Posture-related changes of soleus H-reflex excitability

We investigated whether the modulatory effects of segmental and descending inputs on the soleus H reflex are modified by postural conditions. Fourteen healthy volunteers received a transcranial magnetic stimulus (TMS) or percutaneous electrical stimulation of the posterior tibial nerve (PTN), preceding by 0 to 400 ms the elicitation of the soleus H reflex in supine, sitting, and standing positions. In all positions, TMS induced an early period of facilitation at interstimulus intervals (ISIs) ranging between 5 and 35 ms. In supine and sitting positions, there was a second period of facilitation at ISIs between 60 and 90 ms, which was absent or significantly reduced in the standing position. PTN induced a strong inhibition of the H reflex in all positions up to 125 ms. In supine and sitting positions, inhibition continued up to 400 ms, whereas it was significantly reduced or completely absent beyond 125 ms in the standing position. These results demonstrate posture-related differences in the modulatory effects of descending and segmental inputs on the excitability of the H-reflex circuit.