Over the last 100 years or so there have been a number of advancements in science and technology, and the field of Exercise Physiology is no different.

We have gained a much better understanding of the impact exercise has on the body and an area that has had much attention given to it is fatigue as a result of physical activity.

We have all experienced it in one form another, and it is not only of importance to the elite athletes of the world, but to any one involved in physical activity, because if you are committing any amount of time to exercise and training, you are bound to be affected by fatigue.

Whether you actually care why this fatigue occurs and what causes it, is another story.

Studies into the causes and results of exercise induced fatigue have been carried out by researches since the early 1900's, one of the best known and most respected researches was a guy called A.V. Hill, who was one of the pioneers in developing the traditionally accepted model of fatigue.

His conclusions pointed to Peripheral factors limiting performance at higher intensity as a result of increasing demands being placed on the anaerobic energy pathways.

It was postulated that with maximal efforts, there was a massive interruption in the bodies homeostatic balance, as a result of the bodies maximal physiological and biochemical limits being exceeded. For example, the extreme accumulation of lactic Acid interfered with the muscles ability to function optimally, causing the termination of exercise, or at least a reduction in output (speed, power etc.).

This became known as the "catastrophe model" and has been the foundation for models of fatigue for almost 100 years.

He also postulated that as a result of increased Anaerobic energy metabolism, there will be a corresponding reduction in the volume of oxygen rich blood flow to the heart muscle - Cardiac Ischemia. This increased the potential for damage to the heart muscle tissue to occur.

He felt that there might well be some sort of "governor" that, when cardiac ischemia developed, would limit the amount of work the heart did, thus protecting it from long term damage.

Could this "governor" perhaps be found in the heart itself, or possibly in the brain?

Enter Dr Tim Noakes. It was from Hills original work that Dr. Noakes developed the hypothesis that there might well be an alternative model of fatigue.

He postulated that that fatigue during exercise may form part of a "regulated, anticipatory response coordinated in the brain". The purpose of this ongoing "regulation" is to ultimately preserve the bodies homeostatic balance of all of its physiological systems during exercise, regardless of the intensity, duration or environmental conditions in which the activity is taking place.

In simpler terms, the brain continuously gathers information and feedback from all the bodies systems, about every parameter affected by the activity (temperature, pH level, energy stores, O2 levels etc.), and basically regulates the output permitted by the muscle.

It does not matter how long (duration) you have been exercising for, at what intensity or what the environmental conditions are, by reducing the amount of neural activation of the muscles, the brain regulates stimulation of the muscle, resulting in the sensation of fatigue and thus the perceived need to slow down or reduce the amount of work being done.

There are a number of implications to this hypothesis, to many to discuss in a single post, but the two that I found very interesting relate to our ability to adjust the degree of control the governor exerts through certain training techniques, and the impact the governor has on an individuals pacing strategy during activity (more on this later).

Remember, this is still a work in progress, and as you can imagine, with the brain being such a complex organ, a lot more research is required before any real conclusions can be drawn

James Greenwood is a competitive tri and multisport athlete currently training for Ironman Canada 2009. A level 1 Triathlon Coach, he holds a post graduate degree in Exercise Science, and is a Certified Strength and Conditioning Specialist through the NSCA. James is also currently the resident health and fitness programs expert at MyPypeline.com, and has starred in a number of multisport specific fitness videos.
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