Changes in pinch force with bidirectional load forces.

The forces used to grasp an object were measured while positive (push) and negative (pull) load forces were applied to the hand under varying frictional conditions. Subjects held between the tips of their thumb and index finger a manipulandum composed of two symmetrically mounted disks. The manipulandum was connected to the stage of an electromagnetic linear motor that generated load forces under computer control. In the first experiment, subjects held the position of the manipulandum constant while the motor generated forces in first the positive and then the negative direction. The motor force at which the manipulandum slipped from the fingers was measured in the second experiment. In both experiments, friction was varied by changing the surface (sandpaper, suede, or plastic) of the manipulandum disks. The pinch forces produced by subjects were linearly related to changes in motor force in both the positive and negative directions, with the slope of this relation varying as a function of the surface properties of the manipulandum. The modulation of pinch force with motor force was influenced, however, by the direction of the load force; higher forces were produced in response to negative load forces. Slip forces varied as a function of pinch force and surface texture; higher forces were associated with materials with lower coefficients of friction. These findings suggest that the friction between the skin and an object being grasped changes as a function of the direction of force that the object applies to the skin, possibly due to the anisotropic nature of glabrous skin, and that this mechanical property contributes to variations in pinch force.