Neural processes underlying human balance control

保持平衡的能力对于许多日常活动的执行是必不可少的。当自愿开始运动或预期身体受到可预测的外部扰动时，个体必须依靠前馈机制在运动开始前产生预期的姿势调整(APA)。这涉及到皮质脊髓兴奋性的快速变化，以促进姿势肌肉活动的早期增加。这会导致身体重心的移动，从而将即将到来的运动造成的不稳定降至最低。由于APA的作用是稳定身体，因此APA的时机和大小取决于即将到来的运动的力量和速度，因此当预期更大或更快的运动时，可以观察到更大和更早的APA。然而，有趣的是，老年人衡量他们预期反应的能力降低了，以至于他们在不同的力量水平上表现出更多同质大小的APA。APA伸缩的这一缺陷导致在运动开始前姿势更不稳定，这可能解释了为什么日常生活中经常发生跌倒，原因是在运动准备过程中体重转移不正确。为了更好地了解APA不良伸缩和生成的神经机制，拟议的研究计划将调查中枢神经系统如何在整个寿命期间对预期平衡控制做出贡献。将进行一系列实验，年轻人和老年人将自我发起一个运动(例如，用脚趾站起来)或对身体可预见的扰动做出反应。将引入任务难度和个体限制的不同，以检查中枢神经系统的适应性，以及它是否与任务的要求有关。在这些运动的准备阶段的不同时间(即运动开始之前)，脊髓通路的兴奋性将通过电刺激周围神经在下肢姿势肌肉中诱发Hoffmann反射来评估，而脊髓上区域的参与将通过经颅磁刺激诱发运动诱发电位来确定。平衡表现，以APA大小和起始的形式，将通过身体运动学和运动记录来量化。拟议的研究计划将扩大我们对人类预期平衡控制背后的神经机制的基本理解，并提供关于这些过程如何以及为什么随着年龄的增长而改变的见解。从该计划中获得的知识也可能有助于提高我们确定为什么会因衰老而出现跌倒和平衡赤字的能力。

The ability to maintain balance is essential for the execution of many daily activities. When voluntarily initiating a movement or in anticipation of a predictable external perturbation to the body, individuals must rely on feedforward mechanisms to generate an anticipatory postural adjustment (APA) prior to movement onset. This involves a rapid change in corticospinal excitability to facilitate an early increase in postural muscle activity. This causes a shift in the body's center of mass so that destabilization from the upcoming movement is minimized. Since APAs act to stabilize the body, the timing and magnitude of APAs are dependent on the force and speed of the upcoming movement, whereby larger and earlier APAs are observed when larger or faster movements are expected. Interestingly however, older adults have a reduced ability to scale their anticipatory responses, such that they demonstrate more homogenous-sized APAs across varying levels of force. This deficit in APA scaling leads to greater postural instability prior to movement initiation and may explain why falls in daily life often occur as a result of an incorrect shifting of body weight during movement preparation. To better understand the neural mechanisms underlying the poor scaling and generation of APAs, the proposed research program will investigate how the central nervous system contributes to anticipatory balance control across the lifespan. A series of experiments will be conducted whereby young and older adults will self-initiate a movement (e.g., rising onto their toes) or respond to a predictable perturbation to the body. Variations in task difficulty and individual constraints will be introduced to examine the adaptability of the central nervous system and whether it is associated with the requirements of the task. At various times during the preparatory phase of these movements (i.e., prior to movement onset), the excitability of the spinal pathways will be assessed by electrically stimulating a peripheral nerve to elicit Hoffmann reflexes in the lower limb postural muscles while the involvement of the supraspinal areas will be determined by eliciting motor-evoked potentials using transcranial magnetic stimulation. Balance performance, in the form of APA magnitude and onset, will be quantified through body kinematic and kinetic recordings. The proposed research program will expand our fundamental understanding of the neural mechanisms underlying human anticipatory balance control and provide insight into how and why these processes become altered with advanced age. Knowledge developed from the program may also help to improve our ability to determine why falls and balance deficits arise due to aging.