Abstract：
We tested a hypothesis on force-stabilizing synergies during four-finger accurate force production at three levels: (1) The level of the reciprocal and coactivation commands, estimated as the referent coordinate and apparent stiffness of all four fingers combined; (2) The level of individual finger forces; and (3) The level of firing of individual motor units (MU). Young, healthy participants performed accurate four-finger force production at a comfortable, non-fatiguing level under visual feedback on the total force magnitude. Mechanical reflections of the reciprocal and coactivation commands were estimated using small, smooth finger perturbations applied by the \"inverse piano\" device. Firing frequencies of motor units in the flexor digitorum superficialis (FDS) and extensor digitorum communis (EDC) were estimated using surface recording. Principal component analysis was used to identify robust MU groups (MU-modes) with parallel changes in the firing frequency. The framework of the uncontrolled manifold hypothesis was used to compute synergy indices in the spaces of referent coordinate and apparent stiffness, finger forces, and MU-mode magnitudes. Force-stabilizing synergies were seen at all three levels. They were present in the MU-mode spaces defined for MUs in FDS, in EDC, and pooled over both muscles. No effects of hand dominance were seen. The synergy indices defined at different levels of analysis showed no correlations across the participants. The findings are interpreted within the theory of control with spatial referent coordinates for the effectors. We conclude that force stabilization gets contributions from three levels of neural control, likely associated with cortical, subcortical, and spinal circuitry.
摘要：
我们在三个水平上测试了四指精确力产生过程中的力稳定协同作用的假设：（1）互惠和协同命令的水平，估计为所有四个手指组合的参考坐标和表观刚度；（2）单个手指力的水平；（3）单个运动单位（MU）的击发水平。年轻,健康的参与者在舒适的环境下进行准确的四指力量产生，在视觉反馈下对总力大小的非疲劳水平。倒数和共激活命令的机械反射是使用小的，由“反钢琴”装置施加的平滑手指扰动。使用表面记录估算了指浅屈肌（FDS）和指伸肌（EDC）中运动单元的射击频率。主成分分析用于识别点火频率平行变化的稳健MU组（MU模式）。不受控制的流形假设的框架用于计算参考坐标和表观刚度空间中的协同指数，手指力量,和MU模式幅度。在所有三个层面都看到了稳定部队的协同作用。它们存在于为FDS中的MU定义的MU模式空间中，在EDC中,聚集在两块肌肉上。没有看到手部优势的影响。在不同水平的分析中定义的协同指数显示参与者之间没有相关性。这些发现是在控制理论中用效应器的空间参考坐标来解释的。我们得出的结论是，力稳定来自三个水平的神经控制，可能与皮质有关，皮质下,和脊髓电路。
