Lactate is continuously produced in skeletal muscle, even at rest, but with the onset of exercise, increases in the glycolytic resynthesis of ATP result in a correspondingly greater production of lactate in active fibres.
Lactate metabolism is a dynamic process and while some fibres produce lactate, adjacent fibres simultaneously consume it as an energy source. Nevertheless, during exercise lactate accumulates within the muscle and, although output does not match production, some lactate will diffuse into the blood where, during submaximal exercise, it can be sampled, assayed and analysed to provide an estimate of the anaerobic contribution to exercise and therefore a measure of aerobic fitness.
Lactate is continuously eliminated from the blood by oxidation in the heart or skeletal muscles or through conversion to glucose in the liver and kidneys. The lactate concentration of sampled blood is therefore a function of several dynamic processes including muscle production, muscle consumption, rate of diffusion into the blood and rate of removal from the blood.
Consequently, measures of blood lactate accumulation must be interpreted cautiously as lactate measured in the blood cannot be assumed to reflect a consistent or direct relationship with either muscle lactate production or muscle lactate accumulation.
The lactate threshold is negatively related to age but age effects on maximal lactate steady state remain to be proven. Neither lactate threshold nor maximal lactate steady state have been shown to be related to either sex or maturation.
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