Regulation of multijoint stretch reflexes during interactions with stiff and compliant environments

The purpose of this study was to examine how multijoint stretch reflexes are modulated during interactions with stiff and compliant environments. Reflex responses were elicited using a 3D robotic manipulator to perturb arm posture in the three degrees of freedom relevant to functional behaviors. The robot applied controlled displacements and simulated environments with different stiffnesses. Stiff (10 kN/m) and compliant (10 N/m) environments were used. Perturbation characteristics were matched across environments. Reflex responses were monitored using surface electromyograms from 8 upper limb muscles. Data were collected from 9 subjects. Significant stretch reflex responses were observed in all muscles and these were significantly modulated by the environmental stiffness. This modulation was due to increased levels of background muscle activity in the compliant environment and to increased reflex sensitivity at matched levels of muscle activation. These results suggest that the neuromotor control system uses both feedforward and feedback mechanisms to compensate for changes in environmental compliance.