Ramos C F, Stark L W
Neurology Unit, University of California, Berkeley 94720.
Exp Brain Res. 1990;82(3):651-7. doi: 10.1007/BF00228807.
A recent article by Crenna et al. (1987) has shown that fast, forward bending movements are accompanied by a backwards motion of the hips and lower limbs. The ongoing research presented in this brief note expands upon the experimental data described by Crenna and colleagues, concerning the postural activities associated with rapid forward bending in standing man. Our primary experimental tool is the computer simulation method, with the standing subject being represented by a double-joint system: the trunk is modeled as a rigid link mechanically coupled (via a "hip" joint) to the lower body link fixed to the ground (via an "ankle" joint). Each of the two joints in this system is independently controlled by a neurological control model for single joint movements, consisting of an idealized pair of antagonistic muscles (flexor and extensor), their common load, and proprioception from the muscle spindles. This model thereby integrates descending commands with proprioceptive feedback in controlling the joint movements. Our early simulation experiments determine a "reduced trajectory", that is, the physical perturbation to the postural system, due to the voluntary movement, in the absence of any stabilizing activities. These simulation experiments clearly show that an important component of the backward movements in the hips and lower limbs during forward bending is indeed due to the mechanical (physical) coupling between the upper and lower body segments and thus not solely a consequence of the anticipatory postural muscle activity. Simulations also predict that any postural activities in the hips and lower limbs should be a two-fold process: first, some preprogrammed, descending control to the lower body would be required to actively enhance the passive, backwards motion (this is consistent with, though not strictly identical to, the hypothesis of Crenna and colleagues); secondly, there must be a subsequent activation in the anterior muscles of the lower body in order to arrest this backwards motion, since otherwise the uncountered momentum would carry the body backward to the floor in less than half a second after the upper body movement has terminated.
Crenna等人（1987年）最近发表的一篇文章表明，快速向前弯曲动作伴随着臀部和下肢的向后运动。本简短报告中展示的正在进行的研究扩展了Crenna及其同事所描述的实验数据，这些数据涉及站立男性快速向前弯曲时的姿势活动。我们的主要实验工具是计算机模拟方法，站立的受试者由一个双关节系统表示：躯干被建模为一个刚性连杆，通过一个“髋关节”机械耦合到固定在地面上的下半身连杆（通过一个“踝关节”）。该系统中的两个关节各自由一个用于单关节运动的神经控制模型独立控制，该模型由一对理想化的拮抗肌（屈肌和伸肌）、它们的共同负荷以及来自肌梭的本体感觉组成。因此，该模型在控制关节运动时将下行指令与本体感觉反馈进行了整合。我们早期的模拟实验确定了一条“简化轨迹”，即由于自愿运动，在没有任何稳定活动的情况下，姿势系统所受到的物理扰动。这些模拟实验清楚地表明，向前弯曲时臀部和下肢向后运动的一个重要组成部分确实是由于上半身和下半身节段之间的机械（物理）耦合，因此不仅仅是预期姿势肌肉活动的结果。模拟还预测，臀部和下肢的任何姿势活动都应该是一个双重过程：首先，需要对下半身进行一些预先编程的下行控制，以积极增强被动的向后运动（这与Crenna及其同事的假设一致，但并不完全相同）；其次，下半身前部肌肉必须随后被激活，以阻止这种向后运动，因为否则在身体上部运动终止后不到半秒，未被抵消的动量会使身体向后倒向地面。
