Résumé
Purpose: The purpose of the present study was to examine the time course of neuromuscular fatigue during a low-frequency electrostimulation (ES) session. Methods: Three 17-train stimulation bouts at 30 Hz (duty cycle 40%) were used to electrically induce fatigue in the plantar flexor muscles. Before and after every 17-train bout, torque, electromyographic activity [expressed by Root Mean Square (RMS) and Median Frequency (MF) values] and evoked potentials (M-wave and H-reflex) were recorded, while the twitch interpolation technique was used to assess the level of voluntary activation (LOA). Results: Torque during maximal voluntary contraction decreased significantly from the very first stimulation bout [-6.6 (4.84)%, P<0.001] and throughout the session [-10.32 (7.31)% and -11.53 (5.53)%, P<0.001, for the second and third bouts, respectively]. The LOA and RMS/M values were significantly decreased during the ES session [-2.9 (4.81)%, P<0.05, and -17.5 (26.05)%, P<0.001, respectively, at the end of the protocol], while MF showed no changes. The Hmax/Mmax ratio and M-wave characteristics were not significantly modified during the session. All twitch parameters were significantly potentiated after the first bout and this potentiation was present until the end of the protocol (P<0.001). Conclusion: The maximal torque decrease, evident from the early phase of a low-frequency ES session, appeared without a concomitant inhibition of motoneuron excitability or depression of muscle contractile properties. This is consistent with an early failure of the central drive to the muscle. A possible explanation could be the increased activation of type III and IV afferent fibers, leading to an impairment of the descending command.