid glycolytic ATP re-synthesis (pink area) and reveals a large absolute and relative anaerobic energy expenditure component to total energy expenditure (restoration of ATP/PC stores are represented in the EPOC measurement).