The role of heterogenic spinal reflexes in coordinating and stabilizing a model feline hindlimb

Abstract

In addition to its intrinsic importance during quiet standing, posture also serves as the background for a wide variety of other critical motor tasks. The hierarchical nature of the motor control system suggests that the different layers may be responsible for different aspects of posture. I tested the hypothesis that spinal reflexes are organized according to optimal principles of stability, control accuracy, and energy. I found that there were no globally stable muscle activation patterns for muscles operating near optimal fiber length, suggesting that the intrinsic viscoelastic properties of muscle are insufficient to provide limb stability. However, for stiffer muscles a stable limb could be created by selectively activating muscles based on their moment-arm joint angle relationships. The optimal organization of length and velocity feedback to control and stabilize the endpoint position of a limb could not be produced from a purely muscle controller, but required neural feedback to improve endpoint performance, reduce energetic cost, and produce greater coordination among joints. I found that while muscles at near optimal fiber length were insufficient to provide limb stability, the length feedback provided by the autogenic stretch reflex was sufficient to stabilize. Length feedback was also sufficient to produce the directional tuning of muscle activity and constrained ground reaction forces as is observed in experiments. These results have implications for controlling powered prosthetic devices, suggesting that subdividing the responsibility for stability among hierarchical control structures will simultaneous improve stability and maneuverability of the devices.