The mechanisms of postural control in the coronal plane during perturbed standing

Abstract

Postural stability can be defined as a measure of the body's ability to remain upright in the presence of the frequently occurring small perturbations experienced throughout the course of daily activity. The ability to maintain balance is dependent upon the successful integration of afferent inputs from a number of sensory systems - the vestibular, vision, proprioceptive, and somatosensory systems. One method of studying the balance system is to perturb one sensory system in isolation and then measure the response. Galvanic vestibular stimulation was the method used to challenge postural control. It places a novel signal on the vestibular nerve that is transmitted along the normal vestibular pathways to both peripheral and central areas, and presents a sensory conflict with the vision and proprioception systems that must be resolved for successful balance recovery. The aim of the study was to describe the response movement, determine the control mechanisms behind the recovery movement, and speculate on the role of the vestibular system. Independent variables included the presence or absence of vision, direction of stimulus current, and duration of stimulus. The outcome measures used to describe the response include kinematic variables (linear and angular displacement of body segments - magnitude, duration and onset latencies), kinetic variables (GRF, CofP, resultant joint moments - magnitude and onset latencies), and EMG of selected postural muscles of the lower leg (gluteus medius, adductor magnus, tibialis anterior, and peroneus longus). Eighteen subjects took part in two separate experiments.

The perturbation caused the head, trunk and pelvis to move in a medial/lateral direction in the manner of an inverted pendulum rotating about the pelvis centre of mass. This response was consistent across subjects and all conditions. Kinetic results show a four phase response for the longer duration perturbation and a three phase response for the short perturbation. Analysis of the joint moments confirmed kinematic results, showing most of the control of the movement focussed on the hip abductors/adductors. EMG analysis revealed the use of two strategies for movement: a 'hip dominant' and a 'hip/ankle combination' strategy. The foot placement adopted in the experimental protocol is thought to preclude the use of any possible 'ankle' strategy.

The type of perturbation used in the study does not trigger early postural responses through reflex activity. Its role is to aid in the determination of the orientation of the head and body in space - a role shared with the proprioceptive and vision systems. The vestibular system may have more influence than the proprioception system under the conditions tested. Because of possible VOR involvement, a comparison of the relative importance of vision and the vestibular system could not be performed.