Pharmacological blockade of muscle afferents and perception of effort: a systematic review with meta-analysis

Abstract

The perception of effort (PE) provides information on task difficulty and influences physical exercise regulation and human behavior. This perception differs from other-exercise related perceptions such as pain. There is no consensus on the role of group III-IV muscle afferents as a signal processed by the brain to generate PE. The aim of this meta-analysis was to investigate the effect of pharmacologically blocking muscle afferents on the PE. Six databases were searched to identify studies measuring the ratings of perceived effort (RPE) during physical exercise, with and without pharmacological blockade of muscle afferents. Articles were coded based on the operational measurement used to distinguish studies in which PE was assessed specifically (effort dissociated) or as a composite experience including other exercise-related perceptions (effort not dissociated). Articles that did not provide enough information for coding were assigned to the unclear group. The effort dissociated group (n=6) demonstrated a slight RPE increase with reduced muscle afferents feedback (standard mean change raw (SMCR), 0.39; 95%CI, 0.13 to 0.64). The group effort not dissociated (n=2) did not reveal conclusive results (SMCR, -0.29; 95%CI, -2.39 to 1.8). The group unclear (n=8) revealed a slight RPE decrease with reduced muscle afferents feedback (SMCR, -0.27; 95%CI, -0.50 to -0.04). The heterogeneity in results between groups reveals that the inclusion of other perceptions than effort in its rating influences the RPE scores reported by the participants. The absence of decreased RPE in the effort dissociated group suggests that muscle afferents feedback is not a sensory signal generating PE.

Efforts and their feelings

“…These studies have shown that, when the feeling of effort is measured separately from other performance-related feelings, reports of sensations of effort increase as physical exertion increases even when afferent signals are blocked (Bergevin et al, 2021). Evidence of this kind tips the scale in favor of centralism.…”

“…Empirically, recent research used epidural anesthesia to block afferent signals from the muscles to the nervous system while participants perform effortful physical activities. These studies have shown that, when the feeling of effort is measured separately from other performance‐related feelings, reports of sensations of effort increase as physical exertion increases even when afferent signals are blocked (Bergevin et al., 2021). Evidence of this kind tips the scale in favor of centralism.…”

What is the feeling of effort about?

“…3 FBE-Metabolic and similar views are susceptible to a serious objection: afferent signals are neither sufficient nor necessary for FBE. They are not sufficient, since the addition of lactate and metabolites in the blood (characteristic byproducts of the metabolic processes involved in muscular exertion) generates feelings of muscle fatigue and pain, but not feelings of effort (Pollak, Swenson, et al 2014;Pageaux and Gaveau 2016;Bergevin, Steele, et al 2021). Distinguishing effort from fatigue is important: feelings of fatigue involve a sense of tiredness or heaviness, and are associated 3 There is a long-standing debate between peripheralists and centralists (according to which the feeling of effort arises from commands originating in the central nervous system, identifiable with commands of the will).…”