In addition to its intrinsic importance during quiet standing, posture also serves asthe background for a wide variety of other critical motor tasks. The hierarchical nature ofthe motor control system suggests that the different layers may be responsible fordifferent aspects of posture. I tested the hypothesis that spinal reflexes are organizedaccording to optimal principles of stability, control accuracy, and energy. I found thatthere were no globally stable muscle activation patterns for muscles operating nearoptimal fiber length, suggesting that the intrinsic viscoelastic properties of muscle areinsufficient to provide limb stability. However, for stiffer muscles a stable limb could becreated by selectively activating muscles based on their moment-arm joint anglerelationships. The optimal organization of length and velocity feedback to control andstabilize the endpoint position of a limb could not be produced from a purely musclecontroller, but required neural feedback to improve endpoint performance, reduceenergetic cost, and produce greater coordination among joints. I found that whilemuscles at near optimal fiber length were insufficient to provide limb stability, the lengthfeedback provided by the autogenic stretch reflex was sufficient to stabilize. Lengthfeedback was also sufficient to produce the directional tuning of muscle activity andconstrained ground reaction forces as is observed in experiments. These results haveimplications for controlling powered prosthetic devices, suggesting that subdividing theresponsibility for stability among hierarchical control structures will simultaneousimprove stability and maneuverability of the devices.
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The role of heterogenic spinal reflexes in coordinating and stabilizing a model feline hindlimb
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