Viscoelastic properties of the human wrist during the stabilization phase of a targeted movement

Abstract

Joint stability is a critical factor in the control of human movement.An understanding of the methods by which the neuromuscular system maintains mechanical stability in the face of a wide range of changing mechanical environments is important in the accurate modeling of motor control dynamics.Under normal operating conditions, the neuromuscular system is able to rapidly modulate the mechanical impedance of a joint in order to maintain stability during postural or movement tasks.This is particularly evident following a targeted movement; the mechanics of the limb is controlled so that motion is stopped as quickly as possible with minimal oscillation about the final position.In fact, the human motor control system is very good at this type of task considering that rapid voluntary movements are most often stopped without any noticeable oscillation.The mechanical impedance of a joint consists of inertial, viscous, and elastic components.Its mechanical properties are determined primarily by the activation of the muscles that act about the joint.Stability is therefore effected by intrinsic muscle properties and proprioceptive feedback.It has been suggested that reflex feedback from muscle proprioceptors is responsible for damping oscillations.There is however, a body of evidence that suggests the central nervous system may more readily employ viscoelastic properties intrinsic to the muscle as a means of controlling joint stability.The intrinsic elastic property of muscle has been investigated in detail, especially with respect to its role in postural control.Viscous properties have not yet been studied in great depth.The objective of this study was to investigate the intrinsic and reflexive viscoelastic properties of the human wrist during the stabilization phase of a targeted voluntary movement.The oscillation of the hand about its final position was exaggerated by reducing the viscosity of the wrist joint with the aid of a robot manipulandum.The velocity trace was modeled to estimate the viscoelastic properties of the joint.Wrist stiffness increased when the target width was decreased.Both stiffness and viscosity increased for faster movements and decreased when the distance to the target was increased.I owe a great deal of thanks to my supervisor, Dr. Ted Milner, for his dedication, support, and insistence in high standards of research; for allowing