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The Journal of Neurophysiology Vol. 82 No. 5 November 1999, pp. 2393-2405
Copyright ©1999 by the American Physiological Society
1Department of Integrative Medical Biology, Section for Physiology, Umeå University, SE-901 87 Umeå, Sweden; and 2Department of Psychology, Queen's University, Kingston, Ontario K7L 3N6, Canada
Burstedt, Magnus K. O.,
J. Randall Flanagan, and
Roland S. Johansson.
Control of Grasp Stability in Humans Under Different Frictional
Conditions During Multidigit Manipulation. J. Neurophysiol. 82: 2393-2405, 1999. Control of grasp
stability under different frictional conditions has primarily been
studied in manipulatory tasks involving two digits only. Recently we
found that many of the principles for control of forces originally
demonstrated for two-digit grasping also apply to various three-digit
grasps. Here we examine the control of grasp stability in a
multidigit task in which subjects used the tips of the thumb, index,
and middle finger to lift an object. The grasp resembled those used
when lifting a cylindrical object from above. The digits either all
contacted the same surface material or one of the digits contacted a
surface material that was more, or less, slippery than that contacted
by the other two digits. The three-dimensional forces and torques
applied by each digit and the contact positions were measured along
with the position and orientation of the object. The distribution of
forces among the digits strongly reflected constraints imposed by the
geometric relationship between the object's center of mass and the
contact surfaces. On top of this distribution, we observed changes in force coordination related to changes in the combination of surface materials. When all digits contacted the same surface material, the
ratio between the normal force and tangential load
(Fn:L ratio) was similar
across digits and scaled to provide an adequate safety margin against
slip. With different contact surfaces subjects adapted the
Fn:L ratios at the individual
digits to the local friction with only small influences by the friction
at the other two digits. They accomplished this by scaling the normal
forces similarly at all digits and changing the distribution of load among the digits. The surface combination did not, however, influence digit position, tangential torque, or object tilting systematically. The change in load distribution, rather, resulted from interplay between these factors, and the nature of this interplay varied between
trials. That is, subjects achieved grasp stability with various
combinations of fingertip actions and appeared to exploit the many
degrees of freedom offered by the multidigit grasp. The results extend
previous findings based on two-digit tasks to multidigit tasks by
showing that subjects adjust fingertip forces at each digit to the
local friction. Moreover, our findings suggest that subjects adapted
the load distribution to the current frictional condition by regulating
the normal forces to allow slips to occur early in the lift task, prior
to object lift-off.
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