Evaluation of the Dynamical Structures of Postural Control: New Insights into Motor Adaptation in Ageing and Pain

Abstract

Ageing and pain affect performance of the postural control system. These changes affect the ability to flexibly adapt to challenges with potential consequences for balance and falls risk, and potential to understand the nature of adaptations to pain. The overall aim of this thesis was to provide insight into the dynamical nature of standing and walking postural control quantified using non-linear methods to address these issues. Studies One and Two of this thesis investigated postural control of upright balance in elderly individuals who did or did not report falls in the 12 months after the initial balance assessment and in a group of younger individuals. Study One involved assessment of postural control during normal quiet stance and when postural control was challenged by using combinations of eye closure and standing on a foam surface to challenge sensory input. Study Two assessed standing balance (without vision) in response to sudden changes of proprioceptive information using low amplitude calf muscle vibration. In both studies, center-of-pressure (CoP) motion was used as the output of the postural control system. Findings showed that overall CoP motion was less regular in elderly than in younger individuals. Higher regularity in young than elderly could be explained by fewer postural corrections and CoP motion that reflected longer durations of unattenuated inverted pendulum motion in young than old (Study One). More random fluctuations of CoP motion in elderly was more emphasized in fallers than non-fallers, especially after cessation of muscle vibration (Study Two). Young individuals adapted to postural challenges by increasing regularity of CoP motion, whereas elderly did not. Findings show that regular CoP motion was related to normal postural function. This contrasts some observations in which CoP regularity was greater in ageing and disease than healthy peers. Studies Three and Four investigated postural control during walking on a treadmill in young individuals with nociceptive stimulation in two locations. Walking was perturbed using an acute pain stimulus in the calf or lower back muscle (LBP) and adaptations to this pain stimulus were observed. Study Three used the activation patterns of multiple trunk and leg muscles (at 0.94 ms-1), and Study Four used upper thorax motion as the output of the postural control system (at 0.94 ms-1 and 1.67 ms-1). Study Three examined the common organisation of activity patterns among recorded muscles using synergy analysis (i.e., groups of muscles activated in synchrony) and their amplitude of activation in response to acute pain. Five muscle synergies were identified during walking control, pain and postpain conditions. Cross validation analysis showed that muscle synergies extracted for the control condition could account for >81% of variance of the other conditions. Muscle synergies were altered very little in some participants but were more affected in the other individuals. No systematic differences between pain locations were observed. Considering all part...

Effects of Boundary-Based Assist-as-Needed Force Field on Lower Limb Muscle Synergies During Standing Posture Training

“…Prior studies have proposed that subjects with neuromotor disorders (e.g., CP, SCI, stroke) recruit significantly fewer muscle synergies than healthy subjects during task execution [44]. Due to neuromuscular control deficits and muscle weakness, such a reduction in synergy number is hard to change during training [45], [46]. Although the TAAN force field could alter muscle weightings in each synergy, the synergy number remains stable during training [20].…”