Effect of posture and muscle stretch on spasticity: a dynamic model study

A neuromuscular dynamic model has been developed to simulate the pendulum test of spasticity. The model contains detailed nonlinear dynamics of muscle force generation and stretch reflexes. Kinematic and electromyogram data were recorded from patients during the pendulum test to evaluate the model simulation results. The test was conducted with patients taking either sitting erect or supine position. The model simulation indicates that the stretch reflex thresholds and the gain have different influences on the leg swing in the pendulum test of spasticity. When the simultaneous changes in all three parameters of the stretch reflex loop occurred, a small variation of the gain coupled with changes in both static and dynamic thresholds could produce increasing severity of spasticity. The model successfully simulated the effect of posture changes on spasticity. The effect involved both descending control and longer muscle stretch in quadriceps.