Soleus H-reflex graded depression by contralateral hip afferent feedback in humans

The aim of this study was to establish the effects of contralateral hip oscillations on the ipsilateral soleus H-reflex. A servomotor system was utilized to impose bilateral hip oscillations at 0.5 Hz, while braces stabilized the joint position of both legs. The soleus H-reflex was evoked every 4 s during bilateral synchronous and asynchronous hip movements and during unilateral hip oscillations with the contralateral leg hyperextended or flexed. In all experimental conditions, a stimulus was delivered to the posterior tibial nerve while the ipsilateral leg was moving either into extension or flexion. Torques at the hip and ankle joints and activity from six muscles were recorded from both legs. The ipsilateral soleus H-reflex was profoundly depressed in all conditions. The reflex depression was less during asynchronous than synchronous hip oscillations. Further, the soleus H-reflex depression did not vary with the contralateral hip flexed or extended, but it was larger when the ipsilateral hip was moving in flexion. These findings demonstrate that sensory afferent feedback from the contralateral hip adjusts the amount of inhibition acting on the ipsilateral soleus H-reflex, supporting cross-leg reflex and heteronymous muscle afferents interactions in humans.