Increased reflex contributions to dynamic ankle stiffness in spastic spinal cord injured patients

A parallel-cascade system identification method was used to identify intrinsic and reflex contributions to dynamic ankle stiffness at different positions from plantarflexion in both normal subjects and spastic spinal cord injured (SCI) persons with an incomplete motor function loss. Intrinsic stiffness was well modeled by a linear second-order system between ankle position and torque. Reflex stiffness dynamics were accurately described by a linear third-order system between half-rectified velocity and reflex torque. Intrinsic stiffness increased with ankle dorsiflexion in both normal and SCI subjects and no significant change was found between the two groups. Reflex stiffness also increased in both normal and SCI subjects with ankle dorsiflexion, however, it was significantly higher in patients than in normal subjects at all positions