A model of the motor servo: incorporating nonlinear spindle receptor and muscle mechanical properties

A model for the stretch reflex is proposed incorporating a nonlinear description of muscle receptor behavior, a delay in the reflex loop and a model of muscle mechanical properties. The model adequately describes the nonlinear response properties of EMG and force to constant ramps in loading and unloading direction. The EMG responses during the ramp and at ramp plateau could be simulated adequately for all ramp velocities except for high stretch velocities where EMG activity appeared in bursts, presumably due to spinal nonlinearities. Force responses during ramp stretches could be simulated except at ramp plateau, where the measured force response decayed slower than the simulated responses. The model also explained that EMG and force responses during ramp stretches after a displacement of about 1 cm could be approximately described by a product relationship between a position-related term and a low-fractional power of velocity. During unloading ramps the model did not predict a clear velocity dependence in agreement with the data.