Neuromuscular model for the stretch reflex in passive movement of spastic elbow joint

The objective of this study was to develop a neuromuscular model incorporating the stretch reflex mechanism for the spastic elbow during the pendulum test. A normal subject and three subjects with spasticity participated in this study. The elbow joint trajectories and surface EMG signals of three elbow actuators were simultaneously recorded. A Hill-based musculotendon model was developed to enable prediction of the elbow trajectory during pendulum test. A stretch reflex loop model was incorporated. Three stretch reflex parameters, i.e., the static and dynamic thresholds and the reflex gain were adjusted to fit the recorded elbow trajectory. Sensitive tests showed that a decrease in static threshold would reduce the swing amplitude. A decrease in dynamic threshold would lead to a decrease in swing amplitude, phase shift and an increase in swing frequency. An increase in reflex gain would also reduce swing amplitude. Results indicated that by simultaneously varying the stretch reflex thresholds and gain, the model succeeded in predicting the elbow trajectory during pendulum test. Furthermore, the stretch induced muscle activation predicted by the model coincided with the time course of EMG activation. Compared with normal subjects, the spastic subjects showed decreased stretch reflex thresholds and increased reflex gain