If you are into exercise and fitness, it is likely that you've heard of this before. Did someone explain to you what it is? In short, anaerobic threshold is the point where a metabolic product known as lactic acid begins to accumulate in your body.
A bit more on the matter
Anaerobic threshold is an excellent and reliable indicator of your physical ability, or, to make it more clear, how well you are going to perform in aerobic sports or exercises. Before, we go on, let us clarify what happens when our body is in need of more energy. It produces it, of course, and there are two ways by which energy can be produced in the body. One is the aerobic metabolic pathway (where oxygen is used in the process, similar to combustion) and the other is the anaerobic pathway (where there is literally no time for introduction of oxygen as you need available energy ASAP).
Further explaining would require a blackboard covered with lots of complex chemical formulas and a bunch of "which goes where" arrows so we will leave it as it is. Imagine that your car can run by burning fuel or by doing something else with it if there is no air and the fuel can't burn. That "something else", if we draw a parallel to our bodies, is the anaerobic metabolic pathway. The point is that lactic acid is the end result of the anaerobic pathway. It is removed all the time, at a certain rate. However, if lactic acid is produced at too high a rate, it cannot be removed quick enough and it begins to accumulate. That is the onset of blood lactate accumulation or anaerobic threshold.
Calculating the anaerobic threshold
Unfortunately, this is a complex procedure and basically requires a laboratory setting. That makes it almost "pros only", but there is no reason not to do it if you really want to know where you stand. The tested person will have to do some form of aerobic exercise, like walking on a treadmill or riding of the stationary bike. At first, the resistance or the moving speed is low, but it is gradually increased at certain time intervals, typically in one to three minutes. At each such increase, a blood sample will be taken. As is usually the case, your heart rate and some other parameters of relevance are monitored, and are recorded at the same time when the blood sample is taken.
Levels of lactate in the blood are plotted against each measure of each workload interval. This gives the lactate performance curve. The spot on this curve which features a sudden jump above base level is said to be anaerobic threshold.