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J Neurophysiol (May 9, 2007). doi:10.1152/jn.00058.2007
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Submitted on January 17, 2007
Accepted on May 5, 2007

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

Xun Niu1, Mark L Latash1, and Vladimir M. Zatsiorsky1*

1 Kinesiology, The Pennsylvania State University, University Park, State College, Pennsylvania, United States

* To whom correspondence should be addressed. E-mail: vxz1{at}psu.edu.

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 3-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: fi n= ki 2 ki 1L, where fi n is normal force of digit i, L is the load (newtons), ki 1 is a dimensionless coefficient representing sharing the total tangential force among the digits ({sum} ki 1=1.0 ), and ki 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 ki 2 ki 1 products--and then scaled the template with the load magnitude.







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