Timothy D. Noakes

From catastrophe to complexity: a novel model of integrative central neural regulation of effort and fatigue during exercise in humans: summary and conclusions

It is hypothesised that physical activity is controlled by a central governor in the brain and that the human body functions as a complex system during exercise. Using feed forward control in response to afferent feedback from different physiological systems, the extent of skeletal muscle recruitment is controlled as part of a continuously altering pacing strategy, with the sensation of fatigue being the conscious interpretation of these homoeostatic, central governor control mechanisms.

Evidence for complex system integration and dynamic neural regulation of skeletal muscle recruitment during exercise in humans

A model is proposed in which the development of physical exhaustion is a relative rather than an absolute event and the sensation of fatigue is the sensory representation of the underlying neural integrative processes. Furthermore, activity is controlled as part of a pacing strategy involving active neural calculations in a “governor” region of the brain, which integrates internal sensory signals and information from the environment to produce a homoeostatically acceptable exercise intensity. The end point of the exercise bout is the controlling variable. This is an example of a complex, non-linear, dynamic system in which physiological systems interact to regulate activity before, during, and after the exercise bout.

The Role of Information Processing Between the Brain and Peripheral Physiological Systems in Pacing and Perception of Effort

This article examines how pacing strategies during exercise are controlled by information processing between the brain and peripheral physiological systems. It is suggested that, although several different pacing strategies can be used by athletes for events of different distance or duration, the underlying principle of how these different overall pacing strategies are controlled is similar. Perhaps the most important factor allowing the establishment of a pacing strategy is knowledge of the endpoint of a particular event. The brain centre controlling pace incorporates knowledge of the endpoint into an algorithm, together with memory of prior events of similar distance or duration, and knowledge of external (environmental) and internal (metabolic) conditions to set a particular optimal pacing strategy for a particular exercise bout. It is proposed that an internal clock, which appears to use scalar rather than absolute time scales, is used by the brain to generate knowledge of the duration or distance still to be covered, so that power output and metabolic rate can be altered appropriately throughout an event of a particular duration or distance. Although the initial pace is set at the beginning of an event in a feedforward manner, no event or internal physiological state will be identical to what has occurred previously. Therefore, continuous adjustments to the power output in the context of the overall pacing strategy occur throughout the exercise bout using feedback information from internal and external receptors. These continuous adjustments in power output require a specific length of time for afferent information to be assessed by the brain’s pace control algorithm, and for efferent neural commands to be generated, and we suggest that it is this time lag that crates the fluctuations in power output that occur during an exercise bout. These non-monotonic changes in power output during exercise, associated with information processing between the brain and peripheral physiological systems, are crucial to maintain the overall pacing strategy chosen by the brain algorithm of each athlete at the start of the exercise bout.

Complex systems model of fatigue: integrative homoeostatic control of peripheral physiological systems during exercise in humans

Fatigue is hypothesised as being the result of the complex interaction of multiple peripheral physiological systems and the brain. In this new model, all changes in peripheral physiological systems such as substrate depletion or metabolite accumulation act as afferent signallers which modulate control processes in the brain in a dynamic, non-linear, integrative manner.

The influence of sensory cues on the perception of exertion during exercise and central regulation of exercise performance

AbstractThe perception of effort during exercise and its relationship to fatigue is still not well understood. Although several scales have been developed to quantify exertion Borg’s 15-point ratings of perceived exertion (RPE) scale has been adopted as a valid and reliable instrument for evaluating whole body exertion during exercise. However, Borg’s category-ratio scale is useful in quantifying sensations of exertion related to those variables that rise exponentially with increases in exercise intensity. Previous research has examined the extent to which afferent feedback arising from cardiopulmonary and peripheral variables mediates the perception of exertionHowever, the literature has not identified a single variable that consistently explains exertion ratings. It is concluded that effort perception involves the integration of multiple afferent signals from a variety of perceptual cues. In a process defined as teleoanticipation, the changes in perceived exertion that result from these afferent signals may allow exercise performance to be precisely regulated such that a task can be completed within the biomechanical and metabolic limits of the body. The accuracy with which individuals can regulate exercise intensity based upon RPE values, the decrease in muscle recruitment (central drive) that occurs before fatigue, and the extent to which perceived exertion and heart rate can be altered with hypnosis and biofeedback training all provide evidence for the existence of such a regulatory system. Future research is needed to precisely quantify the extent to which efferent feed forward commands and afferent feedback determine pacing strategies such that an exercise event can be completed without irreversible tissue damage. However, the literature has not identified a single variable that consistently explains exertion ratings. It is concluded that effort perception involves the integration of multiple afferent signals from a variety of perceptual cues. In a process defined as teleoanticipation, the changes in perceived exertion that result from these afferent signals may allow exercise performance to be precisely regulated such that a task can be completed within the biomechanical and metabolic limits of the body. The accuracy with which individuals can regulate exercise intensity based upon RPE values, the decrease in muscle recruitment (central drive) that occurs before fatigue, and the extent to which perceived exertion and heart rate can be altered with hypnosis and biofeedback training all provide evidence for the existence of such a regulatory system. Future research is needed to precisely quantify the extent to which efferent feed forward commands and afferent feedback determine pacing strategies such that an exercise event can be completed without irreversible tissue damage.

From catastrophe to complexity: a novel model of integrative central neural regulation of effort and fatigue during exercise in humans

It is a popular belief that exercise performance is limited by metabolic changes in the exercising muscles, so called peripheral fatigue. Exercise terminates when there is a catastrophic failure of homoeostasis in the exercising muscles. A revolutionary theory is presented that proposes that exercise performance is regulated by the central nervous system specifically to ensure that catastrophic physiological failure does not occur during normal exercise in humans.

The Conscious Perception of the Sensation of Fatigue

In this review, fatigue is described as a conscious sensation rather than a physiological occurrence. We suggest that the sensation of fatigue is the conscious awareness of changes in subconscious homeostatic control systems, and is derived from a temporal difference between subconscious representations of these homeostatic control systems in neural networks that are induced by changes in the level of activity. These mismatches are perceived by consciousness-producing structures in the brain as the sensation of fatigue. In this model, fatigue is a complex emotion affected by factors such as motivation and drive, other emotions such as anger and fear, and memory of prior activity. It is not clear whether the origin of the conscious sensation of fatigue is associated with particular localised brain structures, or is the result of electrophysiological synchronisation of entire brain activity.

The physiological regulation of pacing strategy during exercise: a critical review

The regulation of the pacing strategy remains poorly understood, because much of classic physiology has focused on the factors that ultimately limit, rather than regulate, exercise performance. When exercise is self-paced and work rate is free to vary in response to external and internal physiological cues, then a complex system is proposed to be responsible for alterations in exercise intensity, possibly through altered activation of skeletal muscle motor units. The present review evaluates the evidence for such a complex system by investigating studies in which interventions such as elevated temperature, altered oxygen content of the air, reduced fuel availability and misinformation about distance covered have resulted in alterations to the pacing strategy. The review further investigates how such a pacing strategy might be regulated for optimal performance, while ensuring that irreversible physiological damage is not incurred.

Effect of anticipation during unknown or unexpected exercise duration on rating of perceived exertion, affect, and physiological function

Objectives: To determine the effect of unknown exercise duration and an unexpected increase in exercise duration on rating of perceived exertion (RPE), affect, and running economy during treadmill running. Methods: Sixteen well trained male and female runners completed three bouts of treadmill running at 75% of their peak treadmill running speed. In the first trial, they were told to run for 20 minutes and were stopped at 20 minutes (20 MIN). In another trial, they were told to run for 10 minutes, but at 10 minutes were told to run for a further 10 minutes (10 MIN). In the final trial, they were not told for how long they would be running but were stopped after 20 minutes (unknown, UN). During each of the running bouts, RPE, oxygen consumption (ml/kg/min), heart rate (beats/min), stride frequency (min−1), affect scores (arbitrary units), and attentional focus (percentage associative thought scores) were recorded. Results: RPE increased significantly between 10 and 11 minutes in the 10 MIN compared with the 20 MIN and UN trials (p<0.05). The affect score decreased significantly between 10 and 11 minutes in the 10 MIN compared with the 20 MIN trial (p<0.05). Running economy, as measured by oxygen consumption, was significantly lower in the UN compared with the 20 MIN trial from 10 to 19 minutes (p<0.05). Conclusions: The change in RPE between 10 and 11 minutes in the 10 MIN trial suggests that RPE is not purely a measure of physical exertion, as treadmill speed was maintained at a constant pace both before and after the unexpected increase in exercise duration. The associated changes in affect score at similar times in the 10 MIN trial supports the hypothesis that RPE has an affective component.

A signalling role for muscle glycogen in the regulation of pace during prolonged exercise

Introduction: In this study we examined the pacing strategy and the end muscle glycogen contents in eight cyclists, once when they were carbohydrate loaded and once when they were non-loaded. Methods: Cyclists completed 2 hours of cycling at ∼73% of maximum oxygen consumption, which included five sprints at 100% of peak sustained power output every 20 minutes, followed immediately by a 1 hour time trial. Muscle biopsies were performed before and immediately after exercise, while blood samples were taken during the 2 hour steady state rides and immediately after exercise. Results: Carbohydrate loading improved mean power output during the 1 hour time trial (mean (SEM) 219 (17) v 233 (15) W; p<0.05) and enabled subjects to use significantly more muscle glycogen than during the trial following their normal diet. Significantly, the subjects, kept blind to all feedback except for time, started both time trials at similar workloads (∼30 W), but after 1 minute of cycling, the workload average 14 W higher throughout the loaded compared with the non-loaded time trial. There were no differences in subjects’ plasma glucose and lactate concentrations and heart rates in the carbohydrate loaded versus the non-loaded trial. Of the eight subjects, seven improved their time trial performance after carbohydrate loading. Finishing muscle glycogen concentrations in these seven subjects were remarkably similar in both trials (18 (3) v 20 (3) mmol/kg w/w), despite significantly different starting values and time trial performances (36.55 (1.47) v 38.14 (1.27) km/h; p<0.05). The intra-subject coefficient of variation (CV) for end glycogen content in these seven subjects was 10%, compared with an inter-subject CV of 43%. Conclusions: As seven subjects completed the time trials with the same end exercise muscle glycogen concentrations, diet induced changes in pacing strategies during the time trials in these subjects may have resulted from integrated feedback from the periphery, perhaps from glycogen content in exercising muscles.