Control of Grip Force When Tilting Objects: Effect of Curvature of Grasped Surfaces and Applied Tangential Torque

Abstract

When we manipulate objects in everyday tasks, there are variations in the shape of the grasped surfaces, and the loads that potentially destabilize the grasp include time-varying linear forces and torques tangential to the grasped surfaces. Previous studies of the control of fingertip forces for grasp stability have dealt principally with flat grip surfaces and linear force loads. Here, we studied the regulation of grip force with changes in curvature of grasped surfaces and changes in tangential torque applied by the index finger and thumb when humans lifted an object and rotated it about the horizontal grip axis through an angle of 65 degrees. The curvatures of the matched pair of spherical surfaces varied from -50 m-1 (concave with radius 20 mm) to 200 m-1 (convex with radius 5 mm). The applied tangential torque at the orientation of 65 degrees was varied sixfold. Regardless of the values of curvature and end torque, grip force and tangential torque were coordinated, increasing in parallel throughout the tilt with an approximately linear relationship; the slope of the line increased progressively with increasing surface curvature. This parametric scaling of grip force was directly related to the minimum grip force required to prevent rotational slip, resulting in an adequate safety margin against slip in all cases. We conclude that surface curvature parametrically influences grip force regulation when the digits are exposed to torsional loads. Furthermore, the sensorimotor programs that control the grip force apparently predict the effect of the total load comprising linear forces and tangential torques.

Role of the Primary Motor and Dorsal Premotor Cortices in the Anticipation of Forces during Object Lifting

“…Both shape and size can influence the anticipatory scaling of forces that are applied during the lifting of small objects (Gordon et al, 1991a,b;Johansson, 1997, 2000;Goodwin et al, 1998;Flanagan et al, 2001). The dorsal premotor cortex selects movements based mainly on learned associations as opposed to the more pragmatic visual and somatosensory analyses of shape and size (Geyer et al, 2000).…”

Transfer of Learned Manipulation following Changes in Degrees of Freedom

“…One paradigm that has been found useful for such studies involves lifting an object whose weight distribution is asymmetric, thus requiring the digits to exert not only a net vertical force but also a torque to prevent rolling of the object (Goodwin et al, 1998;Salimi et al, 2000Salimi et al, , 2003Lukos et al, 2007Lukos et al, , 2008. Presented with this task, subjects learn to modulate digit forces as a function of digit placement to exert a compensatory torque in an anticipatory fashion (i.e., before lifting the object) .…”

Effects of friction at the digit-object interface on the digit forces in multi-finger prehension

“…Grip force also increases in response to the tangential torque, i.e. the torque in the tangential plane at the digitsurface interface (Goodwin et al 1998;Kinoshita et al 1997). …”

“…If the object were a cup filled with liquid, the liquid would be spilled. (2) The external torque exerted on the object was either zero or was applied in the tangential plane (Goodwin et al 1998;Kinoshita et al 1997); in the latter case the rotational equilibrium can be maintained by simply exerting larger grip force on the object. (3) In majority of studies a two-digit pinch grip was used.…”