Variation of initial grasping force with, length of an object

We investigate the variation of the initial grasping force of humans with the size of an object. The initial force is defined as the force applied by the fingers just before an object is lifted from the surface. Experiments were carried out in which subjects were asked to lift different objects, using a two-fingered precision grip, while the rest of the arm was stationary. The length or the weight of the objects were changed in each lifting trial. In the absence of visual cues the applied initial finger forces can be attributed to the motor control strategy decided by the brain, based on the afferent sensory information obtained from the finger tip sensors. It is reasonable to expect that there is some functional relation between the initial force and the size of an object. Our goal was to understand more about this relation or reasoning system, which is utilized by the computational process presumed to underlie sensory-motor behaviour. From the experimental results it appears that when humans grasp different objects the initial grasping force increases with object size, although this increase is nonlinear.