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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 (

List of abbreviations: COM: centre of mass; CNS: central nervous system; SCA: Spinal Cerebellar Ataxia; APA: anticipatory postural activity. Human stance is an instable bipedal posture characterized by a high centre of mass (COM) located near the hips. The COM (projected onto ground level) needs to be held within the small area of support (defined by the two feet) to maintain equilibrium. Elderly people and those with neurological deficits have problems with balance. About 30% of community-dwelling adults aged 65 and older fall at least once each year. Falls and fall-related injuries have been shown to be independent determinants of functional decline. Falls occur in different directions and at different speeds depending on the direction and intensity of the perturbation to balance. The way the central nervous system (CNS) responds to an impending fall depends on many factors, direction and velocity of falling are the two most crucial. Thus, when human stance is perturbed, the CNS must utilize and integrate the available sensory and environmental information to select an appropriate response strategy, especially for fast backward falls. In this thesis standing balance was perturbed using servo-controlled multi-directional rotations of the support surface. Balance perturbations consisted of combined pitch and roll rotations (7.5° and 60°/s) presented randomly in different directions. Thus, in a sideways rotation of the support surface to the right the subject’s COM moved to the right side and needed to be corrected to avoid a fall. A visual feedback of COM position based on surface reaction forces was presented prior to stimulus onset in order to standardise stance position. Outcome measures were biomechanical responses (kinematics and kinetics) and surface EMG activity of several muscles. The action of the CNS can be investigated by studying patient groups with clearly defined balance deficits. Thus, patients with spinal cerebellar ataxia (SCA) were the focus of the first study in this thesis. The goals of this study were to investigate the correlations between body segment movements and COM velocity during pathological balance corrections of SCA patients compared to controls, and to relate correlations indicating instability to EMG activity differences. Therefore, activation patterns of several leg and trunk muscles, kinematics and kinetics were compared between a group of SCA patients and age-matched controls. The results showed that, for lateral perturbations, peaks in COM lateral velocity were larger in SCA patients than controls. These peaks were correlated with increased (“hypermetric”) trunk roll downhill and reduced uphill knee flexion velocity. Subsequent arm abduction partially corrected the lateral instability. Excessive posterior COM velocity coincided with marked trunk hypermetric flexion forwards. Early balance correcting responses in knee and paraspinal muscles have reduced amplitudes compared to normal responses, not increased response amplitudes as expected. Later responses were consistent with compensation mechanisms for the lateral instability created by the stiffened knee and pelvis. It was concluded that truncal hypermetria coupled with insufficient uphill knee flexion are the primary causes of lateral instability in SCA patients. Holding the knees and pelvis more rigid possibly permits a reduction in the controlled degrees of freedom and concentration on arm abduction improves lateral instability. For backwards perturbations excessive posterior COM velocity coincided with marked trunk hypermetric flexion forwards. A further conclusion was that this flexion and the ensuing backwards shift of the pelvis results from rigidity which jeopardizes posterior stability. Timing considerations and the lack of confirmatory changes in amplitudes of EMG activity suggest that both lateral and posterior instability in SCA is primarily a biomechanical response to pelvis and knee rigidity resulting from increased muscle background activity rather than changed evoked responses. It has been shown that balance corrections depend on the impending fall direction. Thus direction is crucial in programming muscle activity to recover balance or damping a fall. Muscle activity is controlled by the CNS. Directional sensitivity of the sensory inputs and ensuing responses were shown by Allum et al. (2008) and Carpenter et al., 1999, 2001). The question is whether the CNS independently controls roll and pitch movements of the human body during balance corrections. To help provide an answer to this question, the balance of 16 young healthy subjects using multi-directional rotations of the support surface was perturbed. All rotations had pitch and roll components, for which either the roll or the pitch component were delayed by 150 ms or not at all. Across all perturbation directions, delayed roll caused equally delayed shifts (150 ms) in peak lateral COM velocity. Across directions, delayed pitch did not cause equally delayed shifts in anterior-posterior COM velocity. After 300 ms however, the vector direction of COM velocity was similar to the directions seen in the no delay condition. Trunk, arm and knee joint rotations followed this roll compared to pitch pattern but were different from the no delay rotation synergies after 300 ms, suggesting inter-segmental compensation for the delay effects. Balance correcting responses of muscles demonstrated both roll and pitch directed components regardless of axial alignment. Muscles were categorised into three groups: pitch oriented, roll oriented and mixed. Lower leg muscles were pitch oriented, trunk muscles roll oriented, and knee and arm muscles mixed. The results of this study suggest that roll, but not pitch components, of balance correcting movement strategies and muscle synergies are separately programmed by the CNS. Reliance on differentially activated arm and knee muscles to correct roll perturbations reveals a dependence of the pitch response on that of roll, possibly due to biomechanical constraints, and accounts for the failure of delayed pitch to be transmitted equally in time across all limbs segments. Thus it appears the CNS preferentially programs the roll response of the body and then adjusts the pitch response accordingly. During an impending fall some body segments may be preferentially used to recover balance. As shown in the study of SCA patients, the knees play a critical role for correcting fall in lateral directions - stiff knees impair balance recovery. Thus, training adequate knee flexion would help to recover balance. To determine whether voluntary movements can be effectively incorporated into balance corrections two studies with voluntary body movements were performed. “Knee flexion” and “trunk bending” young healthy subjects had to execute unilateral knee flexion and lateral trunk bending, respectively, simultaneously with support surface tilts. Unilateral uphill knee flexion benefited balance recovery. Subjects rotated their pelvis uphill more than predicted. Downhill knee bending also reduced COM motion. This because of a greater than predicted simultaneous lateral shift of the pelvis uphill. Leg muscle activity of voluntary knee bending showed anticipatory postural activity (APA) with similar profiles to early balance correcting responses. EMG response amplitudes for combined voluntary and compensatory responses were generally not different from just compensatory responses and therefore smaller than predicted. These results suggest that because EMG patterns of APA of voluntary motion and early balance corrections have similar profiles, the CNS is able to incorporate voluntary activation associated with unilateral knee flexion or lateral trunk bending into automatic postural responses. The effect on movement strategies appears to be non-linear. In conclusion, these studies provide crucial insights into central programming of balance reactions useful for developing rehabilitation programs to improve balance. References 1. Allum JHJ, Oude Nijhuis LB, Carpenter MG. Differences in coding provided by proprioceptive and vestibular sensory signals may contribute to lateral instability in vestibular loss subjects. Exp Brain Res 2008; 184:391-410. 2. Carpenter MG, Allum JHJ, Honegger F. Directional sensitivity of stretch reflexes and balance corrections for normal subjects in the roll and pitch planes. Exp Brain Res. 1999; 129:93-113. 3. Carpenter MG, Allum JHJ, Honegger F. Vestibular influences on human postural control in combinations of pitch and roll planes reveal differences in spatiotemporal processing. Exp Brain Res. 2001; 140:95-111.