Abstract
In everyday life, one of the most frequent activities involves accelerating and decelerating an object held in precision grip. In many contexts, humans scale and synchronize their grip force (GF), normal to the finger/object contact, in anticipation of the expected tangential load force (LF), resulting from the combination of the gravitational and the inertial forces. In many contexts, GF and LF are linearly coupled. A few studies have examined how we adjust the parameters–gain and offset–of this linear relationship. However, the question remains open as to how the brain adjusts GF regardless of whether LF is generated by different combinations of weight and inertia. Here, we designed conditions to generate equivalent magnitudes of LF by independently varying mass and movement frequency. In a control experiment, we directly manipulated gravity in parabolic flights, while other factors remained constant. We show with a simple computational approach that, to adjust GF, the brain is sensitive to how LFs are produced at the fingertips. This provides clear evidence that the analysis of the origin of LF is performed centrally, and not only at the periphery.
Highlights
There is evidence that when holding an object with a precision grip, a minimal grip force (GF, normal to the contact surfaces) must be applied to prevent the object from slipping under the influence of load forces (LF, tangential to the contact surfaces)
Figure 2A shows an example of GF and LF over time for a single participant moving a mass of 0.583 kg (M6) at the frequency of 1.33 Hz
A condition in which gravity itself could be manipulated–but not mass– led to different gain adjustments, for very comparable ranges of LF. In this experiment, we evaluated whether the mass and/or frequency of oscillations influence the control of GF as measured through a linear regression between GF and LF
Summary
There is evidence that when holding an object with a precision grip, a minimal grip force (GF, normal to the contact surfaces) must be applied to prevent the object from slipping under the influence of load forces (LF, tangential to the contact surfaces). In a large panel of tasks, humans scale and synchronize their GF in anticipation of the expected LF. This tight coordination between GF and LF has been shown when transporting objects (Flanagan and Tresilian, 1994), during locomotion (Gysin et al, 2003) or when the load at the fingertips depends on position (Descoins et al, 2006), velocity (Flanagan and Wing, 1997), acceleration (Flanagan et al, 1993; Flanagan and Rao, 1995), and even gravity (Augurelle et al, 2003)
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