Postural Coordination Patterns: Visual Rotation and Translation

Recent studies have shown co-existing trunk-leg coordination patterns during quiet stance: in-phase and anti-phase for frequencies below and above 1 Hz, respectively. Two experiments investigated whether the nervous system assumes a multilinked internal model in sensory coupling? In the first experiment, we investigated the influence of the addition or removal of sensory information on these patterns. Trunk-leg coherence decreased with the addition of static vision and light touch, in the AP and ML directions, respectively, at frequencies below 1 Hz, suggesting the in-phase pattern may be more affected by neural control than the anti-phase pattern. In the second experiment, we compared translation of the visual field to a rotation relative to the ankle/hip. Gain and phase between the trunk/leg angles relative to the visual display showed only minor condition differences. The overall results suggest the nervous system adopts a simple control strategy of a single-link internal model at low frequencies

Visual-vestibular compensation in balance recovery: A transfer function model-based analysis

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Identifying Plant and Feedback in Human Posture Control

Human upright bipedal stance is a classic example of a control system consisting of a plant (i.e., the physical body and its actuators) and feedback (i.e., neural control) operating continuously in a closed loop. Determining the mechanistic basis of behavior in a closed loop control system is problematic because experimental manipulations or deficits due to trauma/injury influence all parts of the loop. Moreover, experimental techniques to open the loop (e.g., isolate the plant) are not viable because bipedal upright stance is not possible without feedback. The goal of the proposed study is to use a technique called closed loop system identification (CLSI) to investigate properties of the plant and feedback separately. Human upright stance has typically been approximated as a single-joint inverted pendulum, simplifying not only the control of a multi-linked body but also how sensory information is processed relative to body dynamics. However, a recent study showed that a single-joint approximation is inadequate. Trunk and leg segments are in-phase at frequencies below 1 Hz of body sway and simultaneously anti-phase at frequencies above 1 Hz during quiet stance. My dissertation studies have investigated the coordination between the leg and trunk segments and how sensory information is processed relative to that coordination. For example, additional sensory information provided through visual or light touch information led to a change of the in-phase pattern but not the anti-phase pattern, indicating that the anti-phase pattern may not be neurally controlled, but more a function of biomechanical properties of a two-segment body. In a subsequent study, I probed whether an internal model of the body processes visual information relative to a single or double-linked body. The results suggested a simple control strategy that processes sensory information relative to a single-joint internal model providing further evidence that the anti-phase pattern is biomechanically driven. These studies suggest potential mechanisms but cannot rule out alternative hypotheses because the source of behavioral changes can be attributed to properties of the plant and/or feedback. Here I adopt the CLSI approach using perturbations to probe separate processes within the postural control loop. Mechanical perturbations introduce sway as an input to the feedback, which in turn generates muscle activity as an output. Visual perturbations elicit muscle activity (a motor command) as an input to the plant, which then triggers body sway as an output. Mappings of muscle activity to body sway and body sway to muscle activity are used to identify properties of the plant and feedback, respectively. The results suggest that feedback compensates for the low-pass properties of the plant, except at higher frequencies. An optimal control model minimizing the amount of muscle activation suggests that the mechanism underlying this lack of compensation may be due to an uncompensated time delay. These techniques have the potential for more precise identification of the source of deficits in the postural control loop, leading to improved rehabilitation techniques and treatment of balance deficits, which currently contributes to 40% of nursing home admissions and costs the US health care system over $20B per year

Attentional focus on supra-postural tasks affects postural control: Neuromuscular efficiency and sway characteristics

This study is the first in which both performance production and outcome measurements are incorporated for a postural and supra-postural task, to identify the effect of attentional focus (AF) on muscles distal to the primary action. Postural and shoulder muscles are assessed when 21 participants attempt to minimize aiming error on a distal target while being subject to discrete arm perturbations. During each 60s trial random perturbations were delivered to the right arm and subjects were provided different AF instructions: control (no instruction), internal (focus on finger), and external (focus on laser). Providing an instruction improved both postural and supra-postural performance, i.e. COPnet PL decreased F(2,36)=5.259, p\u3c 0.05, PeakMax of laser marker was lower F(2,36)=11.274, p\u3c0.05. However, based on the current results there is no reason to expect that the type (internal, external) of instruction influences the response to a discrete and external perturbation

Biomechanics of Leaning and Downward Reaching Tasks in Young and Older Women.

Stooping crouching or kneeling (SCK) difficulty is prevalent among older adults yet few studies have explored the mechanisms underlying downward reaching and pick-up difficulty. During targeted movements tradeoffs are expected between the speed and accuracy of center of pressure (COP) movements as balance is maintained. Thus, this research focused on how age-related changes in COP control strategies affect the performance of tasks with a large range of truncal motion and momenta. It was hypothesized that while performing leaning and downward reaching movements, older women, compared to younger women, would exhibit slower but more frequent COP submovements in order to accomplish the task and regain the upright posture. First, we investigated the limiting factors in downward reach and pick-up movements. Using an age-adjusted proportional odds model, increased SCK difficulty was found to be independently associated with balance confidence, leg joint limitations, and knee extension strength. Secondly, we explored the age-related changes in COP control in healthy women. Despite being 27% slower, older women rely on nearly twice as many submovements to maintain a similar level of endpoint accuracy in volitional COP movements, particularly when moving posteriorly. Furthermore, older women used slower primary submovements that more often undershot their target, in comparison to young women, particularly as movement amplitude increased. Lastly, healthy older women were found to lose their balance more often than young women in downward reaching tasks, but rely on similar COP control strategies when successful. Modeling results suggest that a simple forward dynamic model that accounts for changes in musculoskeletal factors may distinguish between healthy young and healthy older women with and without SCK difficulty. We conclude that biomechanical factors can distinguish between older women with and without SCK difficulty. Given the significance of the rate of torque development in arresting downward reaching movements, changes in COP control may be effective tools in evaluating early signs of physical impairment. Undershooting primary submovements and increased secondary submovements are indicative of an increasingly conservative strategy used by older adults near the limits of the base of support that may explain their slower speeds during whole body movements to maintain balance.Ph.D.Biomedical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/91565/1/manueleh_1.pd

Muscle Stimulation via Whole Body Vibration for Postural Control Applications

Although the ability to balance might feel effortless to the most, it should not be given for granted as even the normal process of ageing can compromise it, jeopardising people physical independence. It is therefore important to implement safe training routines that ultimately improve postural control strategies. We evaluated the suitability of whole body vibration (WBV) training –which induces muscle contraction via the stimulation of muscle spindles - for postural control applications. First, we tested the efficacy of different combinations of stimulation frequency and subjects’ posture in eliciting a response from those muscles that play a key role for the implementation of postural responses. Each combination was evaluated by jointly measuring the resulting muscular activation and soft-tissue displacement. Then, we investigated how the selected WBV stimulation affected the balance of healthy subjects. We evaluated the latter by analysing centre of pressure trajectories, muscle and cortex activation and their respective interplay. We found that high frequency vibrations, delivered to participants standing on their forefeet, evoked the greatest contraction of the plantarflexors. Undisturbed balance recorded after such stimulation was characterised by an increased sensitivity of muscle spindles. In line with the latter, the communication between the periphery and the central nervous system (CNS) increased after the stimulation and different muscle recruitment patterns were employed to maintain balance. On the posturography side, stability was found to be compromised in the acute term but seemed to have recovered over a longer term. Together, these findings suggest that, if appropriately delivered, WBV has the potential to stimulate the spindles of the plantarflexors. By doing so, vibration training seems to be able to augment the communication between the proprioceptive organs and the CNS, on which the system relies to detect and react to perturbations, leading to sensorimotor recalibration

Models and Analysis of Vocal Emissions for Biomedical Applications

The MAVEBA Workshop proceedings, held on a biannual basis, collect the scientific papers presented both as oral and poster contributions, during the conference. The main subjects are: development of theoretical and mechanical models as an aid to the study of main phonatory dysfunctions, as well as the biomedical engineering methods for the analysis of voice signals and images, as a support to clinical diagnosis and classification of vocal pathologies