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Spastic hypertonia has reflex/non-reflex and dynamic/static components, which were investigated in ankles of 24 spastic hemiparetic and 32 normal subjects. Furthermore, the spastic ankles were treated with an intelligent stretching device with outcome evaluated in terms of phasic and tonic stretch-reflex gains, reflex threshold, joint elastic stiffness, and viscous damping. We found that joint elastic stiffness was increased in spastic ankles, especially in the plantar flexors. The increase was due to an increase in passive stiffness instead of intrinsic stiffness. Viscous damping was increased in spastic plantar flexors. Reflexively, spastic ankles showed higher dynamic reflex gain in spastic plantar flexors, indicating exaggerated phasic stretch reflex. The static stretch-reflex gain was increased in spastic ankles, indicating hyperactive tonic stretch reflex, and spastic muscles did not regulate tonic stretch reflex as controls did. Reflex hyperexcitability in spasticity was associated with both increased tendon reflex gain and decreased threshold. After stretching, the patients were able to generate higher plantar flexion MVC torque, which was corroborated by increased torques induced by matched electrical stimulation of the spastic muscles. They were also able to move in large ROM. Achilles tendon properties evaluated by ultrasonography corroborated the strength increase.