Postural control is modified by an interaction of psychological and physiological factors

Abstract

This thesis investigated the effects of balance confidence on postural control strategies for maintaining upright stance. Two studies were conducted to examine this relationship in healthy young adults. Balance confidence was modified through the introduction of a threat to posture: standing or voluntarily rising to the toes when positioned away from or at the edge of a platform raised to different surface heights above ground level. For both tasks, the central nervous system (CNS) adjusted the postural response to the level of the postural threat. When standing, amplitude of postural sway decreased progressively while frequency increased progressively as postural threat was increased steadily. Furthermore, this postural response was influenced by the order in which the threat to posture was experienced. For the rise to toes task, the rate and magnitude of the anticipatory postural response was significantly reduced for the most threatening compared to the least threatening condition. These changes in anticipatory postural control resulted in a slower acceleration and reduced forward movement of the centre of mass (COM) to the new position of support over the toes. In both instances, it appeared that the CNS modified control of posture to minimize the disturbance to the body COM; this cautious approach provided for a greater margin of safety especially when the consequences of instability were high. In addition to these changes in postural control, physiological arousal and perceived anxiety increased while perceived confidence and perceived stability decreased when performing the balance tests under more threatening conditions.

The third study investigated the effects of balance confidence on postural control for individuals with Parkinson's disease (PD). A series of eight standing balance tasks were performed; each task provided a different threat to posture. For example, patients were asked to stand with eyes open or closed, feet apart or close together, on a normal or foam support surface or with the possibility of being pushed or pulled off balance. The results suggested that individuals with PD who had less confidence in their ability to perform activities of daily living (ADLs) without falling reported less confidence and more anxiety, felt less stable and demonstrated greater postural sway on the balance tests compared to those individuals who had more confidence. Although significantly related to the severity of the disease, a measure of balance confidence provided added information to explain variation in balance performance. This observation was especially true for the more challenging standing tests, such as standing feet apart with eyes closed or standing feet together with eyes open or eyes closed.

The results of this thesis provide converging evidence identifying balance confidence as a key psychological modulator of postural control. Balance confidence influences the appraisal of postural threat and psychological and physiological responses to this threat. Balance confidence also influences behavioural outcomes, including strategies for postural control. This body of research emphasizes the importance of identifying both psychological and physiological influences on postural control strategies for maintaining upright stance. Although alterations in strategies for postural control may result from an underlying physiological cause, psychological factors, such as fear of falling or low balance confidence, may also contribute to these changes. An understanding of how psychological factors modify strategies for postural control provides direction for balance assessment and fall prevention for individuals who are afraid of falling.