Roles of glabrous skin receptors and sensorimotor memory in automatic control of precision grip when lifting rougher or more slippery objects

Abstract

To be successful, precision manipulation of small objects requires a refined coordination of forces excerted on the object by the tips of the fingers and thumb. The present paper deals quantitatively with the regulation of the coordination between the grip force and the vertical lifting force, denoted as the load force, while small objects were lifted, positioned in space and replaced by human subjects using the pinch grip. It was shown that the grip force changed in parallel with the load force generated by the subject to overcome various forces counteracting the intended manipulation. The balance between the two forces was adapted to the friction between the skin and the object providing a relatively small safety margin to prevent slips, i.e. the more slippery the object the higher the grip force at any given load force. Experiments with local anaesthesia indicated that this adaptation was dependent on cutaneous afferent input. Afferent information related to the frictional condition could influence the force coordination already about 0.1 s after the object was initially gripped, i.e. approximately at the time the grip and load forces began to increase in parallel. Further, "secondary", adjustments of the force balance could occur later in response to small short-lasting slips, revealed as vibrations in the object. The new force balance following slips was maintained, indicating that the relationship between the two forces was set on the basis of a memory trace. Its updating was most likely accounted for by tactile afferent information entering intermittently at inappropriate force coordination, e.g. as during slips. The latencies between the onset of such slips and the appearance of the adjustments (0.06-0.08 s) clearly indicated that the underlying neural mechanisms operated highly automatically.

Modulation of grip force with load force during point-to-point arm movements

“…the observed grip force minus the minimum grip force required to prevent slippage) is also greater for heavy and slippery objects. Indeed, Johansson and Westling (1984) have observed that the ratio of excess to minimum grip force is approximately constant across all objects. They have suggested that the excess grip force guards against random fluctuations in the motor system and that grip force is scaled with load force in order to economize effort.…”

“…For example, when an object is lifted, grip force increases simultaneously with load force prior to lift-off (Johansson and Westling 1984). The rate of increase of grip force and the final grip force depend on the object's weight and its surface texture.…”

“…This is followed by the holding phase where the object is held aloft. Johansson and Westling (1984) have reported that, during the loading phase, the maximum rate of change of grip force depends on the final grip force at lift-off (and during the subsequent holding phase). The greater the final grip force, the larger the rate of change.…”

Finger synergies during multi-finger cyclic production of moment of force

“…Most studies of finger interaction have focused on the production of adequate gripping force (Johansson and Westling 1984;Flanagan and Wing 1995;Burstedt et al 1999;Santello and Soechting 2000;Nowak and Hermsdorfer 2005). Only a handful of studies explicitly addressed the rotational action by the hand (e.g., Johansson et al 1999;Latash et al 2004).…”

Effect of laparoscopic grasper force transmission ratio on grasp control

“…The pinch force modifications were in phase with the changes in load force [19]. Slips rarely occurred as the pinch force exceeded the minimum force required to prevent slippage according to a slip margin determined by the skin object friction [20].…”