A MECHANISTIC APPROACH TO POSTURAL DEVELOPMENT IN CHILDREN

Upright standing is intrinsically unstable and requires active control. The central nervous system's feedback process is the active control that integrates multi-sensory information to generate appropriate motor commands to control the plant (the body with its musculotendon actuators). Maintaining standing balance is not trivial for a developing child because the feedback and the plant are both developing and the sensory inputs used for feedback are continually changing. Knowledge gaps exist in characterizing the critical ability of adaptive multi-sensory reweighting for standing balance control in children. Furthermore, the separate contributions of the plant and feedback and their relationship are poorly understood in children, especially when considering that the body is multi-jointed and feedback is multi-sensory. The purposes of this dissertation are to use a mechanistic approach to study multi-sensory abilities of typically developing (TD) children and children with Developmental Coordination Disorder (DCD). The specific aims are: 1) to characterize postural control under different multi-sensory conditions in TD children and children with DCD; 2) to characterize the development of adaptive multi-sensory reweighting in TD children and children with DCD; and, 3) to identify the plant and feedback for postural control in TD children and how they change in response to visual reweighting. In the first experiment (Aim 1), TD children, adults, and 7-year-old children with DCD are tested under four sensory conditions (no touch/no vision, with touch/no vision, no touch/with vision, and with touch/with vision). We found that touch robustly attenuated standing sway in all age groups. Children with DCD used touch less effectively than their TD peers and they also benefited from using vision to reduce sway. In the second experiment (Aim 2), TD children (4- to 10-year-old) and children with DCD (6- to 11-year-old) were presented with simultaneous small-amplitude touch bar and visual scene movement at 0.28 and 0.2 Hz, respectively, within five conditions that independently varied the amplitude of the stimuli. We found that TD children can reweight to both touch and vision from 4 years on and the amount of reweighting increased with age. However, multi-sensory fusion (i.e., inter-modal reweighting) was only observed in the older children. Children with DCD reweight to both touch and vision at a later age (10.8 years) than their TD peers. Even older children with DCD do not show advanced multisensory fusion. Two signature deficits of multisensory reweighting are a weak vision reweighting and a general phase lag to both sensory modalities. The final aim involves closed-loop system identification of the plant and feedback using electromyography (EMG) and kinematic responses to a high- or low-amplitude visual perturbation and two mechanical perturbations in children ages six and ten years and adults. We found that the plant is different between children and adults. Children demonstrate a smaller phase difference between trunk and leg than adults at higher frequencies. Feedback in children is qualitatively similar to adults. Quantitatively, children show less phase advance at the peak of the feedback curve which may be due to a longer time delay. Under the high and low visual amplitude conditions, children show less gain change (interpreted as reweighting) than adults in the kinematic and EMG responses. The observed kinematic and EMG reweighting are mainly due to the different use of visual information by the central nervous system as measured by the open-loop mapping from visual scene angle to EMG activity. The plant and the feedback do not contribute to reweighting

Sensory Augmentation for Balance Rehabilitation Using Skin Stretch Feedback

This dissertation focuses on the development and evaluation of portable sensory augmentation systems that render skin-stretch feedback of posture for standing balance training and for postural control improvement. Falling is one of the main causes of fatal injuries among all members of the population. The high incidence of fall-related injuries also leads to high medical expenses, which cost approximately $34 billion annually in the United States. People with neurological diseases, e.g., stroke, multiple sclerosis, spinal cord injuries, and the elderly are more prone to falling when compared to healthy individuals. Falls among these populations can also lead to hip fracture, or even death. Thus, several balance and gait rehabilitation approaches have been developed to reduce the risk of falling. Traditionally, a balance-retraining program includes a series of exercises for trainees to strengthen their sensorimotor and musculoskeletal systems. Recent advances in technology have incorporated biofeedback such as visual, auditory, or haptic feedback to provide the users with extra cues about their postural sway. Studies have also demonstrated the positive effects of biofeedback on balance control. However, current applications of biofeedback for interventions in people with impaired balance are still lacking some important characteristics such as portability (in-home care), small-size, and long-term viability. Inspired by the concept of light touch, a light, small, and wearable sensory augmentation system that detects body sway and supplements skin stretch on one’s fingertip pad was first developed. The addition of a shear tactile display could significantly enhance the sensation to body movement. Preliminary results have shown that the application of passive skin stretch feedback at the fingertip enhanced standing balance of healthy young adults. Based on these findings, two research directions were initiated to investigate i) which dynamical information of postural sway could be more effectively conveyed by skin stretch feedback, and ii) how can such feedback device be easily used in the clinical setting or on a daily basis. The major sections of this research are focused on understanding how the skin stretch feedback affects the standing balance and on quantifying the ability of humans to interpret the cutaneous feedback as the cues of their physiological states. Experimental results from both static and dynamic balancing tasks revealed that healthy subjects were able to respond to the cues and subsequently correct their posture. However, it was observed that the postural sway did not generally improve in healthy subjects due to skin stretch feedback. A possible reason was that healthy subjects already had good enough quality sensory information such that the additional artificial biofeedback may have interfered with other sensory cues. Experiments incorporating simulated sensory deficits were further conducted and it was found that subjects with perturbed sensory systems (e.g., unstable surface) showed improved balance due to skin stretch feedback when compared to the neutral standing conditions. Positive impacts on balance performance have also been demonstrated among multiple sclerosis patients when they receive skin stretch feedback from a sensory augmentation walker. The findings in this research indicated that the skin stretch feedback rendered by the developed devices affected the human balance and can potentially compensate underlying neurological or musculoskeletal disorders, therefore enhancing quiet standing postural control

Sensory Augmentation for Balance Rehabilitation Using Skin Stretch Feedback

This dissertation focuses on the development and evaluation of portable sensory augmentation systems that render skin-stretch feedback of posture for standing balance training and for postural control improvement. Falling is one of the main causes of fatal injuries among all members of the population. The high incidence of fall-related injuries also leads to high medical expenses, which cost approximately $34 billion annually in the United States. People with neurological diseases, e.g., stroke, multiple sclerosis, spinal cord injuries, and the elderly are more prone to falling when compared to healthy individuals. Falls among these populations can also lead to hip fracture, or even death. Thus, several balance and gait rehabilitation approaches have been developed to reduce the risk of falling. Traditionally, a balance-retraining program includes a series of exercises for trainees to strengthen their sensorimotor and musculoskeletal systems. Recent advances in technology have incorporated biofeedback such as visual, auditory, or haptic feedback to provide the users with extra cues about their postural sway. Studies have also demonstrated the positive effects of biofeedback on balance control. However, current applications of biofeedback for interventions in people with impaired balance are still lacking some important characteristics such as portability (in-home care), small-size, and long-term viability. Inspired by the concept of light touch, a light, small, and wearable sensory augmentation system that detects body sway and supplements skin stretch on one’s fingertip pad was first developed. The addition of a shear tactile display could significantly enhance the sensation to body movement. Preliminary results have shown that the application of passive skin stretch feedback at the fingertip enhanced standing balance of healthy young adults. Based on these findings, two research directions were initiated to investigate i) which dynamical information of postural sway could be more effectively conveyed by skin stretch feedback, and ii) how can such feedback device be easily used in the clinical setting or on a daily basis. The major sections of this research are focused on understanding how the skin stretch feedback affects the standing balance and on quantifying the ability of humans to interpret the cutaneous feedback as the cues of their physiological states. Experimental results from both static and dynamic balancing tasks revealed that healthy subjects were able to respond to the cues and subsequently correct their posture. However, it was observed that the postural sway did not generally improve in healthy subjects due to skin stretch feedback. A possible reason was that healthy subjects already had good enough quality sensory information such that the additional artificial biofeedback may have interfered with other sensory cues. Experiments incorporating simulated sensory deficits were further conducted and it was found that subjects with perturbed sensory systems (e.g., unstable surface) showed improved balance due to skin stretch feedback when compared to the neutral standing conditions. Positive impacts on balance performance have also been demonstrated among multiple sclerosis patients when they receive skin stretch feedback from a sensory augmentation walker. The findings in this research indicated that the skin stretch feedback rendered by the developed devices affected the human balance and can potentially compensate underlying neurological or musculoskeletal disorders, therefore enhancing quiet standing postural control

The study of individual perception and neural control underlying movement in coordinating postural control when there is an increase in complexity of environmental and task constraints in CAI individuals compared to healthy controls

Individuals with Chronic Ankle Instability (CAI) commonly exhibit postural control (stability, adaptation) deficits and altered gait (walking, running) mechanics (Hertel, 2008; Hertel and Corbett, 2019). These impairments in motor behaviors have been hypothesized to be a result of inadequate, yet inherent interactions between individual perception (i.e., sensory systems) and movement (action) integrated at the central nervous system (CNS), resulting in less flexible and adaptable sensorimotor systems. Flexibility and adaptability of sensorimotor systems reflecting on underlying biological noise (movement variability) are critical to coordinate the sensory reweighting system. The sensory reweighting system assigns a relative weight to each sensory system based on the complexity of organismic, environmental, and task constraints to convey redundant and convergent sensory feedback at the CNS. An adequate sensory reweighting system results in sufficient multisensory integration by filtering all potential distractors, the irrelevant sensory information, to the context (e.g., task goals). Successful multisensory integration allows the CNS to integrate the context-relevant sensory information necessary to manage postural control that is the foundation of motor control to achieve suitable performance and adapt to a sudden environmental change. However, the gap exists in the literature to understand the integration phenomenon on how individual elements (i.e., sensory reweighting system, movement variability) contribute to the interaction between perception and movement, especially when environmental and task constraints increase in the same cohort of participants with and without CAI. Therefore, the primary purpose of this study was to understand the modulation of 1) the sensory reweighting system and postural control, 2) postural adaptation to a sudden change in the environment in the direction of lateral ankle sprain mechanisms, and 3) movement variability, an underlying biological noise pertaining to postural control, when the complexity of environmental and task constraints are manipulated in CAI individuals compared to healthy controls. A total of 44 physically active individuals, consisting of 22 individuals with CAI (13 females, 9 males; age: 26.09 ± 5.76 years; height: 172.25 ± 9.76 cm; weight: 76.18 ± 14.91 kg) and 22 individuals without CAI (13 females, 9 males; age: 25.41 ± 5.92 years; height: 169.70 ± 9.32 cm; weight: 71.98 ± 14.79 kg) volunteered to participate in this mixed-model repeated-measures study. The NeuroCom Sensory Organization Test (SOT) and Adaptation Test (SMART EquiTest, NeuroCom International Inc., Clackamas, OR) were utilized to examine postural control (equilibrium scores), postural adaptation (sway energy scores), the sensory reweighting system (sensory reweighting ratios), and movement variability (sample entropy) while controlling posture in double- and single-limb (injured, uninjured) stances in individuals with and without CAI. Interestingly, CAI individuals controlled posture very similar to healthy controls. The unique finding of this study was that group differences in the sensory reweighting system depended on both task constraints and sensory systems; CAI individuals upweighted on vestibular feedback when the SOT manipulated somatosensory and visual feedback while controlling posture in the injured-limb. Both groups weighted on somatosensory and visual feedback similarly with continuous emphasis on vision during individual tasks (stance limbs: double, injured, uninjured). Therefore, we contend CAI individuals upweighted on vestibular feedback, which is an independent sole veridical reference to self-motion, when sensory conflicts and task constraints became greater standing in the injured-limb. These findings also imply an effective multisensory integration among CAI. CAI individuals exhibited respective superior postural adaptation to a sudden environmental change in a support surface with plantarflexion rotation and in the uninjured-limb than healthy controls. Superior postural adaptation is indicative of pre-programmed feedforward motor control. In addition, lower movement variability in postural control was noted in the uninjured- and injured-limbs in CAI. Group differences in movement variability depended on task constraints: those individuals with CAI lowered variability in the uninjured-limb when no sensory feedback was manipulated, and in both the uninjured- and injured-limbs when they were forced to reweight on vestibular feedback with manipulation of somatosensory and visual feedback. Lowered movement variability exhibited with an increase in task constraints in the injured- and uninjured-limbs may be indicative of a mechanism that CAI implemented to provide a boundary to freeze the degree-of-freedom (redundancy in sensory feedback) to achieve effective multisensory integration. Collectively, our findings of superior postural adaptation and lower movement variability in postural control for CAI may imply an existent change in central organization and implementation of supraspinal mechanisms of postural control. Furthermore, postural control, postural adaptation, and movement variability in individuals with and without CAI depended on the environmental or task constraints. Environment- and task-dependent postural control, postural adaptation, and movement variability contribute to motor behaviors throughout the lifespan. Therefore, taking a multisensory-feedback approach by recognizing when to increase environmental and task constraints may optimize rehabilitation intervention to prevent subsequent ankle sprains in individuals with CAI

STATIC STANDING BALANCE AND STRENGTH MEASUREMENTS BEFORE AND AFTER TWO DIFFERENT GROUP EXERCISE INTERVENTIONS IN INDEPENDENT LIVING OLDER ADULTS

Purpose: The aims of this dissertation were to examine, in older adults: 1) the test-retest reliability of static standing balance performance using an accelerometer and lower extremity strength performance using a uniaxial load cell device; 2) the validity of balance and strength measurements at baseline with different mobility measurements; and 3) the effect of two different exercise programs on standing balance and lower extremity muscle strength. Participants: Thirty-eight participants were enrolled in the reliability testing (89% female, mean age 76 ± 7 years), and a total of 131 subjects (85% female, mean age 80 ± 8 years) were enrolled in the experimental study. Methods: For the balance assessment, an accelerometer was used to collect acceleration data in the anterior-posterior and medial-lateral directions for different standing balance conditions. In addition, lower extremity muscle strength measurements were assessed with a portable load cell for three consecutive trials. Clinical measures of mobility were concurrently tested. Test-retest reliability was assessed over two testing visits occurring one week apart, using the intraclass correlation coefficient. Spearman’s rank correlation coefficient was used to test convergent validity at baseline for the whole sample. A linear mixed model was used to examine the effect of the “On the Move” and standard of care group exercise programs on standing balance and lower extremity muscle strength. Results: Both balance and muscle strength performance showed good to excellent test-retest reliability using the accelerometer and uniaxial load cell device, respectively. The balance and measures were most strongly correlated with the Short Physical Performance Battery, and the strength measures with the repeated chair stands test. Both exercise interventions resulted in a significant change in both balance accelerometry measures and lower extremity muscle strength when compared to a waitlist control group, but did not differ from each other. Conclusion: The dual-axis accelerometer and uniaxial-load cell provide a reliable method for testing standing balance and lower extremity muscle strength, respectively in older adults living independently in the community. Participation in either group exercise intervention would result in improvement in both standing balance and lower extremity strength as compared to not receiving any exercise

Visual Perturbation to Enhance Return to Sport Rehabilitation after Anterior Cruciate Ligament Injury: A Clinical Commentary

Anterior cruciate ligament (ACL) tears are common traumatic knee injuries causing joint instability, quadriceps muscle weakness and impaired motor coordination. The neuromuscular consequences of injury are not limited to the joint and surrounding musculature, but may modulate central nervous system reorganization. Neuroimaging data suggest patients with ACL injuries may require greater levels of visual-motor and neurocognitive processing activity to sustain lower limb control relative to healthy matched counterparts. Therapy currently fails to adequately address these nuanced consequences of ACL injury, which likely contributes to impaired neuromuscular control when visually or cognitively challenged and high rates of re-injury. This gap in rehabilitation may be filled by visual perturbation training, which may reweight sensory neural processing toward proprioception and reduce the dependency on vision to perform lower extremity motor tasks and/or increase visuomotor processing efficiency. This clinical commentary details a novel approach to supplement the current standard of care for ACL injury by incorporating stroboscopic glasses with key motor learning principles customized to target visual and cognitive dependence for motor control after ACL injury. # Level of Evidence

A Model of Falls Risk in Older Adults

Falls are a significant problem in older adults, with 1/3 of people over the age of 65 falling in a given year. Age-related changes in the systems involved with balance lead to increased reaction time to postural perturbations, decreased postural control, and changes in gait, all resulting in an increased risk of falling. Other factors that can lead to imbalance outside of the primary balance systems increase the risk of falls. Previous research focused on producing a clinically-relevant tool for fall risk assessment; a theoretical falls risk model has yet to be produced. Risk factors were identified from literature review and recommendations from multiple clinical practice guidelines in the area of falls risk. Using data from the National Health and Nutrition Examination Survey, a Poisson regression was performed to determine which risk factors are significant predictors of reported problems with falls in older, community-dwelling adults. An additional Poisson analysis was performed, including interaction terms to see if any risk factors combined to increase the falls risk. Analysis showed that including interaction terms was a significantly better fit in the Poisson model than with the terms omitted. The most predictive risk factor for a reported problem with falls is asking the patient if they have problems with their balance. Many other risk factors will cause a feeling of imbalance, so the primary risk factor for falls is likely having a feeling of imbalance. Additional research is needed on intervention for falls risk and identifying threshold values for when risk factors will likely lead to imbalance

Sensory-Related Changes in Two-Segment Dynamics on a Sway-Referenced Support Surface

In its simplest form, the human postural control system can be described as a closed-loop control system consisting of a plant (body segments and musculotendon actuators) and feedback. Previous efforts to understand the contributions of plant and feedback employed techniques to "open the loop" which is problematic with the study of posture because the plant is unstable without feedback. In the present experiment, a closed-loop system identification method was used to "open the loop" without removal of sensory feedback. Subjects stood on a movable platform facing a visual scene, both of which were capable of rotation about an axis coaxial with the subject's ankles. The visual stimulus (present all trials) consisted of a 10-frequency sum-of-sines while movement of the support surface consisted of the following conditions: 1. Stationary; 2. Sway-referenced to the subject's body sway; 3. 10-frequency sum-of-sines; 4. Combined sway-referenced and sum-of-sines. Closed-loop frequency response functions were calculated for visual stimulus to EMG and visual stimulus to body sway angle. The open loop frequency response function for the plant was determined by dividing the frequency response functions, mathematically canceling the effects of feedback. With respect to the visual stimulus, gains for the leg segment showed no differences between the four platform conditions. Phase for the stationary condition was lower at the higher stimulus driving frequencies than for any of the moving platform conditions. In contrast, trunk segment gains were lower for the sway-referenced conditions at lower stimulus frequencies than for the stationary and sum-of-sines conditions. Phase showed a slight lead of the legs over the trunk for the sway-referenced conditions. The phase relationship between the trunk and leg segments, typically in-phase below ~1 Hz and anti-phase above ~1 Hz, showed a gradual transition at a lower frequency for the sway-referenced conditions than for the stationary or sum-of-sines conditions. Complex coherence showed a "legs-leading" coordinative relationship at the phase mode transition for the two sway-referenced conditions. Differences in the frequency response functions demonstrate that the plant changes with platform condition requiring different postural control strategies to maintain stability

The Imbalanced Consumer: The Effect off Physical Imbalance on Brand Recall and Construal Level

The role of physical imbalance in consumer behavior is an understudied topic in consumer psychology. This dissertation investigated the effect of physical imbalance on consumers. In a consumer environment, imbalance can be activated in various ways, such as when consumers struggle to walk on wet floors, icy sidewalks, miss a step, travel, or walk in a virtual space. This dissertation hypothesized that momentary loss of physical balance reduces consumers´ capacity to recall brands from memory. The finding extends research on subjects with balance impairments by testing this proposition with young and healthy consumers. Consumers and practitioners should know whether the number of recalled brands decreases when physical imbalance is experienced. Recognizing that imbalance is a source of cognitive load, consumers and marketers can benefit from understanding whether the remaining capacity becomes subject to limitations. According to Construal level theory, consumers perceive available choice alternatives as more viable when proximal sensations are dominant. This dissertation tested the proposition that imbalance reduces cognitive capacity and demands abrupt proximal action, prompting consumers to prefer low construed proximal choice alternatives. The first study suggested that consumers’ find their mental performance to be reduced during imbalance. The study found scant evidence for a decrease in the retrieval of brands from memory among young and healthy consumers. Subsequent studies tested whether imbalance could instigate a preference for low construal choice alternatives. The following two studies highlighted the need to improve study design and measurement. The final study found that imbalanced participants were more likely to choose certain smaller monetary rewards in the present over higher, more uncertain future rewards. The finding suggests a small effect of proximal sensation prompting a preference for lower construed alternatives. A single-paper meta-analysis suggested that the evidence for the proximal sensation of imbalance on psychological distance is weak. Alternative explanations about consumers´ certainty, mood, and self-efficacy were also tested. None of the alternative relationships were significant. The findings from this dissertation contribute to the literature by pinpointing the complexities of physical balance as a symphony of sensory interactions rather than a mere conceptual metaphor. This was the first study in consumer psychology to test the effect of physical imbalance on young and healthy adults as a demanding sensory state. The dissertation demonstrated that measuring the effect of proximal sensation of imbalance requires technical skill and resources. It contributes to consumer psychology by concluding that imbalance does not have a significant effect on the retrieval of brands or preference among young consumers. The study of imbalance will continue to be relevant for consumer research as aging populations are more likely to suffer from imbalance impairments while the use of balance-demanding virtual reality is increasing in popularity

Effect of Aging on Human Postural Control and the Interaction with Attention

The ability to stand upright and walk is generally taken for granted, yet control of balance utilizes many processes involving the neuromuscular and sensory systems. As we age, balance function begins to decline and can become problematic for many older adults. In particular, adults 65 years of age and older exhibit a higher incidence of falls than younger adults, and falls are a leading cause of injury in older adults, contributing to significant medical costs. Without better understanding of the impact of aging on balance and means to ameliorate those effects, this problem is expected to grow as life expectancy continues to increase.In addition to sensori-motor declines with age that impact balance, another factor known to affect balance, particularly in older adults, is attention, meaning the amount of cognitive resources utilized for a particular task. When two or more tasks vie for cognitive resources, performance in one or more tasks can be compromised (a common example today is driving while talking on a cell phone). Attention has been observed to be a critical factor in many falls reported by older adults. However, it is still not fully understood how aging and attentional demand affect balance and how they interact with each other.In this dissertation, we conducted dual-task experiments and model-based analyses to study upright standing and the interaction of the effects of age and attention on postural control. The effect of age was investigated by testing two age groups (young and older adults) with no evident balance and cognitive impairment and by comparing results of the two groups. The effect of attention and its interaction with age was studied by comparing body sway in the two age groups in response to a moving platform, while either concurrently performing a cognitive task (dual-task) or not (single-task). Our findings highlight postural control differences between young and older adults, as quantified by experimental measures of body motion as well as by model parameter values, such as stiffness, damping and processing delay