Aoki Tomoko, Niu Xun, Latash Mark L, Zatsiorsky Vladimir M
Department of Kinesiology, Biomechanics Lab, The Pennsylvania State University, 39 Recreation Building, University Park, PA 16802, USA.
Exp Brain Res. 2006 Jul;172(4):425-38. doi: 10.1007/s00221-006-0350-9. Epub 2006 Feb 16.
The effects of surface friction at the digit-object interface on digit forces were studied when subjects (n=8) statically held an object in a five-digit grasp. The friction conditions were SS (all surfaces are sandpaper), RR (all are rayon), SR (S for the thumb and R for the four fingers), and RS (the reverse of SR). The interaction effects of surface friction and external torque were also examined using five torques (-0.5, -0.25, 0, +0.25, +0.5 Nm). Forces and moments exerted by the digits on a handle were recorded. At zero torque conditions, in the SS and RR (symmetric) tasks the normal forces of the thumb and virtual finger (VF, an imagined finger with the mechanical effect equal to that of the four fingers) were larger for the RR than the SS conditions. In the SR and RS (asymmetric) tasks, the normal forces were between the RR and SS conditions. Tangential forces were smaller at the more slippery side than at the less slippery side. According to the mathematical optimization analysis decreasing the tangential forces at the more slippery sides decreases the cost function values. The difference between the thumb and VF tangential forces, DeltaF (t), generated a moment of the tangential forces (friction-induced moment). At non-zero torque conditions the friction-induced moment and the moment counterbalancing the external torque (equilibrium-necessitated moment) could be in same or in opposite directions. When the two moments were in the same direction, the contribution of the moment of tangential forces to the total moment was large, and the normal forces were relatively low. In contrast, when the two moments were in opposite directions, the contribution of the moment of tangential forces to the total moment markedly decreased, which was compensated by an increase in the moment of normal forces. The apparently complicated results were explained as the result of summation of the friction-related (elemental) and torque-related (synergy) components of the central commands to the individual digits.
当8名受试者用五指抓握静态握持物体时，研究了手指与物体界面处的表面摩擦力对指力的影响。摩擦条件分为SS（所有表面均为砂纸）、RR（所有表面均为人造丝）、SR（拇指表面为砂纸，其余四指表面为人造丝）和RS（与SR相反）。还使用五种扭矩（-0.5、-0.25、0、+0.25、+0.5 Nm）研究了表面摩擦力和外部扭矩的相互作用。记录手指作用在手柄上的力和力矩。在零扭矩条件下，在SS和RR（对称）任务中，RR条件下拇指和虚拟手指（VF，一个机械作用等同于其余四指的假想手指）的法向力比SS条件下更大。在SR和RS（不对称）任务中，法向力介于RR和SS条件之间。较滑一侧的切向力比较不滑一侧小。根据数学优化分析，减小较滑一侧的切向力会降低成本函数值。拇指和VF切向力之差ΔF(t)产生了切向力力矩（摩擦诱导力矩）。在非零扭矩条件下，摩擦诱导力矩和平衡外部扭矩的力矩（平衡所需力矩）可能方向相同或相反。当这两个力矩方向相同时，切向力力矩对总力矩的贡献较大，法向力相对较低。相反，当这两个力矩方向相反时，切向力力矩对总力矩的贡献显著减小，这由法向力力矩的增加来补偿。这些看似复杂的结果被解释为中央指令中与摩擦相关（基本）和与扭矩相关（协同）成分对各个手指求和的结果。
