Crevecoeur Frédéric, Thonnard Jean-Louis, Lefèvre Philippe
Center for Systems Engineering and Applied Mechanics, Université catholique de Louvain, 1348 Louvain-la-Neuve, Belgium.
J Neurophysiol. 2009 Aug;102(2):786-96. doi: 10.1152/jn.00113.2009. Epub 2009 May 20.
The planning and control of motor actions requires knowledge of the dynamics of the controlled limb to generate the appropriate muscular commands and achieve the desired goal. Such planning and control imply that the CNS must be able to deal with forces and constraints acting on the limb, such as the omnipresent force of gravity. The present study investigates the effect of hypergravity induced by parabolic flights on the trajectory of vertical pointing movements to test the hypothesis that motor commands are optimized with respect to the effect of gravity on the limb. Subjects performed vertical pointing movements in normal gravity and hypergravity. We use a model based on optimal control to identify the role played by gravity in the optimal arm trajectory with minimal motor costs. First, the simulations in normal gravity reproduce the asymmetry in the velocity profiles (the velocity reaches its maximum before half of the movement duration), which typically characterizes the vertical pointing movements performed on Earth, whereas the horizontal movements present symmetrical velocity profiles. Second, according to the simulations, the optimal trajectory in hypergravity should present an increase in the peak acceleration and peak velocity despite the increase in the arm weight. In agreement with these predictions, the subjects performed faster movements in hypergravity with significant increases in the peak acceleration and peak velocity, which were accompanied by a significant decrease in the movement duration. This suggests that movement kinematics change in response to an increase in gravity, which is consistent with the hypothesis that motor commands are optimized and the action of gravity on the limb is taken into account. The results provide evidence for an internal representation of gravity in the central planning process and further suggest that an adaptation to altered dynamics can be understood as a reoptimization process.
运动动作的规划与控制需要了解被控制肢体的动力学特性，以便生成适当的肌肉指令并实现预期目标。这种规划与控制意味着中枢神经系统必须能够应对作用于肢体的力和约束，比如无处不在的重力。本研究调查了抛物线飞行引起的超重对垂直指向运动轨迹的影响，以检验运动指令会针对重力对肢体的影响进行优化这一假设。受试者在正常重力和超重条件下进行垂直指向运动。我们使用基于最优控制的模型来确定重力在以最小运动成本实现的最优手臂轨迹中所起的作用。首先，正常重力下的模拟再现了速度曲线的不对称性（速度在运动持续时间的一半之前达到最大值），这是在地球上进行垂直指向运动的典型特征，而水平运动呈现对称的速度曲线。其次，根据模拟结果，尽管手臂重量增加，但超重条件下的最优轨迹应呈现峰值加速度和峰值速度的增加。与这些预测一致，受试者在超重条件下的运动速度更快，峰值加速度和峰值速度显著增加，同时运动持续时间显著缩短。这表明运动运动学随着重力增加而变化，这与运动指令被优化且考虑了重力对肢体的作用这一假设相一致。研究结果为中枢规划过程中重力的内部表征提供了证据，并进一步表明对改变的动力学的适应可被理解为一个重新优化的过程。
