Stretch reflex system model for spasticity evaluation

Abstract

The quantitative and objective evaluation of spasticity has been desired in rehabilitation and orthopaedics where subjective evaluations such as modified Ashworth scale (MAS) were mostly used. The objective of this work is to develop the knee joint model for representing various pendulum trajectories and for quantifying the spasticity. Knee joint model included the extension and flexion muscles. The joint moment consisted of both the active moment from the stretch reflex and the passive moment from the viscoelastic joint properties. The stretch reflex was modeled as nonlinear feedback of muscle length and the muscle lengthening velocity, which is physiologically-feasible. Moreover, we modeled the spastic reflex as having dynamic threshold to account for the various pendulum trajectories of spastic patients. We determined the model parameters of three patients through minimization of error between experimental and simulated trajectories. The simulated joint trajectories closely matched with the experimental ones, which show the proposed model can predict pendulum motions of patients with different spastic severities. The predicted muscle force from spastic reflex appeared more frequently in the severe spastic patient, which indicates the dynamic threshold relaxes slowly in this patient as is manifested by the variation coefficient of dynamic threshold. The proposed method provides prediction of muscle force and intuitive and objective measure of spasticity and it is expected to be useful in quantitative assessment of spasticity.