LactateLactic acid

Lactate is formed when glycolysis runs faster than the mitochondria can process pyruvate. It is produced continuously, even at rest, and rises with intensity as production outpaces clearance.

It matters because almost everything popularly believed about it is wrong. Lactate does not cause muscle soreness — delayed soreness comes from mechanical damage, and lactate clears within an hour. Nor does it cause the burn: the associated hydrogen ions do. Lactate itself is shuttled to the heart, brain and other muscle fibres and burned as fuel.

In practice it is most useful as a measurement. Blood lactate marks the threshold above which effort becomes unsustainable, and lactate testing remains one of the more informative ways to set endurance training zones.