Control of grip force when tilting objects: effect of curvature of grasped surfaces and of applied tangential torque

When we manipulate objects in everyday tasks, there are vari-ations 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 250 m21 (concave with radius 20 mm) to 200 m21 (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 an