Adaptive control finger force to tangential torque at the finger-object interface during holding and lifting tasks with a precision grip.

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This study examined the effects of torque tangentially acting at the finger-grasped object interface during holding and lifting tasks using a precision grip. A force-torque sensor and a Fast Trak angle/position sensor were installed in a grip apparatus. In the first experiment, the subjects(n = 6)held the apparatus for some period of time and rotated it at the fingertip by decreasing the grip force. The holding grip force was linearly related to torque. The estimated coefficient of friction for rotational slips was about 7 times larger than that for linear slips for varied grip surfaces. In the second experiment, the fingertip was blocked by the injection of Lidocaine during holding the apparatus. The grip force adjustment to torque was abolished. The findings indicated that the grip force adjustment to torque was made by cutaneous afferents at the fingertip. When visual information concerning the kinematic(angle)changes of the object during the rotation task was given under the fingertip anaestheria, the grip force adjustment was improved. In the third experiment, the subjects(n = 11)lilfted the apparatus from a table. The grip apparatus had an additional and movable weight attached inside the apparatus by which varied torque conditions were created. The rate change of grip force was appropriately scaled to torque. When torque was changed without informing the subjects, the time course of initial grip force followed that of the previous lifting trial. The findings suggested that torque was used as a parameter for feedforward control of the grip force, In the fourth experiment, the effect of visual information concerning the weight distribution was given by allowing the subjects to see the additional weight attached to the grip apparatus. It was found that such visual information can actually help in adjustment of feedforward control of the grip force for lifting of objects.