Control of fingertip forces in multidigit manipulation

Previous studies of control of fingertip forces in skilled manipulation hal e focused on tasks involving two digits, typically the thumb and index fing er. Here He examine control of fingertip actions in a multidigit task in wh ich 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 similar to 5.54 cm from either finger. The three-d imensional fords rind torques applied by each digit and die digit contact p ositions were measured along with the position and orientation of the objec t. The vertical forces applied tangential to the grasp surfaces to lift the object were synchronized across the digits, and the contribution by each d igit to the total vertical force reflected intrinsic object properties (geo metric relationship between the object's center of mass and the grasped sur faces). Subjects often applied small torques tangential to the grasped surf aces even though the object could have been lifted without such torques. Th e normal forces generated by each digit increased in parallel with the loca l tangential load (force and torque) providing an adequate safety margin ag ainst slips Lit each digit. In the present task, the orientations of the fo rce vectors applied by the separate digits were not fully; constrained and therefore the motor controller had to choose from a number of possible solu tions. 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 t he digits that included changes in the distribution of vertical forces acro ss 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 co ntact positions. In conclusion, we hale shown that when lifting an object w ith ii three-digit grip. the coordination of fingertip forces, in many resp ects, matches a hat has been documented previously for two-digit grasping. At the same time, our study reveals novel aspects of force control that eme rge only in multidigit manipulative tasks.