LactateLactic acid
A product of fast glucose breakdown: a usable fuel, not a waste product.
Lactate is produced whenever glucose is broken down quickly, which happens continuously, at rest as well as during exercise. It is not a waste product and it is not what makes you sore two days later, despite both ideas being repeated for the best part of a century.
It is a fuel. Muscle, heart and brain all take up lactate and oxidise it, and it shuttles between fibres and between organs as an energy substrate. The liver can also convert it back into glucose, which is one route by which hard exercise recycles its own by-products.
What it does well is mark intensity. Blood lactate rises steeply once production outpaces clearance, and that inflection is used to define training zones and to predict endurance performance, often better than VO2max does among trained athletes.
The soreness it is blamed for is not its doing. Lactate clears within roughly an hour of finishing, long before delayed onset muscle soreness arrives, and the burning sensation during hard work tracks more closely with hydrogen ions and inorganic phosphate accumulating alongside it.
The terminology is a mess and worth treating with suspicion. A major review catalogued a long list of competing lactate threshold definitions and calculation methods, all named similarly and none interchangeable, which is why two sources can give you different threshold numbers from the same test.
Clearance improves with training, and it improves through both easy aerobic volume and hard interval work. Being able to produce a lot of lactate and clear it quickly is a trained capacity rather than a fixed trait.
In practice, you do not need a meter. The intensity you could hold for about an hour, and the point where speaking in full sentences becomes difficult, get most people close enough. Our heart rate zone calculator sets the bands from there.