Do we really need a central governor to explain brain regulation of exercise performance?
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- Marcora, S.M. Eur J Appl Physiol (2008) 104: 929. doi:10.1007/s00421-008-0818-3
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In this paper two different models of brain regulation of exercise performance are critically compared: the central governor model proposed by Noakes and colleagues, and an alternative psycholobiological model based on motivational intensity theory.
KeywordsPerception of effortExercise performanceBrainCentral governorMotivationPsychobiology
To the Editor
In a paper recently published in the European Journal of Applied Physiology, Crewe et al. (2008) present interesting data on the effects of exercise intensity and ambient temperature on rating of perceived exertion (RPE) during prolonged constant-power exercise. On the basis of these data, the authors conclude that the rate of increase in perception of effort is set early on in the exercise bout by a subconscious intelligent system in the brain [the so called central governor (St Clair Gibson and Noakes 2004)] that, on the basis of various afferent sensory inputs, decides in “teleoanticipation” for how long the subject can exercise without threatening his/her whole-body homeostasis. According to this central governor model of exercise tolerance, the function of high perceived exertion at the preset time to exhaustion is to deter the conscious brain from dangerously overriding the subconsciously calculated teleoanticipatory strategy in response to other motives such as verbal encouragement provided by a cheering crowd (St Clair Gibson and Noakes 2004).
Although we admire Noakes and colleagues for challenging the current paradigm of exercise physiology by emphasising the crucial role played by the brain in the regulation of exercise performance, we think that their central governor model is internally inconsistent, unnecessarily complex, and biologically implausible. First of all, it is not clear why the central governor needs to deter the conscious brain by generating high perceived exertion at exhaustion. Indeed, Noakes and colleagues have proposed that the central governor has direct control over maximal neural recruitment of locomotor muscles (Noakes 2000; St Clair Gibson and Noakes 2004; Noakes 2007). If we assume that this is true, then the conscious sensation of effort is, in theory, unnecessary. In fact, subconscious control of the allowable extent of skeletal muscle recruitment could cause exhaustion no matter how strong is the motivation of the individual to continue exercise. Our argument that the central governor model could work even without perceived exertion is supported by Noakes himself who did not include RPE in his original descriptions of the central governor model. Noakes (2000) even stated that conscious regulation of effort (the so called psychological-motivational model) “conflicts with one proposal of the muscle recruitment model, which holds that exercise performance is regulated at a subconscious level and which exists, in part, to prevent conscious override that might damage the human”.
Similarly, the observation that RPE increases linearly during constant-power exercise can be explained by some of the known physiological changes induced by prolonged exercise rather than the action of a hypothetical central governor. Indeed, there is convincing experimental evidence that perception of effort arises from efferent rather than afferent sensory inputs (Marcora 2008). Therefore, the increase in RPE over time can be explained by the “moment-by-moment” increases in central motor commands to the locomotor and respiratory muscles needed to compensate the progressive reductions in cortical, motoneuronal, and/or muscular responsiveness during prolonged submaximal exercise at a fixed workload (Taylor and Gandevia 2008). Furthermore, during prolonged cycling exercise there is a progressive hyperventilation which further increases the required central neural drive to the respiratory muscles. In our opinion, the observation that exercise intensity and ambient temperature affect the rate of RPE increase (Fig. 1 in Crewe et al.) can be better explained by their effects on neuromuscular responsiveness and/or ventilation rather than complex subconscious teleoanticipatory calculations made by an unknown part of the brain based on afferent feedback from temperature- and not yet identified intensity-receptors.
According to the authors, the finding that RPE expressed as a percentage of time to exhaustion raises at a similar rate across all five trials (Fig. 2 in Crewe et al.) provides further support to the central governor model. However, this finding is also consistent with our psychobiological model of exercise tolerance. Indeed, when potential motivation is not affected, motivational intensity theory predicts that subjects consciously decide to stop exercise when they reach the same level of perceived exertion across all five trials regardless of their duration. Therefore, the observation that RPE scales with time when using the same end RPE value and time to exhaustion to normalise RPE during each trial is, in our opinion, a simple arithmetical transformation rather than the result of complex teleoanticipation.
Despite the above criticisms, the finding that RPE predicts time to exhaustion in so many different conditions is encouraging because it suggests that a psychobiological model based on effort-related decision-making may provide a unifying theory of exercise tolerance. This is in stark contrast with the traditional physiological model of fatigue and exercise performance which, despite more than 80 years of intense research, has yet to identify the cardinal “exercise stopper” (Gandevia 2001).