VO2 max, the peak oxygen uptake, is the maximum capacity of an individual's body to transport and use oxygen during incremental exercise, which reflects the physical fitness of the individual.

It is one factor that can determine an athlete's capacity to perform sustained exercise and is linked to aerobic endurance.

The physical limitations that restrict the rate at which energy can be released aerobically are dependent upon the chemical ability of the muscular cellular tissue system to use oxygen in breaking down fuels and the combined ability of cardiovascular and pulmonary systems to transport the oxygen to the muscular tissue system.

In both sexes, a large portion of the age-associated decline in VO2max in non-endurance-trained individuals is explicable by the loss of muscle mass, which is observed with advancing age.

The average rate of decline is generally accepted to be about 1% per year or 10% per decade after the age of 25.

Mitochondrial function could be closely related to the trainability of VO2max in previously sedentary populations and/or sub-VO2maximal endurance performance in athletes.

The decline in age-related VO2 max can also be accounted for by a reduction in maximum heart rate, maximal stoke volume and maximal a-VO2 difference i.e. the difference between oxygen concentration arterial blood and venous blood.

A progressive decline in VO2max generally occurs with advancing age, VO2max showed a strong negative linear relationship with age in both men and women.

Vigorous training at a younger age does not seem to prevent the fall in VO2 max if training is ceased altogether. Elite athletes have been shown to decline by 43% from ages 23 to 50 (from 70 ml/kg/min to 40 ml/kg/min) when they stop training after their careers are over. In some cases, the relative decline is greater than for the average population - as much as 15% per decade or 1.5% per year.

It seems that training can slow the rate of decline in VO2 max but becomes less effective after the age of about 50.