Department of Kinesiology, University of Maryland, College Park, Maryland 20742, USA.
J Neurosci. 2011 Oct 19;31(42):15144-53. doi: 10.1523/JNEUROSCI.1013-11.2011.
A fundamental issue in motor control is how to determine the task goals for a given behavior. Here, we address this question by separately identifying the musculoskeletal and feedback components of the human postural control loop. Eighteen subjects were perturbed by two mechanical perturbations (gentle pulling from behind at waist and shoulder levels) and one sensory perturbation (movement of a virtual visual scene). Body kinematics was described by the leg and trunk segment angles in the sagittal plane. Muscle activations were described by ankle and hip EMG signals, with each EMG signal computed as a weighted sum of rectified EMG signals from multiple muscles at the given joint. The mechanical perturbations were used to identify feedback, defined as the mapping from the two segment angles to the two EMG signals. The sensory perturbation was used to estimate parameters in a mechanistic model of the plant, defined as the mapping from the two EMG signals to the two segment angles. Using the plant model and optimal control theory, we compared identified feedback to optimal feedback for a range of cost functions. Identified feedback was similar to feedback that stabilizes upright stance with near-minimum muscle activation, but was not consistent with feedback that substantially increases muscle activation to reduce movements of the body's center of mass or center of pressure. The results suggest that the common assumption of reducing sway may not apply to musculoskeletal systems that are inherently unstable.
运动控制中的一个基本问题是如何为给定的行为确定任务目标。在这里，我们通过分别识别人体姿势控制回路的肌肉骨骼和反馈组件来解决这个问题。 18 名受试者受到两种机械扰动（腰部和肩部水平的轻柔拉动）和一种感觉扰动（虚拟视觉场景的移动）的干扰。身体运动学由矢状面的腿和躯干节段角度描述。肌肉激活由脚踝和臀部 EMG 信号描述，每个 EMG 信号由给定关节处多个肌肉的整流 EMG 信号的加权和计算。机械扰动用于识别反馈，定义为两个节段角度到两个 EMG 信号的映射。感觉扰动用于估计植物的机械模型中的参数，定义为两个 EMG 信号到两个节段角度的映射。使用植物模型和最优控制理论，我们比较了识别的反馈与一系列成本函数的最优反馈。识别的反馈类似于用最小肌肉激活稳定直立姿势的反馈，但与显著增加肌肉激活以减少身体质心或压力中心运动的反馈不一致。结果表明，减少摇摆的常见假设可能不适用于固有不稳定的肌肉骨骼系统。
