Reduced reciprocal inhibition during assisted stepping in human spinal cord injury

The aim of this study was to establish the modulation pattern of the reciprocal inhibition exerted from tibialis anterior (TA) group I afferents onto soleus motoneurons during body weight support (BWS) assisted stepping in people with spinal cord injury (SCI). During assisted stepping, the soleus H-reflex was conditioned by percutaneous stimulation of the ipsilateral common peroneal nerve at one fold TA M-wave motor threshold with a single pulse delivered at a short conditioning-test interval. To counteract movement of recording and stimulating electrodes, a supramaximal stimulus at 80-100 ms after the test H-reflex was delivered. Stimuli were randomly dispersed across the step cycle which was divided into 16 equal bins. The conditioned soleus H-reflex was significantly facilitated throughout the stance phase, while during swing no significant changes on the conditioned H-reflex were observed when compared to the unconditioned soleus H-reflex recorded during stepping. Spontaneous clonic activity in triceps surae muscle occurred in multiple phases of the step cycle at a mean frequency of 7 Hz for steps with and without stimulation. This suggests that electrical excitation of TA and soleus group Ia afferents did not contribute to manifestation of ankle clonus. Absent reciprocal inhibition is likely responsible for lack of soleus H-reflex depression in swing phase observed in these patients. The pronounced reduced reciprocal inhibition in stance phase may contribute to impaired levels of co-contraction of antagonistic ankle muscles. Based on these findings, we suggest that rehabilitation should selectively target to transform reciprocal facilitation to inhibition through computer controlled reflex conditioning protocols.