In a resting individual the potential OXYGEN supply to the tissues is greater than its consumption. During heavy exercise, the energy required by the tissues is greater than can be supplied by aerobic cellular metabolism and the additional energy is supplied by a biochemical reaction called anaerobic metabolism. There is a build-up of lactate – a product of LACTIC ACID – from anaerobic metabolism which is ultimately oxidised after conversion to citrate and metabolism via the citric acid cycle. The increased amount of oxygen above resting concentrations which needs to be consumed to perform this metabolism is known as the oxygen debt or deficit.