The ‘sensory tolerance limit’: A hypothetical construct determining exercise performance?

Abstract

Neuromuscular fatigue compromises exercise performance and is determined by central and peripheral mechanisms. Interactions between the two components of fatigue can occur via neural pathways, including feedback and feedforward processes. This brief review discusses the influence of feedback and feedforward mechanisms on exercise limitation. In terms of feedback mechanisms, particular attention is given to group III/IV sensory neurons which link limb muscle with the central nervous system. Central corollary discharge, a copy of the neural drive from the brain to the working muscles, provides a signal from the motor system to sensory systems and is considered a feedforward mechanism that might influence fatigue and consequently exercise performance. We highlight recent findings from studies focusing on fatigue-related feedback and feedforward mechanisms and discuss their relevance for the previously proposed hypotheses that a ‘critical threshold of peripheral fatigue’ and/or a ‘sensory tolerance limit’ may regulate neuromuscular fatigue and ultimately exercise performance. The concept of a ‘critical threshold of peripheral fatigue’ is based on the idea that a negative feedback loop operates to protect the exercising limb muscle from severe threats to muscle homeostasis during whole-body exercise. The concept of a ‘sensory tolerance limit’ can be viewed as a more global negative feedback loop suggesting that the sum of all feedback and feedforward signals is processed within the central nervous system which ultimately regulates the intensity of exercise to ensure that voluntary activity remains tolerable.

The magnitude of neuromuscular fatigue is not intensity dependent when cycling above critical power but relates to aerobic and anaerobic capacities

“…Similar changes in muscle metabolic response (i.e. Interestingly, similar changes in evoked twitch forces (∼35%) have also been reported immediately after supra-CP exercise across a wide range of severe exercise intensities, which was described as a 'critical threshold' of peripheral fatigue (Amann & Dempsey, 2008;Amann et al, 2011Amann et al, , 2009Hureau, Romer, & Amann, 2016;Johnson, Sharpe, Williams, & Hannah, 2015;Romer et al, 2007;Thomas et al, 2015). Interestingly, similar changes in evoked twitch forces (∼35%) have also been reported immediately after supra-CP exercise across a wide range of severe exercise intensities, which was described as a 'critical threshold' of peripheral fatigue (Amann & Dempsey, 2008;Amann et al, 2011Amann et al, , 2009Hureau, Romer, & Amann, 2016;Johnson, Sharpe, Williams, & Hannah, 2015;Romer et al, 2007;Thomas et al, 2015).…”

Towards a three-dimensional framework of centrally regulated and goal-directed exercise behaviour: a narrative review

“…However, while of great theoretical interest and congruent with a recent proposal for a combined feedforward and feedback ‘sensory tolerance limit’,67 the meaning and practical value of this differentiation in healthy individuals is unclear and warrants further investigation.…”

Misinterpretation of the Borg’s Rating of Perceived Exertion Scale by patients with panic disorder during ergospirometry challenge

“…One alternative would be to understand the role of the central corollary discharge, a copy of the neural drive from the brain to the working muscles that provides a signal from the motor system to sensory systems and is considered a feedforward mechanism that might influence fatigue and consequently exercise performance. 20 In summary, these findings suggest that in patients with PD, a simple rating of perceived exertion as indexed by the Borg's RPE Scale may not be sufficient to accurately capture the intensity of physical effort. Figure 5 Absolute difference between the effort reported by smokers and non-smokers in the PD group through the RPE scale to 90% VO 2 max and HR measured at 90% VO 2 max.…”

“…These mechanisms have been proposed to interfere with the self-evaluation in the RPE Scale through an altered sensory tolerance limit. 20 In the present study, we hypothesise that patients with panic disorder (PD) overestimate their exertion rendering the RPE Scale scores inappropriate for evaluating cardiovascular (CV) capacity in the patient with PD population.…”

“…The most widely used instrument to measure perceived exertion or exercise intensity is the Borg's Rating of Perceived Exertion (RPE) Scale (6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20). 1 Objective measures of effort, such as heart rate (HR) and oxygen uptake, have be used in conjunction with RPE scores in clinical settings such as cardiac rehabilitation [2][3][4][5] and with patients receiving b-blocker therapy.…”