Stimulation of Muscle Afferents During Muscle Contraction Does Not Impact Perception of Effort

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Perception of effort is a main feature of fatigue and plays an important role in the regulation of exercise performance. However, its neurophysiology is poorly understood. To date, the two main models of perception of effort generation are the corollary discharge (CD) model and the afferent feedback (AF) model. PURPOSE: To investigate the validity of these models using electromyostimulation (EMS) to manipulate the magnitude of central motor command/CD and AF during voluntary (V), evoked (EMS) and combined (V+EMS) contractions at same force output. We hypothesized that perception of effort would reflect the magnitude of the central motor command/CD, independently of AF stimulated by EMS. METHODS: Ten subjects with previous experience of EMS took part in this study. V, EMS and V+EMS contractions where performed during isotonic (5% and 20% maximal voluntary contraction, MVC) and isometric contractions (10 and 20% MVC). Subjects were asked to report perception of effort, perceived force and muscle pain for each contraction. RESULTS: For the same force output, subjects did not report any effort during evoked contractions (no central motor command/CD), but all reported effort during voluntary contractions. Subjects rated the effort lower (isometric: P=0.036, dz=1.062; isotonic: P<0.001, dz=1.725) during combined contractions (low central motor command/CD) than during voluntary contractions (full central motor command/CD). Contrary to perception of effort, subjects perceived the force during all kind of contractions. Muscle pain was higher during EMS (P<0.05). CONCLUSION: Subjects were able to rate independently perception of effort, perceived force and muscle pain. The ability of the subjects to perceive force and muscle pain during evoked contraction confirms that AF contributes to these sensations during evoked and combined contractions. The lower perception of effort during the combined contraction (low central motor command/CD and higher AF) compared to voluntary contraction (full motor command/CD and lower AF) provides strong evidence that AF is not the sensory signal generating perception of effort. These findings provide further support in favor of the proposal that perception of effort is generated by neurocognitive processing of CD, i.e. efferent copies of the central motor command.