As you increase your effort when you exercise, the amount of oxygen you consume to produce energy (and hence the rate at which you exhale carbon dioxide) increases

As you increase your effort when you exercise, the amount of oxygen you consume to produce energy (and hence the rate at which you exhale carbon dioxide) increases. However, there is a maximum level of oxygen consumption, beyond which increases in exercise intensity don’t lead to further increases in oxygen consumption. This level of oxygen consumption is called the VO2 max. (The initials simply stand for volume of oxygen).

Some experts believe that VO2 max is a key physiological determinant of an athlete’s walking performance, and that it is an important objective of a training program to improve it. Other sports scientists argue that the limits to an athlete’s walking performance are determined by a range of factors – such as adaptation of muscles, walking efficiency, metabolism – and that VO2 max is simply a measure of the oxygen that the athlete consumes at the maximum level of energy output. On this view, which I find persuasive, VO2 max is not the critical factor which determines maximum performance, but is rather a consequence of a combination of other limiting factors. Whichever way you look at it there is a measurable level of exercise intensity at which the athlete’s consumption of oxygen reaches a plateau and does not increase further.