Department of Biomedical Engineering, McGill University, Montreal, Canada.
Department of Biomedical Engineering, McGill University, Montreal, Canada.
J Biomech. 2019 Mar 6;85:50-58. doi: 10.1016/j.jbiomech.2019.01.009. Epub 2019 Jan 11.
In standing, the human body is inherently unstable and its stabilization requires constant regulation of ankle torque, generated by a combination of ankle intrinsic properties, peripheral reflexes, and central contributions. Ankle intrinsic stiffness, which quantifies the joint intrinsic properties, has been usually assumed constant in standing; however, there is strong evidence that it is highly dependent on the joint torque, which changes significantly with sway in stance. In this study, we examined how ankle intrinsic stiffness changes with postural sway during standing. Ten subjects stood on a standing apparatus, while subjected to pulse perturbations of ankle position. The mean torque of a short period before the start of each pulse was used as a measure of background torque. Responses with similar background torques were grouped together and used to estimate the parameters of an intrinsic stiffness model. Stiffness estimates were normalized to the critical stiffness and the background torque was transformed to the center of pressure location. We found that in most subjects, the normalized stiffness increased linearly with the movement of center of pressure towards the toes, with an average slope of 2.11 ± 0.80 1/m·rad. This modulation of ankle intrinsic stiffness seems functionally appropriate, since the intrinsic stiffness increases quickly, as the center of pressure moves toward the toes and the limits of stability. These large changes of ankle intrinsic stiffness with postural sway must be incorporated in any model of stance control.
在站立时，人体本质上是不稳定的，需要不断调节踝关节力矩来稳定身体，这一过程依赖于踝关节的固有属性、外周反射和中枢贡献的综合作用。踝关节固有刚度是衡量关节固有属性的指标，通常被认为在站立时保持恒定；然而，有强有力的证据表明，它高度依赖于关节力矩，而关节力矩在站立时的摆动中会发生显著变化。在这项研究中，我们研究了在站立时，踝关节固有刚度如何随姿势摆动而变化。十名受试者站在站立装置上，同时接受踝关节位置脉冲扰动。每个脉冲开始前的短时间段内的平均扭矩被用作背景扭矩的度量。具有相似背景扭矩的响应被分组在一起，用于估计固有刚度模型的参数。刚度估计值被归一化为临界刚度，背景扭矩被转换为压力中心的位置。我们发现，在大多数受试者中，归一化刚度随压力中心向脚趾移动呈线性增加，平均斜率为 2.11±0.80 1/m·rad。这种踝关节固有刚度的调制似乎具有功能适应性，因为当压力中心向脚趾和稳定极限移动时，固有刚度会迅速增加。随着姿势摆动，踝关节固有刚度的这些大幅变化必须包含在任何站立控制模型中。
