Control of grasp stability in humans under different frictional conditionsduring multidigit manipulation

Control of grasp stability in humans under different frictional conditions during multidigit manipulation. J. Neurophysiol. 82. 2393-2405, 1999. Contr ol of grasp stability under different frictional conditions has primarily b een studied in manipulatory tasks involving two digits only. Recently we fo und that many of the principles for control of forces originally demonstrat ed for two-digit grasping also apply to various three-digit grasps. Here we examine the control of grasp stability in a multidigit task in which subje cts used the tips of the thumb, index, and middle finger to lift an object. The grasp resembled those used when lifting a cylindrical object from abov e. The digits either all contacted the same surface material or one of the digits contacted a surface material that was more, or less, slippery than t hat contacted by the other two digits. The three-dimensional forces and tor ques applied by each digit and the contact positions were measured along wi th the position and orientation of the object. The distribution of forces a mong the digits strongly reflected constraints imposed by the geometric rel ationship between the object's center of mass and the contact surfaces. On top of this distribution, we observed changes in force coordination related to changes in the combination of surface materials. When all digits contac ted the same surface material, the ratio between the normal force and tange ntial load (F-n:L ratio) was similar across digits and scaled to provide an adequate safety margin against slip. With different contact surfaces subje cts adapted the F-n:L ratios at the individual digits to the local friction with only small influences by the friction at the other two digits. They a ccomplished this by scaling the normal forces similarly at all digits and c hanging the distribution of load among the digits. The surface combination did not, however, influence digit position, tangential torque, or object ti lting systematically. The change in load distribution, rather, resulted fro m interplay between these factors, and the nature of this interplay varied between trials. That is, subjects achieved grasp stability with various com binations of fingertip actions and appeared to exploit the many degrees of freedom offered by the multidigit grasp. The results extend previous findin gs based on two-digit tasks to multidigit tasks by showing that subjects ad just fingertip forces at each digit to the local friction. Moreover, our fi ndings suggest that subjects adapted the load distribution to the current f rictional condition by regulating the normal forces to allow slips to occur early in the lift task, prior to object lift-off.