Modulation of the soleus H-reflex in prone human subjects using galvanic vestibular stimulation

Objectives: To determine if vestibular-evoked modulation of the soleus H-reflex can be achieved in a muscle that is not being used for postural support. Methods: Ten healthy subjects lay prone while the right leg was supported. In this position soleus H-reflex amplitudes were measured with the head facing forward, coupled with ipsilateral monopolar monaural galvanic stimulation (anode or cathode). To evaluate the interval between the onset of the galvanic stimulus and tibial nerve stimulation, the timing was varied between 0 and 200 ms in 20 ms intervals. A two-way ANOVA and student’s t-test was performed to compare the mean amplitudes of the test and conditioned H-reflexes. Results: Galvanic stimulation significantly modified the amplitude of the H-reflex in a prone lying subject (P<0.05). Furthermore, the peak inhibitory and facilitatory effect occurred when the galvanic vestibular stimulus was delivered 100 ms prior to the H-reflex stimulus. Conclusions: The results of this study demonstrate that galvanic stimulation can modulate the excitability of the soleus motoneuron pool when the muscle is not being used posturally. This suggests that in certain situations, it may be possible to use this type of vestibular stimulation to examine the integrity of descending vestibulospinal pathways in prone human subjects.