Factors influencing variability in load forces in a tripod grasp

The aim of this investigation was to determine the factors that influence the variability in the load force during a precision grip. Subjects grasped an object with three coarse contact surfaces using a tripod grasp of the thumb, index and middle fingers. The orientations of the contact surfaces and their locations relative to the object's center of mass were varied. Forces and torques exerted by each of the three digits were measured or computed from the equilibrium conditions and analyzed during the static hold phase. The static load force depends on the object's weight, but also on the precise location of the center of pressure of each digit, as well as on the magnitude of the tangential torque and horizontal grip force. Variations in these parameters caused deviations in the load forces by as much as 20% or more from their nominal values. For the thumb, the largest source of variability was the vertical location of the center of pressure. For the two fingers, variations in the locations of the centers of pressure (along the horizontal and vertical) and in the magnitudes of the tangential torques had effects that were about equal in magnitude. Since many of the sources of variability in load force are unpredictable, these results point to the importance of online feedback regulation of grip force.