At work rate corresponds to heavy or severe exercise, a slowly developing component of the VO2 response, named VO2 slow component appear. A clear picture of the mechanism underlying the slow-component rise in VO2 remains to be described. We argue in this article that the VO2 slow component could be explained by a progressive lactate and pyruvate availability for mitochondria observed only when oxidative capacity of muscle is challenged (above lactate threshold). In this kind of exercise, the work rate requires the use of a large number of mitochondria that could only be involved when the pyruvate/lactate concentration in the cells is increased. This mechanism would then operate according to the law of mass action. The resynthesize of ATP should become progressively more aerobic than anaerobic. The hypothesis of a pyruvate/lactate concentration mechanism of VO2 slow component needs further experimental validation.
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