To grip and not to slip : sensorimotor mechanisms in reactive control of grasp stability

Abstract: The reactive control of fingertip forces maintaining grasp stability was examined in man during a prehensile task. Blindfolded subjects used the precision grip between the tips of index finger and thumb to restrain an object that was subjected to unpredictable load forces. These were delivered tangential to the parallel grip surfaces of the object. Load forces, grip forces (perpendicular to the grip surfaces) and position of the object were recorded.Subjects automatically adjusted the grip forces to loads of various amplitudes and rates. Thereby they maintained a reliable safety margin against frictional slips without using excessive grip forces. A rapid rise in grip force lasting about 0.2 s was triggered after a short delay following the onset of a sustained ramp load increase. This 'catch-up' response caused a quick restoration of an adequate grip:load force ratio that prevented frictional slips. If the ramp load continued to increase after the catchup response, the grip force also increased in parallel with the load change in a 'tracking' manner. Consequently, during the hold phases of 'ramp-and-hold' loads, the employed grip forces were approximately proportional to the load amplitude. Sensory information about the rate of change of the load force parametrically scaled the 'catchup' and 'tracking' responses.Following anesthetic block of sensory input from the digits, the grip responses were both delayed and attenuated or even abolished. To compensate for these impairments, subjects had to voluntarily maintain exceedingly high grip forces to prevent the object from slipping. The grip control improved slightly during hand and forearm support conditions that allowed marked wrist movements to occur in response to the loading. This indicates that signals from receptors in muscles, joints or skin areas proximal to the digits can to some extent be used to adjust grip forces during impaired digital sensibility. In contrast, these signals had only minor influence on the control during normal digital sensibility.Grip responses to loads delivered in various directions revealed that the load direction, in relation to gravity and to the hand's geometry, represents intrinsic task variables in the automatic processes that maintain a stable grasp. The load direction influenced both the response latencies and the magnitudes of the grip responses. The response latencies were shortest for loads in directions that were the most critical with regard to the consequences of frictional slippage, i.e., loads directed away from the palm or in the direction of gravity. Recordings of signals in cutaneous afferents innervating the finger tips demonstrated that these effects on the response latencies depended on differences in the time needed by the central nervous system to implement the motor responses. The short latencies in the most ‘criticar load directions may reflect the preparation of a default response, while additional central processing would be needed to execute the response to loads in other directions. Adjustments to local frictional anisotropies at the digit-object interface largely explained the magnitude effects.In conclusion, grip responses are automatically adjusted to the current loading condition during unpredictable loading of a hand held object. Subjects call up a previously acquired sensorimotor transform that supports grasp stability by preventing both object slippage and excessive grip forces. Cutaneous sensory information about tangential forces and frictional conditions at the digit-object interface is used to initiate and scale the grip responses to the current loading conditions, largely in a predictive manner.