VO2 max is a measure of the maximum volume of oxygen that an athlete can use. It is measured in millilitres per kilogramme of body weight per minute (ml/kg/min).

The overall message is pretty obvious: You should keep training like an endurance athlete in order to (at least partly) ward off age-related decline in VO2 max.  In doing so, you’ll be keeping your whole system functioning optimally, not just your heart.

In a study, older subjects had a lower whole-body VO2 max than the younger ones by 38 percent. Interestingly, they were also 27 percent lower in single-leg VO2 max, suggesting that peripheral factors like blood circulation and diffusion had declined.

One trait that didn’t decline was the ability of their muscles to use oxygen. Using muscle biopsies, the researchers calculated the VO2 max of the mitochondria in the subjects’ leg muscles, and it was basically the same in both groups of subjects. This result, Gifford says, suggests that oxygen-processing capacity in the muscles “is primarily driven by physical activity, not age.”


In theory, all this excess mitochondrial capacity seems like a waste—or even a violation of the principle of “symmorphosis,” which argues that “the size of the parts of the system must be matched to the overall functional demand.” In this view, there’s no single bottleneck that determines VO2 max. Instead, all the parts of the cascade—the heart, the arteries, the capillaries, the mitochondria—are just the right size for your needs, and together they dictate VO2 max.

So why do endurance athletes develop excess mitochondrial capacity? The authors argue that “it is doubtful that this reserve capacity serves no purpose.” They discuss a few theories, such as the idea that excess mitochondrial capacity might assist in fat burning, which would enhance actual endurance performance without changing VO2 max. There’s also some evidence that it may buffer oxidative stress and reduce cell damage.

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