Does central fatigue exist under low-frequency stimulation of a low fatigue-resistant muscle?

The aim of the present study was to determine whether central fatigue occurs when fatigue is electrically induced in the abductor pollicis brevis muscle. Three series of 17 trains (30 Hz, 450 μs, 4 s on/6 s off, at the maximal tolerated intensity) were used to fatigue the muscle. Neuromuscular tests consisting of electrically evoked and voluntary contractions were performed before and after every 17-train series. Both the force induced by the stimulation trains and maximal voluntary force generation capacity significantly decreased throughout the protocol (-27 and -20%, respectively, at the end of the protocol, P < 0.001). These decreases were accompanied by failure in muscle excitability (P < 0.01), as assessed by the muscle compound action potential (M-wave or Mmax), leading to significant impairment in the muscle contractile properties (P < 0.05), as assessed by the muscle mechanical response (Pt). Central fatigue indices (level of activation, RMS/Mmax and H reflex) were not significantly changed at any point in the protocol. This gives evidence of preserved motor command reaching the motor neurons and preserved spinal excitability. The results indicate that this low-frequency stimulation protocol entails purely peripheral fatigue development when applied to a low fatigue-resistant muscle.