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 stretching and electrically stimulating their calf muscles for 1 h. This was performed using an ankle ergometer. The active fatigue task included a total of 2400 muscle stretches with an intensity of 10% of the maximal voluntary contraction (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 stimulation compensated for this force loss by 4.28% (P < 0.05). Stretch-reflex recordings revealed a notable post-fatigue reduction in the peak-to-peak amplitude (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 recover while the leg was kept ischemic. It is suggested that the existing protocol with minor metabolic loading can induce central fatigue, which seems to be of reflex origin from the fatigued muscle. Although the role of presynaptic inhibition of Ia terminals is possibly reinforced, disfacilitation via reduced spindle sensitivity cannot be excluded.