A comparison of the effect of fatigue caused by electrical induction or voluntary contraction on Ia inhibition in human soleus muscle was investigated in fourteen healthy male subjects. The Hoffmann reflex (H reflex) was employed to elucidate the contribution of fatigue induced contraction to the Ia inhibition. A statistically significant difference (P < 0.05) in H reflex amplitude between the intermittent stimulation at 15 Hz and voluntary contraction was observed. Our study suggested that Ia inhibition in human soleus muscle, caused by electrically induced or voluntary contraction, induced differences in the number of impulses in the group Ia fibers, between the two conditions.
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