Neuromuscular changes after long-lasting mechanically and electrically elicited fatigue

Central fatigue was investigated under an isolated active condition whereby the possible effects of supraspinal fatigue were minimized. Therefore, ten subjects were fatigued by simultaneously and repeatedly mechanically stret ching and electrically stimulating their calf muscles for 1 h. This was per formed using an ankle ergometer. The active fatigue task included a total o f 2400 muscle stretches with an intensity of 10% of the maximal voluntary c ontraction (MVC). This protocol clearly impaired neuromuscular function, as revealed by a significant reduction in MVC (P < 0.01) and the neural input to the muscle (average EMG) (P<0.01-0.001). The interpolated nerve stimula tion compensated for this force loss by 4.28% (P < 0.05). Stretch-reflex re cordings revealed a notable post-fatigue reduction in the peak-to-peak ampl itude (59.1%, P < 0.01) and stretch-resisting force of the muscle (14.1%, P < 0.01). The maximal H-reflex declined by 50.5% (P < 0.001) and did not re cover while the leg was kept ischemic. It is suggested that the existing pr otocol with minor metabolic loading can,induce central fatigue, which seems to be of reflex origin from the fatigued muscle. Although the role of pres ynaptic inhibition of la terminals is possibly reinforced, disfacilitation via reduced spindle sensitivity cannot be excluded.