Abnormal passive and intrinsic stiffness in the spastic ankle

A parallel-cascade system identification method was used to measure dynamic ankle stiffness in normal and spastic spinal cord injured (SCI) subjects. Modulation of passive and intrinsic stiffness gain of 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. Both passive and intrinsic stiffness were described well by a linear second-order model having elastic, viscous and inertia parameters. For passive stiffness: the elastic and viscous parameter increased with ankle dorsiflexion in both groups; in additions, both parameters were larger in SCI than in control subjects as ankle dorsiflexed. For intrinsic stiffness: (I) the elastic parameter increased with ankle position until neutral position (90 degrees) in both groups, it then saturated in control subjects, but decreased as ankle moved to dorsiflexion in SCI subjects; (2) it was always smaller in SCI subjects; (3) the intrinsic viscous parameter increased at the beginning of plantarflexion then decreased slightly until maximum dorsiflexion in both groups. However, it was lower than in control subjects as the ankle moved from neutral position to maximum dorsiflexion. The results indicate that abnormal non-reflex mechanics are due to enhanced passive stiffness and reduced intrinsic stiffness.