A dynamic neuromuscular model for describing the pendulum test of spasticity

Both dynamic and static thresholds, as well as the gain in the stretch reflex loop, affect the sensitivity of motoneurons to muscle stretch. How the variation in each parameter will influence the mechanical behavior of patients with spasticity is not well understood because of the difficulty in experimentally isolating individual parameters. A neuromuscular dynamic model, based on the pendulum test of spasticity, has been developed to study the specific contribution of individual parameter abnormalities in stretch reflex loops to the observed mechanical abnormalities. The model contains detailed nonlinear dynamics of muscle force generation and stretch reflexes. A computer simulation of the model indicates that the stretch reflex thresholds and the gain have different influences on the leg swing in the pendulum test of spasticity. Individual changes in the static stretch reflex threshold, in the dynamic threshold, or in the gain can not stimulate the whole spectrum of spasticity severity. When simultaneous changes in all three parameters of the stretch reflex loop occur, a small variation of the gain coupled with changes in both static and dynamic thresholds can produce increasing severity of spasticity as the thresholds further decrease. The model is also successful in simulating the effect of posture changes on spasticity.