Factors influencing the force control during precision grip.

A small object was gripped between the tips of the index finger and thumb and held stationary in space. Its weight and surface structure could be changed between consecutive lifting trials, without changing its visual appearance. The grip force and the vertical lifting force acting on the object, as well as the vertical position of the object were continuously recorded. Likewise, the minimal grip force necessary to prevent slipping, was measured. The difference between this minimal force and the employed grip force, was defined as the safety margin to prevent slipping. It was found that the applied grip force was critically balanced to optimize the motor behaviour so that slipping between the skin and the gripped object did not occur and the grip force did not reach exceedingly high values. To achieve this motor control, the nervous system relied on a mechanism that measured the frictional condition between the surface structure of the object and the fingers. Experiments with local anaesthesia indicated that this mechanism used information from receptors in the fingers, most likely skin mechanoreceptors. In addition to friction, the control of the grip force was heavily influenced by the weight of the object and by a safety margin factor related to the individual subject. The frictional conditions during the previous trial could also, to some extent, influence the grip force.