H-reflex and motor evoked potential (MEP) induced by transcranial magnetic stimulation (TMS) in soleus muscle of five normal subjects were investigated before and after 20 days 6°head-down bed rest (BR). Soleus H- reflex during standing following bed rest decreased in all five subjects. The size of the maximum H-reflex (Hmax), expressed as a percentage of the maximum M-response (Mmax), decreased from 63.3 ± 23.3% before BR to 26.7 ± 12.5% after BR. However, there were no significant differences between MEPs before and after bed rest. Moreover, MEP/Hmax after bed rest was larger than before bed rest in all subjects. These results indicate a strong inhibition of H- reflex and no adaptation of MEP in soleus muscle during standing following bed rest.
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