Department of Kinesiology, The Pennsylvania State University, University Park, PA 16802, USA.
Exp Brain Res. 2010 May;203(1):101-18. doi: 10.1007/s00221-010-2213-7. Epub 2010 Mar 25.
Accurate control of forces produced by the fingers is essential for performing object manipulation. This study examines the indices of finger interaction when accurate time profiles of force are produced in different directions, while using one of the fingers or all four fingers of the hand. We hypothesized that patterns of unintended force production among shear force components may involve features not observed in the earlier studies of vertical force production. In particular, we expected to see unintended forces generated by non-task fingers not in the direction of the instructed force but in the opposite direction as well as substantial force production in directions orthogonal to the instructed direction. We also tested a hypothesis that multi-finger synergies, quantified using the framework of the uncontrolled manifold hypothesis, will help reduce across-trials variance of both total force magnitude and direction. Young, healthy subjects were required to produce accurate ramps of force in five different directions by pressing on force sensors with the fingers of the right (dominant) hand. The index finger induced the smallest unintended forces in non-task fingers. The little finger showed the smallest unintended forces when it was a non-task finger. Task fingers showed substantial force production in directions orthogonal to the intended force direction. During four-finger tasks, individual force vectors typically pointed off the task direction, with these deviations nearly perfectly matched to produce a resultant force in the task direction. Multi-finger synergy indices reflected strong co-variation in the space of finger modes (commands to fingers) that reduced variability of the total force magnitude and direction across trials. The synergy indices increased in magnitude over the first 30% of the trial time and then stayed at a nearly constant level. The synergy index for stabilization of total force magnitude was higher for shear force components when compared to the downward pressing force component. The results suggest complex interactions between enslaving and synergic force adjustments, possibly reflecting the experience with everyday prehensile tasks. For the first time, the data document multi-finger synergies stabilizing both shear force magnitude and force vector direction. These synergies may play a major role in stabilizing the hand action during object manipulation.
手指产生的力的精确控制对于执行物体操作至关重要。本研究考察了在使用一只手的一个或四个手指产生不同方向的精确时间力曲线时，手指相互作用的指标。我们假设，在早期对垂直力产生的研究中没有观察到的剪切力分量中无意产生力的模式可能涉及到一些特征。特别是，我们预计会看到非任务手指产生的无意力不在指令力的方向上，而是在相反的方向上，以及在与指令方向正交的方向上产生大量的力。我们还测试了一个假设，即使用无控制流假设框架量化的多手指协同作用将有助于减少总力大小和方向的跨试验方差。年轻、健康的受试者被要求通过用右手手指按压力传感器在五个不同的方向上产生精确的力斜坡。食指在非任务手指中产生的无意力最小。当小指是一个非任务手指时，它表现出最小的无意力。任务手指在与预期力方向正交的方向上产生大量的力。在四指任务中，单个力矢量通常指向任务方向之外，这些偏差几乎完全匹配，以产生任务方向上的合力。多手指协同作用指数反映了手指模式（手指指令）空间中的强共变，减少了跨试验的总力大小和方向的可变性。协同作用指数在试验时间的前 30%内增大，然后保持在几乎恒定的水平。与向下按压力分量相比，用于稳定总力大小的协同作用指数对于剪切力分量更高。结果表明，在奴役和协同力调整之间存在复杂的相互作用，这可能反映了日常抓握任务的经验。数据首次记录了多手指协同作用稳定剪切力大小和力矢量方向。这些协同作用可能在物体操作过程中稳定手部动作中发挥重要作用。
