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 (CR). We experimentally identified three parameters that characterized their 2-dimensional stiffnesses - AP stiffness (K_S^AP), and lateral stiffness (K_S^ML), at the ankles and a parameter we refer to as the engagement-disengagement rate (K_ED) of the legs. We performed simulations with our Engaged Leg Model (Bakshi et al. 2019a; b) 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_S^AP has a task dependent dual role in upright balance and is crucial to prevent falling; K_S^ML helps overcome viscous drags but is not instrumental to stability; K_ED 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.

中文翻译：

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主动刚度在直立平衡和多向摇摆中的多重作用

被动和主动机制对于解释小幅度前后 (FB) 自愿摇摆都是必要的。具有刚度控制的平行和对称腿倒立摆模型是一种在安静站立期间复制 FB 摇摆的简单方法。然而，模拟横向左右 (LR) 随意摇摆的难度更大，涉及髋关节负重和踝关节压力分布的双重机制。为了评估这些因素，我们让受试者进行小幅度 FB 和 LR 摇摆以及圆形旋转 (CR)。我们通过实验确定了三个参数，其特征在于它们的2维刚度- AP刚度（K_小号^AP），和横向刚度（K_小号^ML)，在脚踝处，我们将一个参数称为腿部的接合-脱离率 (K _ED )。我们使用我们的 Engaged Leg Model (Bakshi et al. 2019a; b) 进行了模拟，以测试其对三种测试条件下摇摆期间平衡稳定性极限的预测。将模型的预测与实验数据进行比较，我们发现 K _S ^AP在直立平衡中具有任务依赖的双重作用，并且对于防止跌倒至关重要；ķ_小号^ML有助于克服粘阻力，但不是工具性的稳定; ķ _ED在稳定性方面起着关键作用，并且依赖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，特别是对于在平衡控制期间无法使用髋部策略的患者。