Reply: To PMID 24142455.

Abstract

We thank Dr Smirmaul for his interest in our findings, but advise caution not to overinterpret the data to address a question which would have required an entirely different study design. Specifically, the letter to the editor is strongly focused on effort perception, a variable which was, as expected, not considered as a descriptor by the participants when asked to report evoked sensations (Pollak et al. 2014). In fact, given that the perception of effort requires at least the attempt to activate muscle (or brain neurons), an altered perception would have been rather unusual given the absence of any sort of muscle contraction or task in our study (i.e. there was no required effort to be rated). In addition to having missed this critical fact, we disagree with the main postulate of the letter and need to remind the author about existing evidence documenting an independent role for both central command and muscle afferent feedback in determining the perception of effort during exercise. On the one hand, experiments using curare to increase central command while leaving muscle afferent feedback unchanged during a given task have reported a facilitating effect of central command on effort perception (e.g. Gandevia & McCloskey, 1977; Gallagher et al. 2001). On the other hand, studies using fentanyl to block group III/IV muscle afferents while leaving central command unchanged during exercise document a significant contribution of neural feedback to the perception of effort associated with physical activity (e.g. Amann et al. 2011; Gagnon et al. 2012). Taken together, the line of reasoning used in the letter is refuted by the following factors: (i) the fact that subjects were instructed not to attempt to move their thumb excluded any physical effort to be rated/perceived (which explains the lack of this variable on the participant-driven list of descriptors); and (ii) the well-documented contribution of both central command and muscle afferent feedback to effort perception. While we agree with the author that our data suggest that afferent feedback is the main contributor to the sensation of muscle fatigue, we disagree with the claim that our findings suggest that afferent feedback does not contribute to effort perception. Readers are invited to give their opinion on this article. To submit a comment, go to: http://ep.physoc.org/letters/submit/expphysiol;99/5/836.