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-1and 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 post-pain 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 participants, synergies related to propulsion and weight acceptance were largely unaffected by pain, whereas synergies related to other functions (trunk control and leg deceleration) were more affected. Calf muscle activity was less during both calf pain and LBP than control and was further reduced during calf pain.Study Four quantified walking performance by the sensitivity of the postural control system to small perturbations (local stability) that naturally occur during walking, also reflecting regularity. Local stability, stride-to-stride variability and amplitude of thorax motions were determined. At 0.94ms-1gait stability was lower and stride-to-stride variability was higher, during LBP and calf pain than no pain. This was more pronounced during calf pain, which is likely to be explained by the critical biomechanical function of calf muscles in gait. This was supported by observations of greater mediolateral amplitude of upper thorax motion and stance time asymmetry than in low back pain and no pain. At 1.67ms-1, gait stability was greater and stride-to-stride variability was smaller with LBP than no pain and calf pain, suggesting a more protective strategy. Gait stability was not affected during calf pain despite increased mediolateral amplitude of thorax motion and asymmetric stance, possibly due to greater kinematic constraints and smaller effects of calf muscle activity on propulsion at this speed.The four studies of this thesis show that regularity of time series that reflect the output of the postural control system at the level of CoP motion or upper body movements reflects normal healthy postural control function. Compared to young individuals, this finding is confirmed by the observations of reduced regularity in response to perturbations to the postural control system observed in elderly and specifically in fallers than non-fallers. When the postural control system is perturbed by an acute pain stimulus during walking in young individuals both increased and decreased regularity are observed. Pain adaptations depend on task constraints (walking speed) and the ability to adapt muscle coordination depend on available options and the impact of adaptations in relation to overall walking performance.