Reflex behavior during walking in incomplete spinal-cord-injured subjects

Reflexes evoked by stimulation of the posterior tibial nerve at the ankle during walking were investigated in 10 subjects with incomplete injury of the spinal cord. Low-intensity stimuli (1.5 to 1.7 times the motor threshold) were delivered during treadmill walking. Reflexes were recorded by surface electromyography from the tibialis anterior (TA) and soleus (SOL) muscles. Responses were evoked at early, medium, and late latencies in all subjects. The reflex activity at a medium latency (50 to 80 ms) was cyclically modulated in all of the subjects during walking in both muscles, but the pattern of modulation was different from that seen in normal subjects (Yang and Stein, J. Neurophysiol., 63, 1109-1117). Most spinal-cord-injured subjects did not show inhibitory responses in the TA; excitatory responses were seen in the swing phase and occasionally in the stance phase. Inhibitory responses were typically evoked in the SOL muscle during the stance phase, but abnormal excitatory responses were also seen during the swing phase in the majority of subjects. Excitatory responses were seen in some subjects during the stance phase as well. Functional walking ability and clinical measures of muscle tone were not correlated with the degree of reflex modulation in walking. The lack of correlation between these measures could be related to many factors including the small number of subjects, the subjectivity of clinical measures, and the different movement conditions under which these measures were taken. The results demonstrated that most spinal-cord-injured subjects retained the ability to modulate this reflex during walking, but the pattern of modulation was abnormal. Moreover, excitatory responses were more frequently evoked than inhibitory responses.