Function of group IB inhibition during assisted stepping in human spinal cord injury

The aim of this study was to investigate the function of group Ib inhibition exerted between ankle synergists during assisted stepping in people with chronic motor incomplete spinal cord injury (SCI). During stepping, the soleus (SOL) H-reflex was conditioned by percutaneous stimulation of the medialis gastrocnemius (MG) nerve by a bipolar electrode placed distal and medial to the cathode electrode for the tibial nerve. Stimulation was delivered at 0.95 times MG M-wave motor threshold with a pulse train at short conditioning-test intervals. To counteract movement of recording and stimulating electrodes, a supramaximal stimulus at 80 to 100 milliseconds after the test H-reflex was delivered. Stimuli were randomly dispersed across the step cycle, which was divided into 16 equal bins. The conditioned SOL H-reflex was significantly facilitated throughout the stance phase, while it was depressed during the swing phase when compared with the unconditioned SOL H-reflex. Ankle clonus during the stance phase may have affected the excitability state of spinal motoneuronal and interneuronal circuits, but the presence of group Ib inhibition in the swing phase suggests that in the absence of loading this spinal circuit is modulated in a physiologic manner in human incomplete SCI. Repetitive body loading and unloading promotes functional integration of sensory signals mediating load in human SCI.