Grip-force modulation in multi-finger prehension during wrist flexion and extension

Abstract

Extrinsic digit muscles contribute to both fingertip forces and wrist movements (FDP and FPL – flexion, EDC - extension). Hence it is expected that finger forces depend on the wrist movement and position. We investigated the relation between grip force and wrist kinematics to examine whether and how the force: (1) scales with wrist flexion-extension (FE) angle; (2) can be predicted from accelerations induced during FE movement. In one experiment subjects naturally held an instrumented handle using a prismatic grasp and performed very slow FE movements. In another experiment, the same movement was performed cyclically at three prescribed frequencies. In quasistatic conditions, the grip force remained constant over the majority of the wrist range of motion. During the cyclic movements, the grip force changed. The changes were described with a linear regression model that represents the thumb and virtual finger (VF = four fingers combined) normal forces as the sum of the effects of the object’s tangential and radial accelerations and an object-weight-dependent constant term. The model explained 99% of the variability in the data. The independence of the grip force from wrist position agrees with the theory that that the thumb and VF forces are controlled with two neural variables that encode referent coordinates for each digit while accounting for changes in the position dependence of muscle forces, rather than a single neural variable like referent aperture. The results of the cyclical movement study extend the principle of superposition (some complex actions can be decomposed into independently controlled elemental actions) for a motor task involving simultaneous grip force exertion and wrist motion with significant length changes of the grip-force producing muscles.

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

“…While AS varied, this variation was limited to postures in a close proximity to the fully flexed wrist position. This observation is consistent with our earlier study (Ambike et al 2013) wherein the “natural grip force” for the extreme wrist flexion was higher than for all other wrist positions. Other wrist positions had no effect on AS; so, for those positions Hypothesis-1 is rejected by the data.…”

“…Furthermore, we manipulated the wrist flexion-extension angle thus changing the length of extrinsic muscles, and observed variations in the maximal grip strength according to the classic force-length curve (McMahon 1984). Rather unexpectedly, however, the submaximal, ‘natural’ grip force was independent of the wrist position (Ambike et al 2013). …”

Task-specific stability of abundant systems: Structure of variance and motor equivalence

“…In earlier studies, an interpretation has been offered of the force drift (and also of the hand position drift in multi-joint tasks, Zhou et al 2014;Zhou et al 2015) based on the idea of RC control (Ambike et al 2013;Zhou et al 2014;Ambike et al 2015). According to this idea, RC for a performance variable drifts slowly towards the actual value of this variable (RC-back-coupling) reflecting the natural tendency of physical systems to move towards a minimum of potential energy.…”

“…Removing visual feedback during steady-state accurate force production tasks is known to lead to a slow drift in the force level (Slifkin et al 2000;Vaillancourt et al 2001;Vaillancourt and Russell 2002;Ambike et al 2013). In those earlier studies, the subjects were not explicitly instructed to correct target forces, while our subjects were always instructed to keep the F TOT and M TOT values at the target level.…”

Prehension Synergies During Fatigue of a Single Digit: Adaptations in Control With Referent Configurations

“…The principle has been shown to hold in a variety of experimental conditions (Ambike, Paclet, Latash, & Zatsiorsky, 2013; Niu, Latash, & Zatsiorsky, 2012; Robertson & Johnston, 2012; Shim, Latash, & Zatsiorsky, 2005b). In a recently published paper (Singh, Zatsiorsky, & Latash, 2013) we showed that adaptations to fatigue of a single digit violate the principle of superposition.…”