Postural control and adaptation to threats to balance stability

Postural control is the ability to maintain equilibrium and orientation in a gravitational environment. It is dependent on feedback and feedforward mechanisms that generate appropriate corrective movement based on body-sway motion detected primarily by visual, vestibular, and proprioceptive sensory systems. Since information from the various senses is not always accurate (e.g. by disease) or available (e.g. with eyes closed), the postural control system must adapt to maintain stance. This thesis aimed to investigate postural control and adaptation to threats of balance. Effective approaches for the clinical measurement of postural control still remains to be developed. In the past, it has been common to investigate patients’ balance by having them stand upon compliant foam blocks with eyes open and closed since standing on foam is believed to affect the accuracy of information from cutaneous mechanoreceptors on the soles of the feet. However, when assessing balance on foam blocks with different compliances and mechanical properties, it was found that postural sway was larger on firmer compliant surfaces, which also increased the importance of visual information. Postural adaptation was also investigated by repeatedly perturbing balance using muscle vibrations. In healthy, young persons, a slow adaptive change was observed. This adaptation involved decreased costs of standing including decreased energy, body movement and muscle activity and changes to the relationship between muscle activity and movement. The characteristics of the adaptation also depended on the availability of visual information. The elderly had poor postural control with and without being perturbed but were able to adapt to improve their poor balance. However, decreased mechanoreceptive sensation in the elderly prevented them from adapting their balance to the level of younger test subjects. Sleep deprivation decreased attention and alertness and resulted in decreased postural control and adaptation. The findings in this thesis extend what is known about motor learning. The adaptive learning capability of the postural control system, and hence the accurate reconstruction of the kinematics and kinetics of movement, was dependant on ones own mechanoreceptive somatosensation and availability of visual information. Decreasing attention and alertness through sleep deprivation decreased adaptive capabilities, suggesting an important role for sleep in memory and consolidation of a new motor skill

Comprehensive Physical Function Assessment in Elderly People

In elderly people with mobility limitations, abnormalities in posture and gait contribute to the greater decline of physical motor function. The aim of the review article was to determine the comprehensive physical motor function assessment. Muscle function was assessed with the grip strength. Gait function was assessed with walking time tests conducted at a normal pace. Balance function was assessed with one‐legged standing time. The 6‐min walking distance test (6MD) was performed in a 10‐m, straight corridor. Walking efficiency during the 6MD trials was measured using the Cosmed K4b2 (Rome, Italy), an indirect calorimetry system specifically designed to measure energy expenditure in nonlaboratory settings. The center of pressure was recorded using a balance board (Wii; Nintendo Co., Ltd., Kyoto, Japan). A vibratory stimulus was applied alternately to two muscles by fixing two vibrators from the vibration device onto the participant’s gastrocnemius and lumbar multifidus muscle. These findings show that an assessment affecting postural control under proprioceptive stimulation might be a good indicator of elderly people. Also, the objective assessment of walking efficiency might be important for identifying the risk of external activity limitation or functional limitations among the late elderly

Plantar Pressure, Cutaneous Sensation and Stochastic Resonance: An Examination of Factors Influencing the Control and Perception of Posture

The goal of this dissertation was to understand how people control posture in the context of sensory loss. To do so we explored three potential influences on the detection of external information and how they relate to the control of posture and perception of body orientation: 1) does changing posture alter the forces under the foot, and do these changes impact the ability to detect external vibrations? 2) Does decreasing the temperature of the foot influence the ability to detect external vibrations, the perception of body orientation, and the control of posture? And 3) does stochastic resonance (SR) improve the perception of body orientation and the control of posture when the sensory thresholds are elevated to clinical levels through cooling of the feet? The results of the experiments indicate that: 1) increasing the pressure under the feet, elicited by changes in posture, elevates the cutaneous sensory threshold, and that the forefoot appears to be more sensitive than the rearfoot to changes in weighting; 2) decreasing the temperature of the skin elevates cutaneous sensory thresholds, and impacts postural control by constraining the fluctuations of the medial-lateral center of pressure; and 3) applying SR to the soles of the feet improves the ability to perceive body position, with greater amounts of skin cooling resulting in greater improvements in postural performance due to SR. This dissertation demonstrates that decreasing plantar loading lowers cutaneous sensory thresholds, indicating that the changes in postural fluctuations frequently observed among those with clinical sensory loss may serve as a mechanism that allows for improved access to external information if they prove to reduce the pressure under sensory impaired portions of the feet. Additionally, we add to the growing body of literature identifying SR as a means to improve postural performance when cutaneous sensory function is impaired. From a clinical perspective, the results presented here indicate that aids designed to apply SR to the soles of the feet, as a means to improve posture and gait, should modulate their signal such that they apply a signal amplitude appropriate to the amount of loading the foot experiences

A Physiology-Based Approach for Detecting Vibration Perception Threshold in the Plantar Foot

Stochastic-resonance-based vibration therapies have demonstrated the potential to improve balance in persons with somatosensory deficiencies to help prevent fall incidents. These vibrations must remain below vibration perception threshold (VPT) to be safe and effective. Several concerns exist regarding current approaches of detecting VPT, including inconsistent unit scales, limited knowledge of the physiological reliability of the methods, or potential effects they may have on standing balance. Recent assumptions that threshold detection tests have no impact on subsequent vibratory stimulations warrant further investigation. The purpose of this study was (a) to develop a new modified 4-2-1 VPT detection method (M421) based on existing approaches and underlying physiological principles, and (b) to identify potential effects the M421 may have on balance during or after threshold testing. To address the need for greater comparability between patient populations and across vibration systems, a common scale for expressing VPT was also established. Our results indicate that, among healthy adults, the M421 test does not significantly alter balance during or following threshold testing, and that a single trial conducted on both feet is comparable to separate tests of each foot. M421 demonstrates repeatable results and can be completed efficiently. Future studies will seek to further validate M421 through direct comparisons against existing methods to determine the optimal approach for detecting VPT prior to stochastic vibration interventions

EEG-based investigation of cortical activity during Postural Control

The postural control system regulates the ability to maintain a stable upright stance and to react to changes in the external environment. Although once believed to be dominated by low-level reflexive mechanisms, mounting evidence has highlighted a prominent role of the cortex in this process. Nevertheless, the high-level cortical mechanisms involved in postural control are still largely unexplored. The aim of this thesis is to use electroencephalography, a widely used and non-invasive neuroimaging tool, to shed light on the cortical mechanisms which regulate postural control and allow balance to be preserved in the wake of external disruptions to one’s quiet stance. EEG activity has been initially analysed during a well-established postural task - a sequence of proprioceptive stimulations applied to the calf muscles to induce postural instability – traditionally used to examine the posturographic response. Preliminary results, obtained through a spectral power analysis of the data, highlighted an increased activation in several cortical areas, as well as different activation patterns in the two tested experimental conditions: open and closed eyes. An improved experimental protocol has then been developed, allowing a more advanced data analysis based on source reconstruction and brain network analysis techniques. Using this new approach, it was possible to characterise with greater detail the topological structure of cortical functional connections during the postural task, as well as to draw a connection between quantitative network metrics and measures of postural performance. Finally, with the integration of electromyography in the experimental protocol, we were able to gain new insights into the cortico-muscular interactions which direct the muscular response to a postural challenge. Overall, the findings presented in this thesis provide further evidence of the prominent role played by the cortex in postural control. They also prove how novel EEG-based brain network analysis techniques can be a valid tool in postural research and offer promising perspectives for the integration of quantitative cortical network metrics into clinical evaluation of postural impairment.Kerfi stöðustjórnunar er afturvirkt stýrikerfi sem vinnur stöðugt að því að viðhalda uppréttri stöðu líkamans og bregðast við ójafnvægi. Vaxandi þekking á undanförnum árum hefur lýst því að úrvinnsla þessara upplýsinga á sér stað á öllum stigum miðtaugakerfisins, þá sérstaklega barkarsvæði heilahvela. Engu að síður, er nákvæmu hlutverk heilabarkar við stöðustjórnun enn óljóst að mörgu leyti. Tilgangur þessa verkefnis var að rannsaka nánar hlutverk heilabarkar við truflun og áreiti á kerfi stöðustjórnarinnar, með notkun hágæða heilarafrits (EEG). Við byrjuðum á því að mæla heilarit einstaklinga meðan á þekktri líkamsstöðu-æfingu stóð, til þess að skoða svörun líkamans við röð titringsáreita sem beitt var á kálfavöðvana til að framkalla óstöðugleika. Bráðabirgðaniðurstöður fengnar með PSD-aðferð (power spectral analysis) leiddu í ljós aukna virkni á ákveðnum svæðum í heilaberki og sérstakt viðbragðsmynstur við að framkvæma æfinguna, annars vegar með lokuð augu og hins vegar opin augu. Rannsókn okkar hélt áfram með nýrri og þróaðari tækni sem gerði okkur kleift að framkvæma fullkomnari greiningaraðferðir til að túlka, greina og skilja merki frá heilaritnu. Með fullkomnari greiningaraðferðum var hægt að lýsa með nákvæmari hætti staðfræðilega uppbyggingu starfrænna tenginga í heilaberki meðan á líkamsstöðu æfingunni stóð, sem og að draga tengsl á milli megindlegra netmælinga og mælinga á líkamsstöðu. Að lokum bætist við vöðvarafritsmæling við aðferðafræðina, sem gaf okkur innsýn inn í samskipti heilabarka og vöðvana sem stýra vöðvaviðbrögðum og viðhalda líkamsstöðu við utanaðkomandi áreiti. Á heildina litið gefa niðurstöðurnar sem settar eru fram í þessari ritgerð enn sterkari vísbendingar um það áberandi hlutverk sem heilabörkurinn gegnir við stjórnun líkamsstöðu. Niðurstöðurnar sanna einnig hvernig ný aðferð á greiningu á tengslaneti heilans sem byggir á heilariti getur verið gilt tæki í líkamsstöðu rannsóknum og er nytsamlegt tól fyrir mælingar á heilakerfisneti í klínískt mat á skerðingu líkamsstöðu

EEG-based investigation of cortical activity during Postural Control

The postural control system regulates the ability to maintain a stable upright stance and to react to changes in the external environment. Although once believed to be dominated by low-level reflexive mechanisms, mounting evidence has highlighted a prominent role of the cortex in this process. Nevertheless, the high-level cortical mechanisms involved in postural control are still largely unexplored. The aim of this thesis is to use electroencephalography, a widely used and non-invasive neuroimaging tool, to shed light on the cortical mechanisms which regulate postural control and allow balance to be preserved in the wake of external disruptions to one’s quiet stance. EEG activity has been initially analysed during a well-established postural task - a sequence of proprioceptive stimulations applied to the calf muscles to induce postural instability – traditionally used to examine the posturographic response. Preliminary results, obtained through a spectral power analysis of the data, highlighted an increased activation in several cortical areas, as well as different activation patterns in the two tested experimental conditions: open and closed eyes. An improved experimental protocol has then been developed, allowing a more advanced data analysis based on source reconstruction and brain network analysis techniques. Using this new approach, it was possible to characterise with greater detail the topological structure of cortical functional connections during the postural task, as well as to draw a connection between quantitative network metrics and measures of postural performance. Finally, with the integration of electromyography in the experimental protocol, we were able to gain new insights into the cortico-muscular interactions which direct the muscular response to a postural challenge. Overall, the findings presented in this thesis provide further evidence of the prominent role played by the cortex in postural control. They also prove how novel EEG-based brain network analysis techniques can be a valid tool in postural research and offer promising perspectives for the integration of quantitative cortical network metrics into clinical evaluation of postural impairment

The effects of peripheral nerve impairments on postural control and mobility among people with peripheral neuropathy

Approximately 20 million Americans are suffering Peripheral Neuropathy (PN). It is estimated that the prevalence of all-cause PN is about 2.4% in the entire adult population, whereas over 8-10% in the population segment over the age of 55 (Martyn & Hughes, 1997). Peripheral Neuropathy leads to a high risk of falling, resulting from the deficits of postural control caused by the impaired peripheral nerves, especially the degenerative somatosensory system. To date, there is no effective medical treatment for the disease but pain managements. The deficits of postural control decrease the life quality of this population. The degeneration of peripheral nerves reduces sensory inputs from the somatosensory system to central nervous system via spinal reflexive loop, which should provide valuable real-time information for balance correction. Therefore, it is necessary to investigate how PN affects the somatosensory system regarding postural control. Besides that, people with PN may develop a compensatory mechanism which could be reinforced by exercise training, ultimately to improve balance and mobility in their daily life. The neuroplasticity may occur within somatosensory system by relying on relative intact sensory resources. Hence, unveiling the compensatory mechanism in people with PN may help in understanding (a) essential sensations or function of peripheral nerves to postural control, (b) effective strategy of physical treatments for people with PN, and (c) task-dependent sensory information requirements. Therefore, this dissertation discussed the roles of foot sole sensation, ankle proprioception, and stretch reflex on balance as well as gait among people with PN. Furthermore, the discussion of the coupling between small and large afferent reflexive loops may spot the compensatory mechanism in people with PN