Enhanced Stretch Reflex Excitability of the Soleus Muscle in Persons With Incomplete Rather Than Complete Chronic Spinal Cord Injury

Abstract

Nakazawa K, Kawashima N, Akai M. Enhanced stretch reflex excitability of the soleus muscle in persons with incomplete rather than complete chronic spinal cord injury. Objective To compare excitabilities of spinal stretch reflex among clinically complete spinal cord injury (SCI), incomplete SCI, elderly healthy, and young healthy subjects. Design Case comparison. Setting Research laboratory. Participants Volunteer sample of 12 complete SCI, 10 incomplete SCI, 10 elderly, and 11 young subjects. Intervention Mechanically induced stretch reflex, H-reflex, and M response in electromyographic activity of the soleus muscle were recorded in all subjects. Main Outcome Measures Absolute peak-to-peak stretch reflex amplitude and maximum H-reflex (Hmax), and those values relative to the maximum M response (Mmax) amplitude (relative peak-to-peak stretch reflex amplitude) and H/M ratio. Results Both the absolute and relative peak-to-peak stretch reflex amplitudes showed the greatest values in incomplete SCI among the 4 groups. Although absolute and relative peak-to-peak stretch reflex amplitudes of the incomplete SCI group were greater than those of the complete SCI group, the H/M ratios of both groups were comparable, and were greater than those of the younger and elderly groups. Conclusions The results suggest that the greater absolute and relative peak-to-peak stretch reflex amplitudes of incomplete SCI were mostly due to the greater maximum motor potential (Mmax), while the elevated spinal motoneuronal excitability shown by the increased H/M ratio was maintained in the chronic stage after both complete and incomplete SCIs.