Fatigue leading from physical exertion has traditionally been attributed to a depletion of oxygen and carbohydrates or a buildup of lactic acid. Based on this theory, the most promoted model since the mid-1920s has been the Cardiovascular/Anaerobic model proposed by A. V. Hill and associates. Used by scientists, coaches, and athletes, the model indicates that a lack of oxygen to the working muscles limits exercise performance. For this reason, VO2 max, lactate threshold, and running economy are used to discuss training and physiology.
Although the Cardiovascular/Anaerobic model is still used today, a great amount of new research challenges the validity of the model; however, the model survives mainly because there has been no significant contender to be considered as an alternative. The main problem is that the Cardiovascular/Anaerobic model does not explain why endurance athletes do not collapse at the end of a race as would be expected if their stores of oxygen or fuel have depleted. There have only been a few very rare instances where this has happened.
In fact, quite the opposite usually happens: most of the time, athletes develop a surge of energy as they near the end of a race, despite their fatigue. In addition, fatigue is usually experienced when the finish line is in sight no matter the length of the race. None of this can be explained by the theory of depletion or, therefore, the Cardiovascular/Anaerobic model.
Central Governor Model
The issue can, however, be solved by the Hill/Noakes Central Governor model. Developed by exercise physiologist Dr. Noakes, the model is also credited to A. V. Hill for his additional work on the Governor model, although his work was mainly ignored by the supporters of the Cardiovascular/Anaerobic model and did not receive much publicity until the model was refined by Noakes.
To understand the Central Governor model, it is first necessary to define what is meant by fatigue. Fatigue is not an absolute event — this would suggest that a person is either in a state of fatigue or not — rather, fatigue can be expressed on a scale of greater or lesser amounts. An athlete can always continue moving, although he or she may need to slow down.
Noakes suggests that fatigue is generated by the brain as a protective metabolism. He comes to this conclusion using the logic that, should the brain allow the body to near depletion, the subject would come close to dying during physical exertion, which works against one's survival instincts. Instead, by creating a sense of fatigue long before depletion, the body is protected from irreversible damage.
The Central Governor model states, therefore, that the brain monitors the conditions of all the systems in the body and calculates the metabolic costs of continuing at current pace. By doing so, the brain can subsequently ensure that the body moves at optimum pace. This explains why heart rate may fluctuate when no external changes are present. These changes are found to be consistent with the brain calculations.
This also explains the why pace becomes slowed during exposure to high temperatures, a phenomenon that cannot be rationalized by the depletion theory. The Central Governor model posits that this occurs because the brain is computing heat buildup and commanding the body to use a slower pace in order to limit internal body heat.
Understanding the concept of fatigue can help endurance athletes to overcome its effects and subsequently improve athletic performance. The Central Governor model allows for increased accuracy and is therefore more useful for improving training practices than the Cardiovascular/Anaerobic model.