Posture-dependent modulation of reciprocal inhibition upon initiation of ankle dorsiflexion in man

The present study tested whether soleus H-reflex depression recorded from normal subjects during isotonic ankle dorsiflexion is due to reciprocal inhibition and this might be modulated by varying postures. The soleus H-reflex amplitude of eight healthy adults was investigated at various times prior to and during tibialis anterior discharge while subjects were seated and when standing. Results showed that the amount of soleus H-reflex depression was significantly larger in the standing than that in the sitting posture in spite of the same dorsiflexion movement. Furthermore, the depression upon initiation of dorsiflexion movement appeared earlier in the standing than in the sitting. The results suggest that increasing amounts of reciprocal inhibition are correlated with and dependent upon the preceding postural conditions for voluntary movement, i.e., modulation of reciprocal inhibition seems to be dictated by the difference in functional demand between sitting and standing posture.