No abstract

Fatigue is a common experience in both health and disease. Yet, pathological (i.e., prolonged or chronic) and transient (i.e., exertional) fatigue symptoms are traditionally considered distinct, compounding a separation between interested research fields within the study of fatigue. Within the clinical neurosciences, nascent frameworks position pathological fatigue as a product of inference derived through hierarchical predictive processing. The metacognitive theory of dyshomeostasis (Stephan et al., 2016) states that pathological fatigue emerges from the metacognitive mechanism in which the detection of persistent mismatches between prior interoceptive predictions and ascending sensory evidence (i.e., prediction error) signals low evidence for internal generative models, which undermine an agent’s feeling of mastery over the body and is thus experienced phenomenologically as fatigue. Although acute, transient subjective symptoms of exertional fatigue have also been associated with increasing interoceptive prediction error, the dynamic computations that underlie its development have not been clearly defined. Here, drawing on the metacognitive theory of dyshomeostasis, we extend this account to offer an explicit description of the development of fatigue during extended periods of (physical) exertion. Accordingly, it is proposed that a loss of certainty or confidence in control predictions in response to persistent detection of prediction error features as a common foundation for the conscious experience of both pathological and nonpathological fatigue.

The experience of fatigue is ubiquitous in both health and disease. However, the common separation of chronic or pathological fatigue from the transient symptoms arising through (cognitive/physical) exertion has forged a disconnect between these research domains. Within the clinical neurosciences, nascent frameworks position fatigue as a product of inference based on the principle of predictive coding. These accounts propose that the brain principally functions as a self-evidencing inference machine. Here, the perception of fatigue is proposed to emerge through metacognitive processes identifying a persistent mismatch (or error) between issued sensory expectations (i.e. predictions) and the true sensory feedback received within allostatic-control circuits. Continued prediction error signals low evidence for held internal models controlling bodily states. In turn, self-efficacy in allostatic control policies (i.e. predictions) wanes and is ultimately perceived as fatigue. However, the same predictive mechanisms have previously been excluded as a theoretical account for the transient experience of exertional fatigue, on the basis that action (i.e. rest) alleviates fatigue and thus prevents the agent from experiencing the necessary decline in allostatic control mastery. Here we contest this proposition and offer a more parsimonious account of fatigue in which the same mechanism (i.e. a loss of certainty or confidence in allostatic predictions) is at the core of both the pathological and exertional symptoms of fatigue. We discuss how fatigue emerging from acute exertion may conform to this theory, highlighting supporting evidence from the exercise sciences, before providing testable hypotheses for the outlined theoretical framework.

Background The subjective experience of fatigue impairs an individual’s ability to sustain physical endurance performance. However, precise understanding of the specific role perceived fatigue plays in the central regulation of performance remains unclear. Here, we examined whether the subjective intensity of a perceived state of fatigue, pre-induced through prior upper body activity, differentially impacted performance and altered perceived effort and affect experienced during a sustained, isometric contraction in lower body. We also explored whether (cardiac) interoception predicted the intensity of experienced perceptual and affective responses and moderated the relationships between constructs during physical activity. Methods Using a repeated-measures study design, thirty male participants completed three experimental conditions, with the intensity of a pre-induced state of fatigue manipulated to evoke moderate (MOD), severe (SEV) and minimal (control; CON) intensity of perceptions prior to performance of the sustained contraction. Results Performance of the sustained contraction was significantly impaired under a perceived state of fatigue, with reductions of 10% and 14% observed in the MOD and SEV conditions, respectively. Performance impairment was accompanied by greater perceived effort and more negative affective valence reported during the contraction. However, effects were limited to comparisons to CON, with no difference evident between the two experimental trials (i.e. MOD vs. SEV). Individuals’ awareness of their accuracy in judging resting heartbeats was shown to predict the subjective intensity of fatigue experienced during the endurance task. However, interoception did not moderate the relationships evident between fatigue and both perceived effort and affective valence. Conclusions A perceived state of fatigue limits endurance performance, influencing both how effortful activity is perceived to be and the affective experience of activity. Though awareness of interoceptive representations of bodily states may be important to the subjective experience of fatigue, interoception does not modulate the relationships between perceived fatigue and other perceptual (i.e. effort) and affective constructs.

Background: The subjective experience of fatigue impairs an individual’s ability to sustain physical endurance performance. However, precise understanding of exactly how the perception of fatigue limits performance remains unclear. Methods: Here, we examined whether subjective intensity of perceived fatigue, pre-induced through prior upper body activity, differentially impacted performance and altered perceptual (effort) and affective responses during a sustained, isometric contraction in lower body. We also explored whether (cardiac) interoceptive awareness moderated the relationship between the perception of fatigue and effort. Using a repeated-measures study design, thirty male participants completed three experimental conditions, with the intensity of the pre-induced state of fatigue manipulated to evoke severe (SEV), moderate (MOD) and minimal (control; CON) perceptions prior to the performance of a sustained, sub-maximal contraction in the dominant knee extensors. Results: Participants’ capacity to sustain the isometric contraction was significantly impaired (vs. CON, MOD: -9.5 ± 19.1%, p=0.007; SEV: -13.7 ± 17.2%, p<0.001), with perception of effort higher (SEV, MOD vs. CON: b=-0.90, p<0.001), when perceptions of fatigue were elevated. However, the effect on performance (SEV vs. MOD: -2.5 ± 19.9%, p=0.455) and effort perception (SEV vs. MOD: b=-0.06, p=0.612) was not dependent upon the manipulated intensity of the pre-induced state of fatigue. Perceived fatigue was shown to predict the perception of effort during the endurance task. However, disassociation of perceptual constructs was evidenced with perceived effort, but not fatigue, associated with endurance time (CON: r=-0.57, p=0.004; MOD: r=-0.69, p<0.001; SEV: r=-0.59, p=0.003). Interoceptive awareness was shown to attenuate the relationship between perceived fatigue and effort, but only when fatigue was elevated within the experimental manipulations (b=-4.45, p=0.005). Conclusions: Perception of fatigue indirectly limits endurance performance by exerting influence over sensory processes making a physical task appear more effortful. Moreover, this effect is influenced by individual awareness of interoceptive representations of bodily states.