Grip force adjustments evoked by load force perturbations of a grasped object

Abstract

1. Brief increases or decreases in vertical load force were applied to an object held between the thumb and finger. Grip force increases occurred consistently from 60 to 90 ms after onset of the load force increase. These responses did not adapt and were typically from 100 to 200 ms in duration. Reductions in object load force yielded rapid reductions in grip force at latencies comparable to those for load increases. 2. Response magnitude was proportional to the size or velocity of the load force increment, but did not vary with the level of the preexisting grip force. Thus these responses did not maintain the grip force at a specified level above the object's slip point. 3. Grip force responses were abolished or substantially reduced when loads were delivered directly to the hand rather than to the object. In contrast, force responses were not always abolished upon anesthetization of the thumb and finger. These results are discussed in relation to the role of cutaneous mechano-receptors of the digital pulps and proprioceptors of the arm and hand for providing necessary afferent information utilized in load-related grip force modulation. 4. Rapid and automatic grip force adjustments to load force variations may contribute importantly to grasp tasks in which the load forces vary dynamically and without complete predictability, such as in the manipulation of tools or objects that contact the environment.

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

“…When a brief and unexpected load force is applied to a grasped object, an increase in grip force is observed 60-90 ms later (Johansson and Westling 1987;Cole and Abbs 1988). This is about the time expected for a supraspinal sensorimotor reflex loop.…”

“…If changes in grip force during movement were subserved by reflexes, then we would have expected the peak grip force to lag behind the peak load force by some 60-90 ms (Johansson and Westling 1987;Cole and Abbs 1988). We would argue that the coupling between grip force and load force reflects motor commands rather than grasp mechanics.…”

“…An increase in grip force was seen some 90 ms later. This is about the time expected for a supraspinal sensorimotor reflex loop to complete (Johansson and Westling 1987;Cole and Abbs 1988). For comparison, the forces during an upward movement are shown in the top panel.…”

“…Correspondence to: J. R. Flanagan arise, after a delay, when unexpected loads are applied to grasped objects (Johansson and Westling 1987;Cole and Abbs 1988).…”

Maintaining rotational equilibrium during object manipulation: linear behavior of a highly non-linear system

“…In particular, G is modulated by the inertial forces acting during lifting an object (Johansson and Westling 1984), shaking and point-to-point arm movements Wing 1993, 1995;Tresilian et al 1993;Flanagan and Tresilian 1994;Kinoshita et al 1996) and during locomotion (Gysin et al 2003). It is also modulated by the weight of the object (Johansson and Westling 1984;Winstein et al 1991), abrupt load perturbations (Cole and Abbs 1988;Eliasson et al 1995, Serrien et al 1999, friction conditions (Cole and Johansson 1993;Cadoret and Smith 1996;Burstedt et al 1999), tangential torques (Kinoshita et al 1997) and gravity changes during parabolic flights (McIntyre et al 1998;Hermsdorfer et al 1999;Augurelle et al 2003). G has been assumed to be adjusted to expected L (Johansson and Westling 1984;Flanagan and Wing 1995).…”