Prehension synergies in the grasps with complex friction patterns: local versus synergic effects and the template...

We studied adjustments of digit forces to changes in the friction. The subjects held a handle statically in a three-digit grasp. The friction under each digit was either high or low, resulting in eight three-element friction sets (such grasps were coined the grasps with complex friction pattern). The total load was also manipulated. It was found that digit forces were adjusted not only to the supported load and local friction, but also to friction at other digits (synergic effects). When friction under a digit was low, its tangential force decreased and the normal force increased (local effects). The synergic effects were directed to maintain the equilibrium of the handle. The relation between the individual digit forces and loads agreed with the triple-product model: f(i)(n) = k(i)((2))k(i)((1))L, where f(i)(n) is normal force of digit i, L is the load (newtons), k(i)((1)) is a dimensionless coefficient representing sharing the total tangential force among the digits (summation operator k(i)((1)) = 1.0), and k(i)((2)) is a coefficient representing the relation between the tangential and normal forces of digit i (the overall friction equivalent, OFE). At each friction set, the central controller selected the grasping template -- a three-element array of k(i)((2))k(i)((1)) products -- and then scaled the template with the load magnitude.