Are Complex Control Signals Required for Human Arm Movement?

• Published in: Journal of neurophysiology 1998-03, Vol.79 (3), p.1409-1424
• Main Authors: Gribble, Paul L, Ostry, David J, Sanguineti, Vittorio, Laboissiere, Rafael
• Format: Article
• Language: English
• Subjects: Arm - innervation; Cognitive science; Hand; Humans; Joints - physiology; Mathematics; Models, Biological; Motor Activity - physiology; Movement - physiology; Muscle Contraction; Muscle, Skeletal - innervation; Muscle, Skeletal - physiology; Neuroscience; Reaction Time; Reflex; Space life sciences
• ISSN: 0022-3077; 1522-1598
• DOI: 10.1152/jn.1998.79.3.1409

Paul L. Gribble 1 , David J. Ostry 1 , Vittorio Sanguineti 2 , and Rafael Laboissière 3 1  Department of Psychology, McGill University, Montreal, Quebec H3A 1B1, Canada; 2  Department of Informatics, Systems and Telecommunications, University of Genova, Genova, Italy 16145; and 3  Institut de la Communication Parlée, Grenoble, France 38031 Gribble, Paul L., David J. Ostry, Vittorio Sanguineti, and Rafael Laboissière. Are complex control signals required for human arm movement? J. Neurophysiol. 79: 1409-1424, 1998. It has been proposed that the control signals underlying voluntary human arm movement have a "complex" nonmonotonic time-varying form, and a number of empirical findings have been offered in support of this idea. In this paper, we address three such findings using a model of two-joint arm motion based on the  version of the equilibrium-point hypothesis. The model includes six one- and two-joint muscles, reflexes, modeled control signals, muscle properties, and limb dynamics. First, we address the claim that "complex" equilibrium trajectories are required to account for nonmonotonic joint impedance patterns observed during multijoint movement. Using constant-rate shifts in the neurally specified equilibrium of the limb and constant cocontraction commands, we obtain patterns of predicted joint stiffness during simulated multijoint movements that match the nonmonotonic patterns reported empirically. We then use the algorithm proposed by Gomi and Kawato to compute a hypothetical equilibrium trajectory from simulated stiffness, viscosity, and limb kinematics. Like that reported by Gomi and Kawato, the resulting trajectory was nonmonotonic, first leading then lagging the position of the limb. Second, we address the claim that high levels of stiffness are required to generate rapid single-joint movements when simple equilibrium shifts are used. We compare empirical measurements of stiffness during rapid single-joint movements with the predicted stiffness of movements generated using constant-rate equilibrium shifts and constant cocontraction commands. Single-joint movements are simulated at a number of speeds, and the procedure used by Bennett to estimate stiffness is followed. We show that when the magnitude of the cocontraction command is scaled in proportion to movement speed, simulated joint stiffness varies with movement speed in a manner comparable with that reported by Bennett. Third, we address the related claim that nonmonotonic equilibrium shi