In practice, as this involves expensive and complex equipment, and, as there is a direct relationship between physical work done by person and the amount of oxygen needed for the job, an individual is subjected to an ongoing effort, on a treadmill or a bicycle, and his maximum effort is measured. Based on this relationship between work and oxygen, and by using reliable formulas we can estimate the maximum oxygen consumption of that person. The method is indirect, but predictable, and is used every day in the Exercise Tests to measure fitness.

The values that show the results of the stress test, are calculated in milliliters of oxygen per kilo of weight per minute (ml.kg.min). This is usually called maximum oxygen consumption (VO2max, i.e. a maximum volume of oxygen) among physicians. Also used, is the term METs, which comes from the English metabolic equivalents. Thus, one MET is the amount of oxygen consumed at rest (which is about 3.5 milliliters of oxygen per kilo of weight per minute)

Thus, in practice, the number of MET used to measure fitness. 1 MET reflects a certain amount of oxygen used, therefore, the more METs reached during the Exercise Test, the better physical condition we have.
According to the METs achieved we can classify a persons physical condition.
Here is an example of METs Levels by Age, in groups of women of low, moderate and high cardiorespiratory fitness (CFC)

I am 55 and I reach 10,9 METs, a high CFC for a man. This indicates a healthy heart and a lower probability of future heart problems. And you? How many METs can you reach?