Energy is released within the body by oxidation: 1 ml of O2 releases 20 J. For most of the time our intake of oxygen is in balance with the rate at which our bodies generate energy: this is called aerobic energy production. For short periods when high muscular effort is required the body can operate anaerobically with the rate of energy production exceeding the oxygen intake. This creates an oxygen debt which is repaid by our being out of breath for a while after the effort. Only about 25% of the energy released during the activity can appear as mechanical work.