A parallel-cascade system identification method was used to measure dynamic ankle stiffness in normal and spastic spinal cord injured (SCI) subjects. Modulation of reflex stiffness gain of the ankle extensor muscles (GS) was studied by applying perturbations to the ankle at different positions under passive (relaxed) and active (10% extensor maximum voluntary contraction) conditions. In SCI subjects: (1) modulation of reflex gain with ankle position was abnormal in both passive and active conditions; (2) reflex stiffness gain was always significantly higher than in normal subjects; (3) reflex gain was similar under active and passive conditions except at plantarflexion where reflex gain was slightly higher under active conditions; (4) the reflex latency was shorter. The findings demonstrate that the reflex mechanics are abnormal in SCI spastic patients. The abnormalities are likely due not only to a decrease in presynaptic inhibitory mechanisms but also to a disruption in the motoneuron recruitment order.
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