A Review of Two- and Three-Phase Models of Anaerobic Threshold Estimation

The purpose of the present study was to review the various methods, invasive and non-invasive, used to determine the anaerobic threshold (AT).  AT is the highest training intensity at which energy requirements of exercise are mainly provided by aerobic energy system, and the rate of lactate appearance in the blood is equal to the rate of its disappearance.  Since AT occurrence is associated with aerobic to anaerobic transition, it has a significant contribution to precise exercise prescription, determination of exercise intensity domains, and prediction of performance especially in endurance athletes. Direct measurement of blood lactate is the most accurate method to determine AT, but it has an invasive, expensive nature, and exposes the subject to the risk of blood sampling. As a result, other non-invasive methods that mainly use heart rate and workload relationship or changes in respiratory indexes such as oxygen and carbon dioxide equivalent, have been considered to determine AT. Meanwhile, the traditional method of AT determination usually uses two-phase models (one turn point). While recent researches prefer three-phase models (two turn points) which provide better estimation of training domains compared to those offered by two-phase models. Therefore, the present study intends to review the most common methods of AT determination and their pros and cons in the form of two- and three-phase models.