PURPOSE: The aim of this study was to determine if the linear and nonlinear components of the energy expenditure-walking speed relationship are influenced by body mass index (BMI; kg/m2). The secondary aims were to determine if the relationship was influenced by age, height, and sex. METHODS: Subjects (n=182) walked at 2, 3, and 4 mph for six minutes each with oxygen consumption (V̇O2; ml/kg/min) and measured via indirect calorimetry and converted to energy expenditure (EE; W/kg). Because of the curvilinear change in metabolic rate with increase in walking speed, polynomial random coefficient regression (PRCR) was employed to produce a model which captures the slope of change. Individual level linear and quadratic coefficients were analyzed for relationships with BMI, age, height, and sex. RESULTS: The net V̇O2 regression formula for walking was 1.79(x-3)2+4.97(x-3)+9.32 where x is speed in mph. BMI was modestly correlated with the quadratic coefficients (r = 0.15 to 0.17, p = 0.02 to 0.04) but not the linear coefficients (r =0.02- 0.07, p = 0.36-0.78) for V̇O2 and EE. There was no difference in coefficients between normal BMI (18.5-<25.0 kg/m2), overweight (25-<30.0 kg/m2) and obese (>30.0 kg/m2) groups (H = 1.5-4.0, p = 0.13-0.48). Delta V̇O2 for 2-3 mph, 3-4 mph, and 2-4 mph were not correlated with BMI (r = -0.02 - 0.13, p = 0.11 - 0.41). Height was inversely correlated with the linear and quadratic coefficients (r = -0.32 to -0.14, p = 0.09). Age was not correlated to coefficients (r = -0.16 to 0.32, p = 0.06-0.44). The coefficients for sex were not different after controlling for height in ANCOVA (F(1,179)=0.3-2.9, p >0.09). Age was not correlated to coefficients (r = -0.16 to –0.32, p = 0.06-0.44). CONCLUSION: Although BMI had a modest relationship with the quadratic coefficient, it explained less than 3% of the variance in V̇O2 or EE. Combined with the absence of a delta V̇O2 or a linear component, BMI does not influence the energy expenditure-walking speed relationship. Height explained up to 9% of the variance in the coefficients and eliminated apparent sex differences. Age was not related to the coefficients.

Falls are a public health concern for older adults with or without cognitive impairment, including clinical Mild Cognitive Impairment (MCI) and Alzheimer’s Disease (AD) dementia. Executive function (EF) is linked to falls and is notably impaired in individuals with MCI and AD dementia. However, it is unclear which EF assessments are associated with falls in older adults with intact cognition, MCI, and AD dementia. Apolipoprotein E4 (ApoE4) increases the risk of MCI and AD dementia, with limited information suggesting decreased EF and gait impairment in ApoE4 carriers. The purpose of this study was to 1) investigate the relationships between EF assessments and falls (fall history and future fall) by cognitive status (intact cognition, MCI, AD dementia) and 2) determine if ApoE4 moderates the relationship between EF and falls across cognitive status. EF assessments included Digit Span (DS), Trail Making Test (ΔTMT), Stroop Interference Test, Controlled Oral Word Association assessments of phonemic and semantic fluency (COWAC), and Clock Drawing Test. Binary logistic regression was used with secondary data to analyze the EF and falls relationship. Results showed a significant association of the ΔTMT with fall history but may not be appropriate for use with people with AD dementia due to a floor effect. Significant interaction effects were shown between AD dementia and DS, ΔTMT, and EF as a latent variable, where individuals with a fall history had better EF performance. The interaction effects are influenced by the lower percentage of reported falls in people with severe AD dementia. In the second aim, a confounding effect exposed a potential four-way interaction where higher EF in ApoE4 non-carriers with AD dementia experienced a future fall without a fall history. Overall, more research is needed to determine which EF assessments are best suited for fall risk assessment and whether ApoE4 plays a role in the relationship between EF and falls. Since the low percentage of reported falls in people with severe AD dementia exposed a differing trend between EF and falls, more research is needed to develop valid and reliable tools for collecting fall data in individuals with severe AD dementia.

Parkinson's Disease (PD) is a progressive neurodegenerative disorder that affects movement and balance control. Falls are a common and often debilitating consequence of PD, and reactive balance control is critical in preventing falls. This dissertation aimed to determine the adaptability and neural control of reactive balance responses in people with PD. Aim 1 investigated whether people with PD at risk for falls can improve their reactive balance responses through a 2-week, 6-session training protocol. The study found that reactive step training resulted in immediate and retained improvements in stepping, as measured by the anterior-posterior margin of stability (MOS), step length, and step latency during backward stepping. The second aim explored the neural mechanisms behind eliciting and learning reactive balance responses in PD. The study investigated the white matter (WM) correlates of reactive stepping and responsiveness to step training in PD. White matter was not significantly correlated with any baseline stepping outcomes. However, greater retention of step length was associated with increased fractional anisotropy (FA) within the left anterior corona radiata, left posterior thalamic radiation, and right and left superior longitudinal fasciculi. Lower radial diffusivity (RD) within the left posterior and anterior corona radiata were associated with retention of step latency improvements. These findings highlight the importance of WM microstructural integrity in motor learning and retention processes in PD. The third aim examined the role of the somatosensory system in reactive balance control in people with PD. The tactile and proprioceptive systems were perturbed using vibrotactile stimulation during backward feet-in-place balance responses. The results showed that tactile and proprioceptive stimulation had minimal impact on reactive balance responses. Small effects were observed for delayed tibialis anterior (TA) onsets with proprioceptive stimulation at a medium intensity. Overall, this dissertation provides insights into improving reactive balance responses and the underlying neural mechanisms in PD, which can potentially inform the development of targeted interventions to reduce falls in people with PD.

The ability to walk while completing a secondary task, dual-task walking (DTW), poses notable challenges for individuals affected by neurological disorders, such as multiple sclerosis (MS), who experience both cognitive and motor problems secondary to their disease. However, DTW is an everyday activity that has putative importance for optimal function. Although some research in the past decade has begun to examine changes in DTW in MS, there is still limited work to understand the predictors of DTW, the factors that might moderate relationships between baseline cognitive and motor function and DTW ability, and its consequences (e.g., for quality of life [QoL] or fall risk). To contribute to the understanding of these phenomena and their intersections, three secondary data analyses of two relatively large data sets in the area were conducted to address five major aims. The first step was to identify of the most relevant of these inherently involved domains (cognitive [aim 1] and motor [aim 2] abilities). Lasso regression for inference was performed to address this question for both cognitive (South Shore Neurologic Associates, PC data) and motor (University of Kansas Medical Center [KUMC] data) domains. Next, evaluations to explore the moderating role of the psychological impacts that are common in MS (e.g., depression and falls self-efficacy) were undertaken to determine whether the relationships between cognitive and motor function and DTW ability are different for individuals with different levels of these factors using regression with factor scores performed with each data set (aim 3). As a final step, relationships between DTW and distal outcomes like QoL (cross-sectionally using both data sets and factor score regression; aim 4) and falls (cross-sectionally and longitudinally using KUMC data and negative binomial regression; aim 5). These studies contribute to the corpus of knowledge about DTW in MS in needed ways.

INTRODUCTION: As people age, they become increasingly susceptible to falls, particularly when their attention is divided. Cognitive domains such as executive functioning and processing speed also decline over time and are associated with falls. A critical aspect to reducing falls are reactive steps, which are used to recover balance after a perturbation. Characterizing the relationship between cognition, dual tasking, and prioritization is necessary in order to decrease fall risk in older adults. Thus, the purpose of this analysis was to determine the effects of baseline cognitive status on dual task interference and prioritization of postural and cognitive tasks while reactive stepping. METHODS: 30 participants (Parkinson's disease (PD) n=16, healthy controls (HC) n=14) were divided into two groups based on their baseline cognitive status: the high-cog group (n=18) or the low-cog group (n=12). All participants experienced 7 perturbation trials where they were solely tasked with reactive stepping, 2 cognitive trials where they were solely tasked with verbally responding to an auditory Stroop test, and 7 trials that combined the two tasks. Cognitive and protective stepping performance was calculated for dual task interference and prioritization across groups. RESULTS: There were no outcome variables that showed the log-cog group performing worse than the high-cog group from single to dual task conditions. While examining the dual task interference between groups, the only significant outcome was that the low-cog group exhibited a subtle improvement in their step length performance under dual task conditions while the high-cog group did not. When comparing the prioritization scores, there was no statistically significant difference in prioritization between the high-cog and low-cog group. Albeit not significant across groups, the prioritization score for all outcomes was negative, indicating a stepping prioritization under dual task conditions for both groups. This analysis provides preliminary evidence that baseline cognitive status does not significantly affect dual task interference nor prioritization while reactive step dual tasking. While these effects should be treated with caution, these results would suggest that baseline cognitive status may not play a critical role in dual task interference or attentional allocation in both people with PD and healthy older adults.

There are many inconsistencies in the literature regarding how to estimate the Lyapunov Exponent (LyE) for gait. In the last decade, many papers have been published using Lyapunov Exponents to determine differences between young healthy and elderly adults and healthy and frail older adults. However, the differences in methodologies of data collection, input parameters, and algorithms used for the LyE calculation has led to conflicting numerical values for the literature to build upon. Without a unified methodology for calculating the LyE, researchers can only look at the trends found in studies. For instance, LyE is generally lower for young adults compared to elderly adults, but these values cannot be correlated across studies to create a classifier for individuals that are healthy or at-risk of falling. These issues could potentially be solved by standardizing the process of computing the LyE.

This dissertation examined several hurdles that must be overcome to create a standardized method of calculating the LyE for gait data when collected with an accelerometer. In each of the following investigations, both the Rosenstein et al. and Wolf et al. algorithms as well as three normalization methods were applied in order to understand the extent at which these factors affect the LyE. First, the a priori parameters of time delay and embedding dimension which are required for phase space reconstruction were investigated. This study found that the time delay can be standardized to a value of 10 and that an embedding dimension of 5 or 7 should be used for the Rosenstein and Wolf algorithm respectively. Next, the effect of data length on the LyE was examined using 30 to 1300 strides of gait data. This analysis found that comparisons across papers are only possible when similar amounts of data are used but comparing across normalization methods is not recommended. And finally, the reliability and minimum required number of strides for each of the 6 algorithm-normalization method combinations in both young healthy and elderly adults was evaluated. This research found that the Rosenstein algorithm was more reliable and required fewer strides for the calculation of the LyE for an accelerometer.