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Control of Fingertip Forces in Multidigit Manipulation
Ist Teil von
Journal of neurophysiology, 1999-04, Vol.81 (4), p.1706-1717
Ort / Verlag
United States: Am Phys Soc
Erscheinungsjahr
1999
Link zum Volltext
Quelle
MEDLINE
Beschreibungen/Notizen
1 Department of Psychology, Queen's
University, Kingston, Ontario K7L 3N6, Canada; and
2 Department of Physiology, University of Umeå,
S-901 87 Umea, Sweden
Flanagan, J. Randall,
Magnus K. O. Burstedt, and
Roland S. Johansson.
Control of fingertip forces in multidigit manipulation.
Previous studies of control of fingertip forces in skilled manipulation have focused on tasks involving two digits, typically the thumb and
index finger. Here we examine control of fingertip actions in a
multidigit task in which subjects lifted an object using unimanual and
bimanual grasps engaging the tips of the thumb and two fingers. The
grasps resembled those used when lifting a cylindrical object from
above; the two fingers were some 4.25 cm apart and the thumb was
~5.54 cm from either finger. The three-dimensional forces and torques
applied by each digit and the digit contact positions were measured
along with the position and orientation of the object. The vertical
forces applied tangential to the grasp surfaces to lift the object were
synchronized across the digits, and the contribution by each digit to
the total vertical force reflected intrinsic object properties
(geometric relationship between the object's center of mass and the
grasped surfaces). Subjects often applied small torques tangential to
the grasped surfaces even though the object could have been lifted
without such torques. The normal forces generated by each digit
increased in parallel with the local tangential load (force and
torque), providing an adequate safety margin against slips at each
digit. In the present task, the orientations of the force vectors
applied by the separate digits were not fully constrained and therefore the motor controller had to choose from a number of possible solutions. Our findings suggest that subjects attempt to minimize (or at least
reduce) fingertip forces while at the same time ensure that grasp
stability is preserved. Subjects also avoid horizontal tangential forces, even at a small cost in total force. Moreover, there were subtle actions exerted by the digits that included changes in the
distribution of vertical forces across digits and slight object tilt.
It is not clear to what extent the brain explicitly controlled these
actions, but they could serve, for instance, to keep tangential torques
small and to compensate for variations in digit contact positions. In
conclusion, we have shown that when lifting an object with a
three-digit grip, the coordination of fingertip forces, in many
respects, matches what has been documented previously for two-digit
grasping. At the same time, our study reveals novel aspects of force
control that emerge only in multidigit manipulative tasks.