Trunk stiffness is an important parameter for trunk stability analysis and needs to be evaluated accurately. Discrepancies regarding the dependence of trunk stiffness on the direction of movement in the sagittal plane suggest inherent sources of error that require explanation. In contrast to the common assumption that the muscle stiffness remains constant prior to the induction of a reflex during position perturbations, it is postulated that muscle-stiffness changes of nonneural origin occur and alter the experimental trunk stiffness, causing it to depend on the sagittal direction. This is confirmed through reinterpretation of existing test data for a healthy subject, numerical simulation, and sensitivity analysis using a biomechanical model. The trunk stiffness is determined through a static approach (in forward and backward directions) and compared with the model stiffness for assumed scenarios involving deactivated muscles. The difference in stiffness between the opposite directions reaches 17.5% without a preload and decreases when a moderate vertical preload is applied. The increased muscle activation induced by preloads or electrical stimuli explains the apparent discrepancies observed in previous studies. The experimental stiffness invariably remains between low and high model-stiffness estimates based on extreme scenarios of the postulated losses of muscle activation, thereby confirming our hypothesis.

中文翻译：

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实验躯干刚度的方向依赖性：非神经起源的肌肉刚度变化的作用

躯干刚度是躯干稳定性分析的重要参数，需要准确评估。关于躯干刚度对矢状面运动方向的依赖性的差异表明了固有的误差来源，需要进行解释。与通常的假设相反，即在位置扰动期间，肌肉刚度在反射发生之前保持恒定，而假定非神经起源的肌肉刚度发生变化并改变实验躯干刚度，从而导致其取决于矢状方向。通过对健康受试者的现有测试数据进行重新解释，数值模拟以及使用生物力学模型的敏感性分析，可以证实这一点。躯干刚度是通过静态方法确定的（向前和向后），并将其与模型刚度进行比较，以用于涉及停用肌肉的假定场景。在没有预紧力的情况下，相反方向之间的刚度差达到17.5％，而在施加适当的垂直预紧力时则减小。预紧力或电刺激引起的肌肉激活增加解释了先前研究中观察到的明显差异。根据假定的肌肉激活损失的极端情况，实验刚度始终保持在模型刚度估计值的高低之间，从而证实了我们的假设。无预紧力的情况下为5％，当施加适度的垂直预紧力时为5％。预紧力或电刺激引起的肌肉激活增加解释了先前研究中观察到的明显差异。根据假定的肌肉激活损失的极端情况，实验刚度始终保持在模型刚度估计值的高低之间，从而证实了我们的假设。无预紧力的情况下为5％，当施加适度的垂直预紧力时为5％。预紧力或电刺激引起的肌肉激活增加解释了先前研究中观察到的明显差异。根据假定的肌肉激活损失的极端情况，实验刚度始终保持在模型刚度估计值的高低之间，从而证实了我们的假设。