Modulation of reflex contributions to ankle stiffness with perturbation velocity in normal and spastic spinal cord injured subjects

A parallel-cascade system identification method was used to identify modulation of reflex contributions to dynamic ankle stiffness with position stimuli frequency in both normal subjects and spastic spinal cord injured (SCI) persons with an incomplete motor function loss. Reflex stiffness dynamics were accurately described by a linear, third-order system between half-rectified velocity and reflex torque, Reflex stiffness decreased in both normal and SCI subjects as r.m.s velocity increased; however, it was significantly higher in patients than in normal subjects at all velocities. On the other hand, normalized reflex gains of patients were well marched to those of normal subjects, implying that abnormal reflex mechanics are not due only to presynaptic inhibitory mechanisms.