DISCHARGE PROPERTIES OF SINGLE MOTOR UNITS IN PATIENTS WITH SPINAL-CORD INJURIES

To study neurophysiological correlates of spastic paresis, we analyzed the discharge pattern of single motor units (SMUs) during sustained v oluntary contraction from muscles weakened by spinal cord injury (SCI) and from muscles of near normal strength just at or above the level o f injury. The average firing rate of SMUs was reduced in patients' bic eps brachii and tibialis anterior muscles compared with controls, but not in the triceps brachii. Floating serial correlation coefficients o btained from successive interdischarge intervals were significantly mo re positive in patients than in controls in all three muscles. One sta tistical measure of regularity of discharge, akin to a coefficient of variation, was best able to differentiate patient and control SMUs. In creased discharge variability in muscles just above the level of injur y suggested that subtle effects of traumatic SCIs were more extended t han was clinically apparent. Although consistent statistical differenc es could be measured, these changes were not specific to SCI, nor were all SMUs equally affected.