Finley James M, Dhaher Yasin Y, Perreault Eric J
Northwestern University, Evanston, IL 60201, USA.
Annu Int Conf IEEE Eng Med Biol Soc. 2009;2009:7265-8. doi: 10.1109/IEMBS.2009.5334730.
The dynamics of sway during quiet stance have often been approximated by the movement of an unstable inverted pendulum. Controlling this unstable load requires the nervous system to balance the reliance on feed-forward volitional activation and feedback mechanisms such as stretch reflexes. It has been demonstrated that reflex excitability is heightened when postural stability is threatened by destabilizing forces in the environment. However, the relationship between postural stability, volitional activation, and stretch reflex excitability remains unclear. We addressed this question by characterizing feed-forward and feedback activation strategies during balance of a simulated inverted pendulum. We hypothesized that feed-forward co-contraction and stretch reflex amplitude would scale together as the external support provided by the environment was reduced. Electromyographic (EMG) responses in 5 muscles of the lower limb were used to characterize co-contraction patterns and stretch reflex amplitude as subjects stabilized the simulated loads. Our results revealed that co-contraction magnitude did indeed scale with increasingly destabilizing torques; however reflex amplitude was attenuated as stability was reduced. These findings suggest that the contribution of feedback mechanisms to postural stability depends on both the level of stability provided by the environment and how the environment influences the pattern of volitional activation.
安静站立时的摇摆动态通常用不稳定倒立摆的运动来近似。控制这种不稳定负荷需要神经系统平衡对前馈自主激活和反馈机制（如牵张反射）的依赖。已经证明，当环境中的不稳定力威胁到姿势稳定性时，反射兴奋性会增强。然而，姿势稳定性、自主激活和牵张反射兴奋性之间的关系仍不清楚。我们通过描述模拟倒立摆平衡过程中的前馈和反馈激活策略来解决这个问题。我们假设，随着环境提供的外部支撑减少，前馈协同收缩和牵张反射幅度会一起变化。当受试者稳定模拟负荷时，下肢5块肌肉的肌电图（EMG）反应被用来描述协同收缩模式和牵张反射幅度。我们的结果显示，协同收缩幅度确实随着不稳定扭矩的增加而变化；然而，随着稳定性降低，反射幅度会减弱。这些发现表明，反馈机制对姿势稳定性的贡献既取决于环境提供的稳定性水平，也取决于环境如何影响自主激活模式。
