Vlutters M, Boonstra T A, Schouten A C, van der Kooij H
Department of Biomechanical Engineering, University of Twente, The Netherlands.
Department of Biomechanical Engineering, University of Twente, The Netherlands.
J Biomech. 2015 May 1;48(7):1258-63. doi: 10.1016/j.jbiomech.2015.03.004. Epub 2015 Mar 19.
Ankle stiffness contributes to standing balance, counteracting the destabilizing effect of gravity. The ankle stiffness together with the compliance between the foot and the support surface make up the ankle-foot stiffness, which is relevant to quiet standing. The contribution of the intrinsic ankle-foot stiffness to balance, and the ankle-foot stiffness amplitude dependency remain a topic of debate in the literature. We therefore developed an experimental protocol to directly measure the bilateral intrinsic ankle-foot stiffness during standing balance, and determine its amplitude dependency. By applying fast (40 ms) ramp-and-hold support surface rotations (0.005-0.08 rad) during standing, reflexive contributions could be excluded, and the amplitude dependency of the intrinsic ankle-foot stiffness was investigated. Results showed that reflexive activity could not have biased the torque used for estimating the intrinsic stiffness. Furthermore, subjects required less recovery action to restore balance after bilateral rotations in opposite directions compared to rotations in the same direction. The intrinsic ankle-foot stiffness appears insufficient to ensure balance, ranging from 0.93±0.09 to 0.44±0.06 (normalized to critical stiffness 'mgh'). This implies that changes in muscle activation are required to maintain balance. The non-linear stiffness decrease with increasing rotation amplitude supports the previous published research. With the proposed method reflexive effects can be ruled out from the measured torque without any model assumptions, allowing direct estimation of intrinsic stiffness during standing.
踝关节僵硬度有助于维持站立平衡，抵消重力的不稳定作用。踝关节僵硬度与足部和支撑面之间的顺应性共同构成了踝足僵硬度，这与安静站立相关。踝足固有僵硬度对平衡的贡献以及踝足僵硬度幅度依赖性在文献中仍是一个有争议的话题。因此，我们制定了一个实验方案，以直接测量站立平衡期间双侧踝足固有僵硬度，并确定其幅度依赖性。通过在站立过程中施加快速（40毫秒）斜坡-保持支撑面旋转（0.005-0.08弧度），可以排除反射性贡献，并研究踝足固有僵硬度的幅度依赖性。结果表明，反射活动不会使用于估计固有僵硬度的扭矩产生偏差。此外，与同向旋转相比，受试者在反向双侧旋转后恢复平衡所需的恢复动作更少。踝足固有僵硬度似乎不足以确保平衡，范围为0.93±0.09至0.44±0.06（相对于临界僵硬度“mgh”进行归一化）。这意味着需要改变肌肉激活来维持平衡。随着旋转幅度增加，僵硬度呈非线性下降，这支持了先前发表的研究。使用所提出的方法，可以在没有任何模型假设的情况下从测量扭矩中排除反射效应，从而直接估计站立期间的固有僵硬度。
