xiii, 109 p. : ill. / Falls are a significant source of physical, social, and psychological suffering among elderly adults. Falls lead to morbidity and even mortality. Over one-third of adults over the age of 65 years will fall within a calendar year, with almost 10,000 deaths per year attributed to falls. The direct cost of falls exceeds $10 billion a year in the United States. Fall incidents have been linked to multiple risk factors, including cognitive function, muscle strength, and balance control. The ability to properly identify balance impairment is a tremendous challenge to the medical community, with accurate assessment of fall risk lacking. Therefore, the purpose of this study was to assess balance control during gait among young adults, elderly adults, and elderly fallers; determine which biomechanical measures can best identify fallers retrospectively; demonstrate longitudinal changes in elderly adults and prospectively assess fall risk; and provide a method for mapping clinical variables to sensitive balance control measures using artificial neural networks. The interaction of the whole body center of mass (CoM) in relation to the base of support (BoS) assessed static and dynamic balance control throughout gait. Elderly fallers demonstrated reduced balance control ability, specifically a decreased time to contact with the boundary of the BoS, when compared to young adults at heel strike. This decreased time might predispose older adults to additional falls due to an inability to properly respond to perturbations or slips. Inclusion of these balance control measures along with the Berg Balance Scale and spatiotemporal measures demonstrated sensitivity and specificity values of up to 90% when identifying 98 elderly fallers and non-fallers, respectively. Additionally, 27 older adults were followed longitudinally over a period of one year, with only the interaction of the CoM with the BoS demonstrating an ability to differentiate fallers and non-fallers prospectively. As the collection and analysis of these biomechanics measures can be time consuming and expensive, an artificial neural network demonstrated that clinical measures can accurately predict balance control during ambulation. This model approached a solution quickly and provides a means for assessing longitudinal changes, intervention effects, and future fall risk. This dissertation includes both previously published and unpublished co-authored material. / Committee in charge: Dr. Li-Shan Chou, Chair; Dr. Andrew Karduna, Member; Dr. Marjorie Woollacott, Member; Dr. Ronald Stock, Member; Dr. Arthur Farley, Outside Member

La valve aortique est une composante anatomique centrale du cœur, sujette à de hautes pressions. Les conséquences d’un dysfonctionnement sont graves, notamment l’insuffisance cardiaque qui elle-même peut causer plusieurs symptômes et un impact sur la qualité de vie. Pour prévenir cette complication, il est possible de remplacer la valve par une prothèse. Il en existe plusieurs types parmi lesquelles l’équipe chirurgicale et le patient peuvent choisir. Les dernières lignes directrices américaines n’ont pas de recommandations claires pour les patients entre 50 et 70 ans. Ces patients, âgés de 65 ans et moins et surnommés jeunes adultes dans notre étude, reçoivent de moins en moins de prothèses mécaniques, au profit de celles de type biologique. Ce mouvement semble fondé sur certaines études suggérant une survie comparable entre ces 2 alternatives. De plus, on déconseille souvent la valve mécanique car elle nécessite un traitement anticoagulant à vie. Or, peu d’études ont suivi à long terme ces patients plus jeunes en analysant l’impact sur la qualité de vie du traitement anticoagulant ainsi que le risque de saignement. Notre étude visait donc surtout à analyser l’incidence, la sévérité et l’impact de ces saignements majeurs et la qualité de vie suite à un remplacement de valve aortique chez ces patients. Après un suivi moyen de 11 ans, les résultats suggèrent un taux incident de saignement majeur de 0.8% par patient-année et la mortalité associée à ceux-ci est faible à 3.3%, soit 2 hémorragies intracrâniennes. En tout, 48 patients ont eu un saignement majeur (8.9%). D’un autre côté, l’impact sur la qualité de vie obtenu par un questionnaire spécifique aux valves est faible. En outre, seulement 10.5% des patients utilisaient l’automesure pour surveiller leur anticoagulation, suggérant beaucoup de place à l’amélioration à ce niveau. Cette étude permet donc de mieux orienter la prise de décision au moment de la chirurgie et mieux informer les patients. / The aortic valve is a central component of the heart, experiencing high strain. The consequences of any dysfunctions are usually important, notably heart failure, which in itself is associated with many symptoms and lower quality of life. To prevent this complication, it is possible to replace the valve with a prosthesis. There are many options for the surgeon’s team and the patient to choose from. The latest American guidelines do not have clear recommendations for patients aged between 50 and 70 years. These patients aged 65 years and less, categorized as non-elderly adults, are receiving less and less mechanical prosthesis in profit of the biological ones. This trend seems to be based in part on some studies suggesting comparable survival between these alternatives. Moreover, the mechanical valve is frequently not recommended because it necessitates a lifelong anticoagulant treatment. However, few studies have reported a long-term follow-up of these younger patients analyzing the impact on the quality of life and the risk of major bleeding. The goal of our study was to analyze the incidence, severity and impact of major bleedings and the quality of life after aortic valve replacement in these patients. After a mean follow-up of 11 years, results show a linearized rate of 0.8% per patient-year and the associated mortality is low at 3.3%, consisting of 2 intracranial hemorrhages. Overall, 48 patients experienced a major bleeding (8.9%). On the other hand, the impact on the quality of life measured with a valve-specific questionnaire is low. Also, only 10.5% of the patients were using self-management or self-medication to monitor their anticoagulation, suggesting a lot of room for improvement in this regard. This study helps better define long-term outcomes in this patient population in order to better inform patients about surgical options.