Soleus H-reflex inhibition during gait initiation in Parkinson's disease

The soleus H-reflex excitability during gait initiation was investigated in Parkinson's disease. Eleven patients participated in this study. Patients stepped forward as soon as the start signal flashed. Soleus H-reflex was evoked from the trailing leg 100, 300, or 600 msec after the start signal. The electromyographic activity in the soleus muscle immediately before evoking the H-reflex and the ankle joint motion were recorded. The soleus H-reflex was inhibited 300 msec after the start signal. The amount of the soleus H-reflex inhibition was inversely correlated with the Hoehn and Yahr stage; Items 14, 29, and 31 of the Unified Parkinson's Disease Rating Scale; and the delay of the onset of the ankle dorsiflexion from the start signal. In contrast, the amount of electromyographic activity immediately before evoking the H-reflex was not significantly correlated with those measures but was significantly correlated with Item 22 of the Unified Parkinson's Disease Rating Scale. Those findings indicate that the amount of soleus H-reflex inhibition during gait initiation depends on the severity of the disease. Abnormality of descending command may be related to the severity-dependent H-reflex inhibition.