Control of Movement Variability and the Regulation of Limb Impedance

Abstract

Humans routinely make movements to targets that have different accuracy requirements in different directions. Examples extend from everyday occurrences such as grasping the handle of a coffee cup to the more refined instance of a surgeon positioning a scalpel. The attainment of accuracy in situations such as these might be related to the nervous system's capacity to regulate the limb's resistance to displacement, or impedance. To test this idea, subjects made movements from random starting locations to targets that had shape-dependent accuracy requirements. We used a robotic device to assess both limb impedance and patterns of movement variability just as the subject reached the target. We show that impedance increases in directions where required accuracy is high. Independent of target shape, patterns of limb stiffness are seen to predict spatial patterns of movement variability. The nervous system is thus seen to modulate limb impedance in entirely predictable environments to aid in the attainment of reaching accuracy.

Anticipatory Adjustments to Abrupt Changes of Opposing Forces

“…The involuntary movements that can be observed with the tasks of the present experiment should be affected by the anisotropy of limb impedance (Darainy, Towhidkhah, & Ostry, 2007;Gomi & Kawato, 1997;Gordon, Ghilardi, Cooper, & Ghez, 1994;Lametti, Houle, & Ostry, 2007;Tsuji, Morasso, Goto, & Ito, 1995;Wong, Wilson, Malfait, & Gribble, 2009). Given that anticipatory adjustments could also serve to modulate the impedance of the arm (Biryukova et al, 1999), their effects should be stronger in those directions in which impedance is small without such modulations.…”

“…For example, there can be cocontractions that modulate the impedance of the limb in a way similar to how impedance varies across movement directions (Durainy et al, 2007;Gomi & Kawato, 1997;Lametti et al, 2007;Tsuji et al, 1995;Wong et al, 2009). The presence of such modulations is suggested by the observation that anticipatory adjustments in Experiment 1 had smaller effects for movement directions for which limb impedance is high anyway than for movement directions with smaller impedance.…”

The concept of mobility in single- and double handed manipulation

“…See, for instance Sabes et al (1998), Hamilton and Wolpert(2002), Lametti et al (2007), Selen et al (2009), andCos et al (2011). The experimental condition has typically involved a model of the arm as a planar three-link mechanism.…”

Patterns of hypermetria and terminal cocontraction during point-to-point movements demonstrate independent action of trajectory and postural controllers

“…If agonist/ antagonist coactivity increases at the end of a reach to increase joint stiffness (Gribble et al 2003;Lametti et al 2007; see also Hannaford and Stark 1985), for example in anticipation of a possible environmental perturbation (cf. Hogan 1984; Loeb et…”

“…If agonist/ antagonist coactivity increases at the end of a reach to increase joint stiffness (Gribble et al 2003;Lametti et al 2007; see also Hannaford and Stark 1985), for example in anticipation of a possible environmental perturbation (cf. Hogan 1984;Loeb et al 1990), then muscle activity early in the movement must also change if movement kinematics are to be preserved primarily because agonist/antagonist coactivation increases joint viscosity as well as joint stiffness (Gomi and Osu 1998;Lacquaniti et al 1993). Such change in the feedforward control of reaches should have no impact on subsequent reversals if subjects use independent unitary control actions for the two tasks (cf.…”