Metrics of multi-muscle synergies in Parkinson’s disease: Analysis of variance and motor equivalence

Over the past years, we have developed a test for postural stability based on the theory of synergies stabilizing salient performance variables. In this study, effects of Parkinson's disease (PD) and dopamine-replacement therapy on multi-muscle synergies stabilizing the center of pressure (COP) coordinate were explored between: (1) a cohort of 11 patients without clinically identifiable postural problems (Hoehn-Yahr stage II) and 11 age-matched controls, and (2) a cohort of 10 patients tested off- and on-medication, with and without postural problems (stage II and III, n = 5 per stage). Participants stood on a force platform and performed cyclical body sway at 0.5 Hz along the anterior-posterior direction. Electromyographic signals from 13 leg and trunk muscles were used to compute: (1) the amount of inter-cycle variance that did not affect (VUCM) and affected (VORT) COP coordinate, and (2) the magnitude of the cycle-to-cycle motion that did not change (motor equivalent: ME) and changed (non-motor equivalent: nME) the COP coordinate. We hypothesized that both methods would produce indices sensitive to PD and dopaminergic medications. Compared to controls, patients showed significantly smaller inter-cycle VUCM and ME components suggesting a less flexible, and hence less stable, behavior. Moreover, inter-cycle variance within/orthogonal to the UCM correlated with ME/nME displacements. Results suggest clinical utility of variance and motor equivalence analyses of postural instability in early stages of PD and quantifying the effects of dopamine-replacement drugs. The analysis of motor equivalence is particularly attractive because it requires only a handful of trials (observations).
Listed In: Neuroscience

Locomotor Stability Control and Vestibular Function among Older Adults: Implications for Falls Prevention and Research

In the following project, we explored the relationships between age, vestibulopathy and stability control, in order to determine the age and vestibulopathy-related effects on stability control, and to establish if a relationship existed between static and dynamic stability task performance. The first study examined the response to repeated trip perturbations of healthy middle aged adults and vestibulopathy patients, the second examined feedforward adaptation of gait in young, middle aged and older adults to a sustained mechanical perturbation and the third examined the relationship between standing balance and recovery following a tripping perturbation in vestibulopathy patients. The results showed that vestibulopathy is related to a diminished ability to control and recover gait stability after an unexpected perturbation, and to a deficient reactive adaptation potential. With ageing, the ability to recalibrate locomotor commands to control stability is preserved, although this recalibration may be slower in old age compared to middle and young age. Given that a decline in vestibular function is seen with increasing age, we suggest that assessment of vestibular function may be necessary when investigating locomotor stability and falls risk in both research and clinical settings. Finally, despite static balance tasks and parameters being commonly used in clinical settings, we did not find a consistent relationship between static and dynamic stability task performance, indicating the importance of dynamic stability tests when assessing falls risk in clinical settings.
Listed In: Biomechanics, Gait, Posturography

Static postural control does not strongly predict dynamic gait stability recovery following a trip in adults with and without vestibular dysfunction

Unilateral peripheral vestibular disorder (UPVD) negatively affects upper and lower body motor performance, but postural control during quiet stance in UPVD patients has not been directly compared with dynamic stability control after an unexpected perturbation during locomotion. We analysed centre of pressure (COP) characteristics during static posturography in UPVD patients and healthy controls and compared this with performance of a trip recovery task. 17 UPVD patients and 17 healthy controls were unexpectedly tripped while walking on a treadmill. The margin of stability (MoS) was calculated at touchdown (TD) of the perturbed step and the first six recovery steps. Posturography was used to assess postural stability during 30 seconds of standing with eyes open and closed using a force plate. The trip reduced the MoS of the perturbed leg (p<0.05) with no significant differences in MoS between the groups. Controls returned to MoS baseline level in five steps and patients did not return within the six steps. UPVD patients showed a greater total COP sway path excursion (closed eyes only), anterior-posterior range of COP distance and a more posterior COP position in relation to the posterior boundary of the base of support. There were no significant correlations between COP sway path excursion and MoS values. We concluded that UPVD patients have a diminished ability to control and recover dynamic gait stability after an unexpected trip and lower static postural stability control compared to healthy matched controls, but that trip recovery and static postural control rely on different control mechanisms.
Listed In: Biomechanics, Gait, Neuroscience, Physical Therapy, Posturography

Postural Responses to Noisy Support Surface Translations in Stroke Survivors

Healthy standing posture is characterized by the ability to interact with a changing environment while maintaining upright stance. Being adaptable to changing environments affords flexibility and allows the system to encounter novel environments without losing control of posture. The purpose of this research was to determine if stroke survivors could adapt to support surface translations with differing temporal structures. Methods: Eight stroke survivors participated in this research. Participants stood on a force platform on the Neurocom Balance Manager (Neurocom Intl., Clackamas, OR, USA). The support surface was translated in the anteroposterior direction according to waveforms with different temporal structures, this included white noise, pink noise, brown noise, and a sine wave. They also performed a normal standing trial where the platform did not move. Root mean square and detrended fluctuation analysis of the center of pressure signal were calculated to determine amount and temporal structure of variability respectively. Results: During normal standing the stroke survivors’ posture exhibited lack of adaptability. The stroke survivors had increased amount of variability in all conditions compared to normal standing, regardless of the inherent structure of the support surface translations. The temporal structure of variability indicated weakened long-range correlations in all conditions compared to normal standing. This indicates that regardless of the temporal structure of the support surface movement the amount of movement increased while the structure of movement became more random. Previous work has demonstrated that healthy posture is able to adapt to the temporal structure of support surface translations, this adaptability was not seen in a population of stroke survivors. This lack of adaptability makes interactions with environmental perturbations difficult and impacts functionality. Focusing rehabilitation protocols towards regaining healthy temporal structures in postural control could improve functionality in chronic stroke survivors.
Listed In: Biomechanics, Posturography