Head and trunk movement strategies in quiet stance : from the deficit of vestibular loss to the expertise of tightrope walkers via prosthetic feedback

Is the head more locked to the trunk or stabilised in space during quite stance? Does prosthetic vestibular feedback have a positive impact on movement strategies and muscle synergies of those with vestibular loss? Does training in tandem stance lead to improved movement strategies and/or reweighting of sensory inputs? These questions have not been answered to date. This thesis attempted to answer these questions with appropriate, but new, techniques. The coordination of the head with respect to the trunk and pelvis during quiet, feet side by side, stance in normal and vestibular loss subjects was examined as well as the effect of prosthetic feedback on the strategies and synergies used by vestibular loss subjects. Changes in movement strategies and sensory feedback in tight-rope walkers with considerable training in tandem stance (one foot before the other), were also investigated.
Subjects performed the stance tasks under different sensory conditions: with eyes open or closed, and on either a firm or foam surface. Stance was either side by side stance or tandem stance. For one experiment, vibrotactile and auditory balance feedback of trunk sway was used in addition. Subject groups were bilateral vestibular loss (BVL) patients, trained tightrope walkers and age matched controls. Two further groups of young and elderly healthy subjects were used to characterise differences in head movements with aging. In all studies roll and pitch angular velocities were recorded with six body-worn gyroscopes; a set of two worn at the upper trunk, an identical set at the hips and another lighter set worn on a head band. In one study with BVL subjects, another of the lighter gyroscopes was strapped onto the lower leg. For the balance feedback study surface EMGs were recorded from pairs of antagonistic muscles at the lower leg, trunk and upper arm. Data from all experiments was analysed in both time and frequency domains. For the analysis of tandem stance an estimate of centre of mass movement was calculated as well as its time to reach a virtual stability boundary.
The results indicated that under most sensory conditions, two legged, feet side by side stance conditions, head sway at the head for both the roll and pitch direction is greater than at the upper trunk and the pelvis. For low and mid-frequencies (<0.3 Hz) the head is locked to the trunk i.e. there is a tendency for the head and trunk to move as one unit but the head movement is always more than expected from a pure inverted pendulum movement mode. For the BVL subjects the head on trunk locking is more rigid and characterized by higher resonant frequencies. Prosthetic feedback reduced pelvis sway angle displacements in BVL subjects to values of age-matched healthy controls for all stance tasks. Movement strategies in BVL subjects were reduced in amplitudes with feedback but otherwise not changed. Reduced amplitudes are achieved with improved antagonistic muscle synergies. As we observed with feet side by side stance, tandem stance is also multisegmental. Keeping balance while standing on a tightrope appears to require similar intersegmental movement strategies for the head, trunk and pelvis to those used with other, less difficult tandem stance tasks. The difference with respect to untrained normal subjects is that faster trunk movements are used by tightrope walkers as they explore the limits of the base of support. At the same time they reduce relative head and pelvis movements to those of the trunk via changed proprioceptive weightings.