A parallel-cascade system identification method was used to identify the modulation of reflex contributions to dynamic ankle stiffness with position stimuli amplitude and frequency in both normal and spastic spinal cord injured (SCI) subjects. As amplitude increased, reflex stiffness first increased and then decreased in both normal and SCI subjects. Reflex gain was significantly higher in patients than in normal subjects at all conditions. The reflex threshold, the minimum amplitude required to elicit a stretch reflex response, was smaller in SCI than in normal subjects. As frequency increased, reflex stiffness decreased in both normal and SCI subjects; however, it was significantly higher in SCI than in normal subjects at all frequencies. These results demonstrate that the reflex mechanics are abnormal in SCI spastic patients due to both an increase in reflex gain and a decrease in reflex threshold.
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