Task-Dependent Viscoelasticity of Human Multijoint Arm and Its Spatial Characteristics for Interaction with Environments

Abstract

Human arm viscoelasticity is important in stabilizing posture, movement, and in interacting with objects. Viscoelastic spatial characteristics are usually indexed by the size, shape, and orientation of a hand stiffness ellipse. It is well known that arm posture is a dominant factor in determining the properties of the stiffness ellipse. However, it is still unclear how much joint stiffness can change under different conditions, and the effects of that change on the spatial characteristics of hand stiffness are poorly examined. To investigate the dexterous control mechanisms of the human arm, we studied the controllability and spatial characteristics of viscoelastic properties of human multijoint arm during different cocontractions and force interactions in various directions and amplitudes in a horizontal plane. We found that different cocontraction ratios between shoulder and elbow joints can produce changes in the shape and orientation of the stiffness ellipse, especially at proximal hand positions. During force regulation tasks we found that shoulder and elbow single-joint stiffness was each roughly proportional to the torque of its own joint, and cross-joint stiffness was correlated with elbow torque. Similar tendencies were also found in the viscosity-torque relationships. As a result of the joint stiffness changes, the orientation and shape of the stiffness ellipses varied during force regulation tasks as well. Based on these observations, we consider why we can change the ellipse characteristics especially in the proximal posture. The present results suggest that humans control directional characteristics of hand stiffness by changing joint stiffness to achieve various interactions with objects.

A model of open-loop control of equilibrium position and stiffness of the human elbow joint

“…Indeed, the K ilf values derived in this study were in the lower region of the wide range of total joint stiffness values (14-126 N m·rad −1 ) reported in the literature (e.g. Bennet et al 1992;Lacquaniti et al 1993;Gomi and Osu 1998). The large variation of stiffness values supports the claim of Latash and Zatsiorsky (1993) that neither the definition nor the experimental determination of stiffness is without problems.…”

“…The large variation of stiffness values supports the claim of Latash and Zatsiorsky (1993) that neither the definition nor the experimental determination of stiffness is without problems. In particular, Gomi et al (Gomi and Kawato 1997;Gomi and Osu 1998) suggested that the large variation in reported stiffness values is related to differences in experimental setup and instructions. As one example, in the study of Lacquaniti et al (1982), in which stiffness values up to 126 N m·rad −1 were reported, subjects were instructed to maximally resist a deterministic perturbation.…”

“…Because subjects knew beforehand that a perturbation was going to occur, it is quite likely that they modified the supra-spinal input to the muscles, which may explain why the "stiffness" reported was twice the maximal stiffness value reported in other studies (e.g. Gomi and Osu 1998;Popescu et al 2003). As another example, stiffness is typically estimated by fitting a second-order model to the response to quasi-random perturbations.…”

Learning the dynamics of reaching movements results in the modification of arm impedance and long-latency perturbation responses

“…A number of previous reports have estimated human arm impedance during multi-joint voluntary movements (Gomi and Kawato 1997;Gomi and Osu 1998;Dolan et al 1993;Lacquaniti et al 1993). In these works, small perturbations were applied to the limb and the restoring forces were approximated as a function of displacement.…”

Learning to Control Arm Stiffness Under Static Conditions

“…A number of studies have examined muscle co-contraction in single joint movement in the context of unstable mechanical loads (e.g., De Serres and Milner 1991;Milner 2002a). In work on two-joint movement, evidence that subjects can alter the pattern of arm impedance has been reported in studies that have assessed stiffness during movement (Burdet et al 2001; and in studies in which the limb is stationary but when the subject is actively applying force Gomi and Osu 1998;McIntyre et al 1996;Perreault et al 2002). However, in the two-joint arm, little is known about the control of impedance when the limb is stationary and net force output is zero.…”