Task-dependence of ankle stretch reflex

An investigation was undertaken to determine whether ankle stretch reflexes vary with the task performed by a subject. We measured reflex gain in three subjects while they performed position-matching and torque-matching tasks. In the position-matching task, subjects were required to hold their foot at a desired position while a hydraulic actuator, configured to act like a compliant spring, applied random torque perturbations to their ankle. In the torque-matching task, subjects were required to maintain a desired torque while their foot position was perturbed randomly by the actuator which behaved as a very stiff position control servo. Care was taken to ensure that the movements resulting from the perturbations were similar in both tasks. A nonlinear identification procedure was used to identify the reflex and intrinsic stiffness. Reflex stiffness was greater in the position-matching task than in the torque-matching task in all three subjects. In contrast, intrinsic stiffness was task-independent. Thus, subjects appeared to modulate their reflexes in a way appropriate to the task, supporting the hypothesis that the nervous system has the ability to modulate the stretch reflex characteristics to meet the needs of the task.