Influence of the bed rest for 20 days on postural modulation of soleus H-reflex and presynaptic inhibition

The purpose of present study was to examine the effect of the bed rest (BR) on the postural modulation of soleus H-reflex. Six health male subjects, age 19-32 years, underwent continuous bed rest in 6 degree head down tilt position for 20 days. The H-reflex in soleus was evoked by stimulating the tibial nerve in the popliteal fossa. The H-reflex activity was expressed as a percentage of Hmax to Mmax. The presynaptic inhibition was calculated as the change rate in the level of monosynaptic Ia facilitation on the soleus elicited by a stimulation of heteronymous Ia volley on the quadriceps(3)). During the measurement, the subjects, were fixed on a tilting bed for leaning body position from supine to standing. The rate of change in HM ratio with the postural change from supine to standing after the BR was greater than that before the BR. The presynaptic inhibition in standing after the BR was significantly decreased compared with before the BR. It was concluded that the postural modulation of stretch reflex was disturbed by the inactivity condition during the BR for 20 days.