A person's anaerobic threshold has been reached when their body is under extreme conditions and the burning of oxygen can not keep up with the body's demands. At this point, the body begins to burn stored fuels, like sugars, instead of oxygen as the primary fuel. The word anaerobic literally means "without oxygen." This threshold can be expanded by training, so athletes are most often concerned with where this threshold is and how to expand it.
When the body is under regular conditions, it burns oxygen as the primary fuel to continue motion. This kind of activity is called aerobic. Carbon dioxide is produced as a byproduct of the process and is expelled through the lungs. The aerobic system is active for most daily tasks and light exercise.
When a person demands more from their body than the aerobic system can keep up with, the body switches to the anaerobic system to burn fuels in order to keep the body in motion. This anaerobic threshold is reached after long periods of activity or during a quick burst of activity such as a runner's sprint. The primary difference between the aerobic and anaerobic systems is in the byproduct produced. The aerobic byproduct, carbon dioxide, can be easily expelled by regular breathing, while the anaerobic byproduct is lactic acid. This lactic acid build up in the muscles quickly and causes fatigue.
Many athletes are interested in learning how to expand their anaerobic threshold in order to work longer and more efficiently. It is possible to train the body to have a higher threshold. Repeated training near the threshold will increase the body's efficiency at burning oxygen and therefore increase the threshold.
In order to expand the anaerobic threshold, it is important to discover exactly where it is for each individual. Doctors can discover the exact threshold through blood testing during physical exertion tests, since lactic acid will be evident in the blood. Smaller versions of the laboratory equipment used by doctors are available for coaches and trainers to carry for periodic testing of their athletes.
Some choose to estimate the anaerobic threshold by finding the heart rate. To the best of scientific knowledge, the threshold occurs between 85-90% of the maximum heart rate. To determine the maximum heart rate, take 220 and subtract the age of the athlete, then multiply the result by 0.85 and 0.90. For a 31 year old, the maximum heart rate would be 189 beats per minute, with the threshold kicking in between 160 and 170 beats per minute. This method is the least accurate, but can provide some idea of where to aim for someone not willing to go through clinical testing.