Wrist Vibration Increases Presynaptic Inhibition of the Soleus Muscle During Step Initiation in People With Freezing of Gait and Parkinson's Disease: A Pilot Study

Presynaptic inhibition (PSI) at the spinal cord level is crucial for coordinating postural preparation with step initiation. People with freezing of gait and Parkinson's disease (PD + FOG) show loss of PSI of the soleus muscle during step initiation that is associated with abnormal anticipatory postural adjustments (APA). Here, we hypothesize that increasing PSI of the soleus muscle during step initiation via wrist vibration in PD + FOG would decrease abnormally large APA. Fifteen PD + FOG performed self-initiated steps on a force platform without electrical stimulation and with test or conditioned Hoffman reflexes (H-reflex) to measure PSI of the soleus muscle under three conditions: OFF medication, OFF medication with vibration, and ON medication without vibration. Soleus H-reflexes were recorded during quiet stance (a control task) and when the amplitude of the APA under the same leg exceeded 10%-20% of the mean baseline mediolateral displacement. Vibration consisted of 200-300 Hz applied to the wrist when the ipsilateral leg during APA (same leg where H-reflexes were evoked) was on the ground. PD + FOG showed decreased PSI during APA in OFF and ON medication, but PSI was increased during vibration (p < 0.05). Increased PSI was associated with smaller APA during vibration (p < 0.05). Smaller APAs were associated with lower subjective freezing of gait severity (p < 0.05). These preliminary results show that wrist vibration decreases abnormal APA during step initiation by increasing ipsilateral PSI levels of the soleus muscle. Because PSI is modulated by cortical and brainstem areas related to FOG and APA, proprioceptive drive during vibration may reorganize these brain circuits.