Effects of changes in hip position on actions of spinal inhibitory interneurons in humans

The aim of the present study was to establish whether in healthy human subjects the actions of group I muscle afferents arising from the same spinal segments as the soleus innervation (e.g., common peroneal nerve; CPN) or from more proximal spinal segments (femoral nerve; FN) on the soleus H-reflex are modified by changes in hip position. Control and conditioned soleus H-reflexes were elicited and recorded via conventional methods. In seated subjects, CPN and FN stimulation resulted in similar effects to the soleus H-reflex to that previously reported in healthy subjects. However, during hip angle changes, CPN stimulation at the C-T interval of 2 ms resulted in soleus H-reflex depression only when the hip was flexed at 30 degrees , whereas with the hip flexed or extended at 10 degrees the H-reflex was facilitated. CPN stimulation delivered at 100 ms also induced soleus H-reflex facilitation regardless of the hip angle tested. The heteronymous reflex facilitation (conditioned H-reflex with FN stimulation) did not vary systematically with hip angle changes. These findings indicate that hip proprioceptors interact with spinal inhibitory interneurons to enhance spinal reflex excitability under static conditions. This neural switch might constitute an important feature of movement regulation in humans.