Stretch reflex latency & stiffness of the spastic ankle

A parallel-cascade system identification method was used to measure dynamic ankle stiffness quantitatively and objectively in normal and spastic spinal cord injured (SCI) subjects. Modulation of reflex stiffness gain of ankle extensor muscles (GS) was studied by applying perturbations to ankle at different positions and with different levels of voluntary contraction. In spastic subjects: (I) reflex stiffness gain was always significantly higher than in normal subjects; (2) modulation of reflex gain with both ankle position and voluntary activation was abnormal; (3) the reflex latency was shorter. The finding demonstrate that the reflex mechanics are abnormal in SCI spastic patients. The abnormalities can be due not only to a decrease in presynaptic inhibitory mechanisms but also to a disruption in the motoneuron recruitment order.