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The Journal of Neuroscience, December 15, 1998, 18(24):10724-10734
Control of Grip Force When Tilting Objects: Effect of Curvature
of Grasped Surfaces and Applied Tangential Torque
Antony W.
Goodwin2,
Per
Jenmalm1, and
Roland S.
Johansson1
1 Department of Physiology, Umeå University, S-901 87 Umeå, Sweden, and 2 Department of Anatomy and Cell
Biology, University of Melbourne, Parkville, Victoria 3052, Australia
When we manipulate objects in everyday tasks, there are variations
in the shape of the grasped surfaces, and the loads that potentially
destabilize the grasp include time-varying linear forces and torques
tangential to the grasped surfaces. Previous studies of the control of
fingertip forces for grasp stability have dealt principally with flat
grip surfaces and linear force loads. Here, we studied the regulation
of grip force with changes in curvature of grasped surfaces and changes
in tangential torque applied by the index finger and thumb when humans
lifted an object and rotated it about the horizontal grip axis through
an angle of 65°. The curvatures of the matched pair of spherical
surfaces varied from  50 m 1 (concave with
radius 20 mm) to 200 m1 (convex with radius 5 mm).
The applied tangential torque at the orientation of 65° was varied
sixfold. Regardless of the values of curvature and end torque, grip
force and tangential torque were coordinated, increasing in parallel
throughout the tilt with an approximately linear relationship; the
slope of the line increased progressively with increasing surface
curvature. This parametric scaling of grip force was directly related
to the minimum grip force required to prevent rotational slip,
resulting in an adequate safety margin against slip in all cases. We
conclude that surface curvature parametrically influences grip force
regulation when the digits are exposed to torsional loads. Furthermore,
the sensorimotor programs that control the grip force apparently
predict the effect of the total load comprising linear forces and
tangential torques.
Key words:
object shape; grasp stability; fingertip forces; human
hand; precision grip; fingertip torque; grip force; friction; rotational slip; tangential torsional loads
Copyright © 1998 Society for Neuroscience 0270-6474/98/182410724-11$05.00/0
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