Adjustments to Local Friction in Multifinger Prehension

Abstract

The authors studied the effects of surface friction at the digit-object interface on digit forces and moments when 12 participants statically held an object in a 5-digit grasp. The authors changed lowfriction contact (LFC) with rayon and high-friction contact (HFC) with sandpaper independently for each digit in all 32 possible combinations. Normal forces of the thumb and virtual finger (VF), an imagined finger with a mechanical effect equal to that of the 4 fingers, increased with the thumb at LFC or with an increase in the number of fingers at LFC. When the thumb was at LFC, the thumb tangential force decreased. The VF tangential force decreased when the number of fingers at LFC increased. The interaction of the local responses to friction and the synergic responses necessary to maintain the equilibrium explain the coordination of individual digit forces.Keywords finger forces; friction; grasp stability; grasping; prehension When grasping an object, people adjust digit forces to the friction: More-slippery objects are grasped more strongly, resulting in a higher ratio of grip force (digit force normal to the grip surface) to load force (digit force tangential to the grip surface; Cadoret & Smith, 1996;Cole & Johansson, 1993; Johansson, 1996;Johansson & Westling, 1984b, 1987. Some researchers have examined participants' force adjustments when they used two or three digits to grasp objects that had a different friction for each digit (Burstedt, Edin, & Johansson, 1997;Burstedt, Flanagan, & Johansson, 1999;Edin, Westling, & Johansson, 1992;Quaney & Cole, 2004). The researchers found that a grip-load force ratio for individual digits was scaled on the basis of current local friction at those digits. Participants made the adjustments by modulating normal (grip force) and tangential (load force) digit forces. Similar force adjustments occur when two individuals do the grasping (Burstedt et al., 1997). It has therefore been concluded that coordination of individual digit-tip forces during human manipulation emerges from independent neural networks that control each engaged digit. That low level of control must be subordinate to a higher level of control related to other aspects of manipulative tasks. The investigators mentioned such aspects of the higher level control as (a) temporal synchronization of digit actions before substantial manipulative forces are applied to the object, (b) selecting grasp configurations, and (c) coping with various constraints imposed by the physical properties of the object, such as its weight and size. NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author ManuscriptCompared with the two-digit and tripod grasps, in grasps performed by all five digits, performers undoubtedly have greater freedom to select interdigit force sharing. During multifinger prehension, however, the forces and moments exerted on the object by individual digits always change in a highly coordinated manner (Baud-Bovy & Soechting, 2001; Iberall, 1987;Kinoshita, Kawai, & Ikuta, 1995;Santello & Soecht...

Temporary Nerve Block at Selected Digits Revealed Hand Motor Deficits in Grasping Tasks

“…Local and non-local responses have been examined in some earlier studies during submaximal grip force tasks in healthy hands. Aoki et al (2007) investigated the role of tactile information in adapting grip object properties, and found that changes in texture at a given digit elicit force adjustments at the same as well as other digits, indicating that sensory information at one digit affects the force modulation at non-stimulated digits. Our current results further implied that individual sensory input is shared across all the digits and the disturbed signal from local sensory channel(s) has a more comprehensive impact on the process of the motor output execution in the sensorimotor integration.…”

Factors affecting grip force: anatomy, mechanics, and referent configurations

“…Since the handle was not perfectly symmetrical and there was a residual digit moment (see Appendix A), changes in the normal forces could lead to violation of the handle orientation requirement, in particular because of the changes in the lever arms for the vertical tangential forces. Earlier studies have shown so-called chain effects (Gao et al 2005; Aoki et al 2007; Zatsiorsky and Latash 2008) when a change in only a subset of digit forces and/or other features of the task (such as handle geometry and friction conditions) lead to changes in many other variables caused by the static constraints.…”

Human Hand as a Parallel Manipulator

“…an effect of friction at a given digit on the force exerted by this digit. The term synergic refers to changes in a finger's force in response to changes in friction under other finger(s) (Aoki et al 2006(Aoki et al , 2007Zatsiorsky et al 2006). An example of the local and synergic reactions could be seen in experiments in which the friction under each digit was different, either high or low, resulting in eight friction conditions (for the three-digit grasps, Niu et al 2007) or 32 friction conditions (Aoki et al 2007).…”

“…For instance, to keep the total tangential force constant, the tangential forces of the thumb and fingers changed in opposite directions (Figure 10). Not only the VF tangential force but also the tangential forces of the individual fingers are affected by the local friction conditions (Aoki et al 2007;Niu et al 2007). The tangential force adjustments to the local friction support the notion that the VF tangential force sharing is under neural control; the sharing percentage is not determined solely by the passive mechanical properties of the individual fingers and joints.…”