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 perc utaneous electrical stimulation of the posterior tibial nerve (PTN), preced ing by 0 to 400 ms the elicitation of the soleus H reflex in supine, sittin g, 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 facilitatio n at ISIs between 60 and 90 ms, which was absent or significantly reduced i n 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 co ntinued up to 400 ms, whereas it was significantly reduced or completely ab sent beyond 125 ms in the standing position. These results demonstrate post ure-related differences in the modulatory effects of descending and segment al inputs on the excitability of the H-reflex circuit. (C) 2000 John Wiley & Sons, Inc.