Department of Physical Education, Seoul National University, Seoul, South Korea.
Department of Sports Science, Korean Institute of Sports Science, Seoul, South Korea.
Hum Mov Sci. 2022 Oct;85:102994. doi: 10.1016/j.humov.2022.102994. Epub 2022 Aug 17.
In the referent control theory, grip force emerges by designating the referent aperture (R) as a threshold position inside the object. This study quantified R and investigated whether the synergistic control of digit referent coordinate (RC) and apparent stiffness (k) depend on the external mechanical constraints on the hand-held object. Subjects held a motorized handle capable of adjusting the grip width and performed static multi-digit prehension tasks in which the handle was free and externally fixed in different conditions. The RC and k of individual digits were reconstructed from the changes in digit normal forces and the positions as the grip width was modulated. RCs of the thumb and virtual finger were used to calculate the width and midpoint of R, and synergy indices quantifying the task-specific covariation in the space of the digit normal forces and {RC, k} variables were computed. We found that the k and width of the R were larger when holding a free handle than the fixed handle. The higher stiffness in the free condition could be a strategy to ensure grip stability. The midpoint of R was skewed toward the virtual finger, reflecting different magnitudes of k for the two digits. Further, the normal forces and control variables {RC, k} displayed synergistic covariation for stabilization of the total grasping force. Finally, the synergies were weaker when the handle was externally fixed, demonstrating the dependence of synergies on external constraints. These results add to the current literature by demonstrating that grasp control involves modulation of digit apparent stiffness in addition to the referent coordinate and by identifying the synergistic organization of the control variables during static grasp.
在参照控制理论中，通过将参照孔径 (R) 指定为物体内部的阈值位置来产生握力。本研究量化了 R，并研究了数字参照坐标 (RC) 和表观刚度 (k) 的协同控制是否取决于对手持物体的外部机械约束。受试者握住一个可调节握把宽度的电动手柄，并在自由和外部固定的不同条件下执行静态多指抓握任务。通过改变指力和位置来重建各个手指的 RC 和 k。使用拇指和虚拟手指的 RC 来计算 R 的宽度和中点，并计算量化指力和 {RC，k} 变量空间中特定任务协同变化的协同指数。我们发现，自由握持手柄时，k 和 R 的宽度比固定手柄时更大。在自由条件下更高的刚度可能是确保握持稳定性的一种策略。R 的中点偏向虚拟手指，反映了两个手指的 k 值不同。此外，为了稳定总抓取力，正常力和控制变量 {RC，k} 显示出协同变化。最后，当手柄外部固定时，协同作用较弱，这表明协同作用取决于外部约束。这些结果通过证明抓握控制除了参照坐标外还涉及数字表观刚度的调制，并通过识别静态抓握过程中控制变量的协同组织，为当前文献增添了内容。
