Ashton Graybiel Spatial Orientation Laboratory, Brandeis University, Waltham, Massachusetts.
J Neurophysiol. 2020 Dec 1;124(6):1995-2011. doi: 10.1152/jn.00612.2019. Epub 2020 Sep 30.
Both passive and active mechanisms are necessary to explain small amplitude forward-backward (FB) voluntary swaying. Parallel and symmetric leg inverted pendulum models with stiffness control are a simple way to replicate FB swaying during quiet stance. However, it has been more difficult to model lateral left-right (LR) voluntary swaying involving the dual mechanisms of hip loading-unloading and ankle pressure distribution. To assess these factors, we had subjects perform small amplitude FB and LR sways and circular rotation. We experimentally identified three parameters that characterized their two-dimensional stiffnesses: AP stiffness (K), and lateral stiffness (K), at the ankles and a parameter we refer to as the engagement-disengagement rate (K) of the legs. We performed simulations with our engaged leg model (Bakshi A, DiZio P, Lackner JR. 121: 2042-2060, 2019; Bakshi A, DiZio P, Lackner JR. 121: 2028-2041, 2019) to test its predictions about the limits of balance stability during sway in the three test conditions. Comparing the model's predictions with the experimental data, we found that K has a task-dependent dual role in upright balance and is crucial to prevent falling; K helps overcome viscous drags but is not instrumental to stability; K has a key role in stability and is dependent on the biomechanical geometry of the body, which is invariant across balance tasks. These findings provide new insights into balance control that have important clinical implications for falling, especially for patients who are unable to use a hip strategy during balance control. Our previously published Engaged Leg Model here shows how stiffness plays complex multicausal roles in balance. In one role, it is crucial to stability, with task contingent influences over balance. In another, it overcomes viscous drag. Task-dependent stiffness alone does not explain stable balance; geometrical, invariant aspects of body biomechanics also matter. Our model is fully applicable to clinical balance pathologies involving asymmetries in movement and balance control.
被动和主动机制都需要解释小幅度前后（FB）自愿摆动。具有刚度控制的平行和对称腿部倒立摆模型是复制安静站立时 FB 摆动的一种简单方法。然而，更难以模拟涉及髋关节加载卸载和踝关节压力分布的双重机制的侧向左右（LR）自愿摆动。为了评估这些因素，我们让受试者进行小幅度 FB 和 LR 摆动和圆形旋转。我们通过实验确定了三个特征其二维刚度的参数：脚踝处的 AP 刚度（K）和侧向刚度（K），以及我们称为腿部结合-分离率（K）的参数。我们使用我们的结合腿模型（Bakshi A、DiZio P、Lackner JR. 121: 2042-2060, 2019; Bakshi A、DiZio P、Lackner JR. 121: 2028-2041, 2019）进行模拟，以测试其对摆动时平衡稳定性极限的预测在三种测试条件下。将模型的预测与实验数据进行比较，我们发现 K 在直立平衡中有一个依赖任务的双重作用，对于防止跌倒至关重要；K 有助于克服粘性阻力，但对稳定性没有帮助；K 在稳定性中起着关键作用，并且取决于身体的生物力学几何形状，这在平衡任务中是不变的。这些发现为平衡控制提供了新的见解，对跌倒具有重要的临床意义，特别是对那些在平衡控制过程中无法使用臀部策略的患者。我们之前发表的《Engaged Leg Model》在此展示了刚度在平衡中如何发挥复杂的多因果作用。在一个角色中，它对稳定性至关重要，对平衡有任务相关的影响。在另一个角色中，它克服了粘性阻力。仅依赖任务的刚度并不能解释稳定的平衡；身体生物力学的几何不变方面也很重要。我们的模型完全适用于涉及运动和平衡控制不对称的临床平衡病理学。
