Reimann Hendrik, Fettrow Tyler, Grenet David, Thompson Elizabeth D, Jeka John J
Department of Kinesiology and Applied Physiology, University of Delaware, Newark, DE, United States.
Department of Kinesiology, Temple University, Philadelphia, PA, United States.
Front Sports Act Living. 2019 Sep 27;1:25. doi: 10.3389/fspor.2019.00025. eCollection 2019.
The human body is mechanically unstable during walking. Maintaining upright stability requires constant regulation of muscle force by the central nervous system to push against the ground and move the body mass in the desired way. Activation of muscles in the lower body in response to sensory or mechanical perturbations during walking is usually highly phase-dependent, because the effect any specific muscle force has on the body movement depends upon the body configuration. Yet the resulting movement patterns of the upper body after the same perturbations are largely phase-independent. This is puzzling, because any change of upper-body movement must be generated by parts of the lower body pushing against the ground. How do phase-dependent muscle activation patterns along the lower body generate phase-independent movement patterns of the upper body? We hypothesize that when a sensory system detects a deviation of the body in space from a desired state that indicates the onset of a fall, the nervous system generates a functional response by pushing against the ground in any way possible with the current body configuration. This predicts that the changes in the ground reaction force patterns following a balance perturbation should be phase-independent. Here we test this hypothesis by disturbing upright balance in the frontal plane using Galvanic vestibular stimulation at three different points in the gait cycle. We measure the resulting changes in whole-body center of mass movement and the location of the center of pressure of the ground reaction force. We find that the magnitude of the initial center of pressure shift in the direction of the perceived fall is larger for perturbations late in the gait cycle, while there is no statistically significant difference in onset time. These results contradict our hypothesis by showing that even the initial CoP shift in response to a balance perturbation depends upon the phase of the gait cycle. Contrary to expectation, we also find that the whole-body balance response is not phase-independent. Both the onset time and the magnitude of the whole-body center of mass shift depend on the phase of the perturbation. We conclude that the central nervous system recruits any available mechanism to generate a functional balance response by pushing against the ground as fast as possible in response to a perturbation, but that the different mechanisms available at different phases in the gait cycle are not equally strong, leading to phase-dependent differences in the overall response.
人体在行走过程中机械上是不稳定的。维持直立稳定性需要中枢神经系统不断调节肌肉力量，以便推压地面并以期望的方式移动身体质量。在行走过程中，下半身肌肉对感觉或机械扰动的激活通常高度依赖于相位，因为任何特定肌肉力量对身体运动的影响取决于身体构型。然而，在相同扰动后上半身产生的运动模式在很大程度上与相位无关。这令人困惑，因为上半身运动的任何变化都必须由下半身推压地面的部分产生。下半身依赖相位的肌肉激活模式是如何产生上半身与相位无关的运动模式的呢？我们假设，当感觉系统检测到身体在空间中偏离期望状态，表明跌倒开始时，神经系统会利用当前身体构型以任何可能的方式推压地面来产生功能性反应。这预测平衡扰动后地面反作用力模式的变化应该与相位无关。在这里，我们通过在步态周期的三个不同点使用前庭电刺激在额平面干扰直立平衡来检验这一假设。我们测量全身质心运动的相应变化以及地面反作用力压力中心的位置。我们发现，在步态周期后期的扰动中，压力中心在感知跌倒方向上的初始偏移幅度更大，而起始时间没有统计学上的显著差异。这些结果与我们的假设相矛盾，表明即使是对平衡扰动的初始压力中心偏移也取决于步态周期的相位。与预期相反，我们还发现全身平衡反应并非与相位无关。全身质心偏移的起始时间和幅度都取决于扰动的相位。我们得出结论，中枢神经系统会利用任何可用机制，通过在受到扰动时尽快推压地面来产生功能性平衡反应，但步态周期不同阶段可用的不同机制强度并不相同，导致整体反应存在相位依赖性差异。
