Fatigue is a Brain-Derived Emotion that Regulates the Exercise Behavior to Ensure the Protection of Whole Body Homeostasis

Abstract

An influential book written by A. Mosso in the late nineteenth century proposed that fatigue that "at first sight might appear an imperfection of our body, is on the contrary one of its most marvelous perfections. The fatigue increasing more rapidly than the amount of work done saves us from the injury which lesser sensibility would involve for the organism" so that "muscular fatigue also is at bottom an exhaustion of the nervous system." It has taken more than a century to confirm Mosso's idea that both the brain and the muscles alter their function during exercise and that fatigue is predominantly an emotion, part of a complex regulation, the goal of which is to protect the body from harm. Mosso's ideas were supplanted in the English literature by those of A. V. Hill who believed that fatigue was the result of biochemical changes in the exercising limb muscles - "peripheral fatigue" - to which the central nervous system makes no contribution. The past decade has witnessed the growing realization that this brainless model cannot explain exercise performance. This article traces the evolution of our modern understanding of how the CNS regulates exercise specifically to insure that each exercise bout terminates whilst homeostasis is retained in all bodily systems. The brain uses the symptoms of fatigue as key regulators to insure that the exercise is completed before harm develops. These sensations of fatigue are unique to each individual and are illusionary since their generation is largely independent of the real biological state of the athlete at the time they develop. The model predicts that attempts to understand fatigue and to explain superior human athletic performance purely on the basis of the body's known physiological and metabolic responses to exercise must fail since subconscious and conscious mental decisions made by winners and losers, in both training and competition, are the ultimate determinants of both fatigue and athletic performance.

Keywords: anticipation; brain; central governor model; central nervous system; fatigue; feedback; feedforward; skeletal muscle.

Figure 1

The complete A. V. Hill Cardiovascular/Anaerobic/Catastrophic Model of Human Exercise Performance. The governor component causing a “slowing of the circulation” was lost from the model some time after the 1930s.

Figure 2

The Central Governor Model of Exercise Regulation proposes that the brain regulates exercise performance by continuously modifying the number of motor units that are recruited in the exercising limbs. This occurs in response to conscious and subconscious factors that are present before and during the exercise, and those which act purely during exercise. The goal of this control is to insure that humans always exercise with reserve and terminate the exercise bout before there is a catastrophic failure of homeostasis. The brain uses the unpleasant (but illusory) sensations of fatigue to insure that the exercise intensity and duration are always within the exerciser’s physiological capacity. This model therefore predicts that the ultimate performances are achieved by athletes who best control the progression of these illusory symptoms during exercise. (For more details see St Clair Gibson et al., ; Noakes et al., , ; St Clair Gibson and Noakes, ; Tucker, ; Tucker and Noakes, ; Noakes, 2011a,b).