The second approach for quantifying anaerobic fitness requires expensive laboratory equipment and also requires some assumptions to be made. Anaerobic energy expenditure can be estimated if the oxygen uptake–power output relationship is quantified during submaximal exercise. This relationship can then be used to estimate how much oxygen uptake would be required for any type of high-intensity exercise assuming the task was 100 per cent aerobic in nature. In cycling, the accumulated oxygen deficit (AOD) is calculated by subtracting the actual amount of oxygen consumed during a high-intensity exercise task from the amount of oxygen consumed assuming the task was performed entirely aerobically. By measuring the actual oxygen uptake and estimating the required oxygen uptake for the high-intensity effort it is possible to quantify the accumulated oxygen deficit. During a maximal high-intensity effort this measurement can become a useful index of anaerobic fitness.