Maximal capacity for aerobic exercise

In the figure shown, it is clear that when the subject cycled at 300W, the VO2 response was not different from that achieved at 250W. This indicates that the subject had reached a maximal limit of this ability to increase his VO₂. This peak value is referred to as aerobic capacity, maximal oxygen uptake, or VO₂max. VO₂max is regarded by most at the best single measurement of cardiorespiratory endurance and aerobic fitness. This concept is further illustrated in other figure below, which compares the VO₂max of a trained and untrained man.

Although some sport scientists have suggested that VO₂max is a good predictor of success in endurance events, the winner of a marathon race cannot be predicted from the runner’s laboratory-measured VO₂max. Likewise, an endurance running performance test is only a modest predictor of one’s VO₂max. This suggests that a good performance requires more than a high VO₂max.

Also, research has documented that VO₂max increases with physical training for only 8 to 12 weeks and that this value then plateaus despite continued higher-intensity training. Although VO₂max does not continue to increase, the participants continue to improve their endurance performance. It appears that these individuals develop the ability to perform at a higher percentage of their VO₂max. Most runners, for example, can complete a 42km marathon at an average pace that requires them to use approximately 75% to 80% of their VO₂max.

Consider the case of Alberto Salazar, a near world record holder in the marathon. His measured VO₂max was 70 ml x kg^-1 x min^-1. That is below the VO₂max expected based on his best marathon performance of 2h 8min. He was, however, able to run marathon on 86% of his VO₂max when performing at his racing pace, a percentage considerably higher than that of other world-class runners. This may partly explain his world-class running ability.

Because individuals’ energy requirements vary with body size, VO₂max generally is expressed relative to body weight, in milliliters of oxygen consumed per kilogram of body weight per minute(ml x kg^-1 x min^-1). This allows a more accurate comparison of different-sized individuals who exercise in weight-bearing events, such as running. In non-weight-bearing activities, such as swimming and cycling, endurance performance is more closely related to VO₂max measured in liters per minute.

Normally active, but untrained, 18-to-22-year-old college students have average VO₂max values of 38 to 42 ml x kg^-1 x min^-1 for women and 44 to 50 ml x kg^-1 x min^-1 for men. After the age of 25 to 30 years, inactive people VO₂max values decrease about 1% per year. This is probably attributable to a combination of biological aging sedentary lifestyle. In addition, adult women generally have VO₂max values considerably below those of adult men. Two reasons for this sex difference are body composition differences(women generally have less fat-free mass and more fat mass) and blood hemoglobin content(women have less, thus they have less oxygen-carrying capacity). But it is unclear how much of the sex difference in VO₂max is attributable to actual physiological differences and how much might be caused by a more sedentary lifestyle.