Abstract
During high intensity exercise, metabosensitive muscle afferents are thought to inhibit the motor drive command to restrict the level of peripheral fatigue to an individual’s critical threshold. No evidence exists of an individual relationship between peripheral fatigue and the decrease in voluntary activation reached after prolonged all-out exercise. Moreover, there is no explanation for the previously reported large decrease in voluntary activation despite low metabolic stress during high force contractions. Thirteen active men completed two maximal intensity isokinetic knee extension tests (160 contractions) under conditions of low force – high velocity and high force – low velocity. Neuromuscular testing including maximal torque, evoked torque and voluntary activation, was done every 20 contractions. The exponential modeling of these variables over time allowed us to predict the stable state (asymptote) and the rate of decrease (curvature constant). For both high and low force contractions the evoked torque and voluntary activation asymptotes were negatively correlated (R2 = 0.49 and R2 = 0.46, respectively). The evoked torque asymptotes of the high and low force conditions were positively correlated (R2 = 0.49). For the high force contractions, the evoked torque and voluntary activation curvature constant were negatively correlated (R2 = 0.43). These results support the idea that a restrained central motor drive keeps peripheral fatigue under this threshold. Furthermore, an individual would show similar fatigue sensibility regardless of the force generated. These data also suggest that the decrease in voluntary activation might not have been triggered by peripheral perturbations during the first high force contractions.
Highlights
Muscle fatigue can be defined as an exercise-induced reduction in maximal voluntary muscle force (Gandevia, 2001)
The purpose of the present study was to analyze the individual time course of central and peripheral contributions to force loss during prolonged exercise performed at maximal voluntary intensity with low vs. high force using high vs. low velocity isokinetic knee extensions, respectively
The major results were that (i) the decrease in muscle contractility is limited to different critical thresholds in low vs. high force conditions, in the context of data modeled on an individual basis; (ii) the significant correlation between the stable states for voluntary activation and evoked torque modeled in each condition reinforces the idea that central inhibitions keep the peripheral fatigue under a critical threshold; (iii) the inverse correlation between the rate of voluntary activation and the decrease in evoked torque during high force contractions suggests that, in this condition, central inhibition was not triggered by peripheral perturbations at the start of exercise
Summary
Muscle fatigue can be defined as an exercise-induced reduction in maximal voluntary muscle force (Gandevia, 2001). It has been found that group Ia, Ib, and II spindle afferents are able to modulate their discharge rate during sustained or repeated contractions and reduced excitatory or increase inhibitory inputs to the motor neurons (Gandevia, 2001) Their role in muscle fatigue appears to be minimal (McNeil et al, 2011). Group III and IV afferents have clearly been shown to significantly influence the development of fatigue (Amann and Dempsey, 2016; Blain et al, 2016; Matsuura, 2016) These muscle afferents are free nerve endings activated by contraction-induced mechanical and chemical stimuli (Rowell and O’Leary, 1990; Kaufman et al, 2002). Group IV afferents are the most sensitive to metabolic by-products and respond quickly to perturbations in muscle homeostasis (Kaufman et al, 2002)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
