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The human body has substantial kinetic and kinematic degrees-of-freedoms, so redundant solutions are available for the central nervous system (CNS) to perform a repetitive task. Due to these redundancies, inherent variations exist in human movement, called motor variability (MV). Current evidence suggests that MV can be beneficial, and that there is an inverse association between MV and risk of injury. To better understand how the CNS manipulates MV to reduce injury risks, we investigated the effects of individual differences, task-relevant aspects, and psychological factors as modifiers of MV. Earlier work found that experienced workers adapted more stable movements than novices in repetitive lifting tasks. To expand on this, we quantified how MV differs between experienced workers and novices in different lifting conditions (i.e., lifting asymmetry and fatigue). Three different measures (cycle-to-cycle SD, sample entropy, and the goal equivalent manifold) were used to quantify MV. In a symmetric lifting task, experienced workers had more constrained movement than novices, and experienced workers exhibited more consistent behavior in the asymmetric condition. Novices constrained their movements, and could not maintain the same level of variability in the asymmetric condition. We concluded that experienced workers adapt stable or flexible strategies depending on task difficulty. In a prolonged lifting task, both groups increased their MV to adapt to fatigue; they particularly increased variability in a direction that had no effects on their main task goal. Developing fatigue also makes it difficult for individuals maintain the main goal. Based on these results, we conclude that increasing variability is an adaptive strategy in response to fatigue. We also assessed variability in gait parameters to compare gait adaptability using a head-worn display (HWD) compared with head-down displays for visual information presentation. An effective strategy we observed for performing a cognitive task successfully during walking was to increase gait variability in the goal direction. In addition, we found that head-up walking had smaller effects on MV, suggesting that HWDs are a promising technology to reduce adverse events during gait (e.g., falls). In summary, these results suggest that MV can be a useful indicator for evaluating some occupational injury risks. / Ph. D. / Whenever an individual performs a repetitive task, we can observe variations in their movement patterns. The magnitude of these variations, which are called motor variability, may be related to the risk of injury. To better understand this relationships, we investigated how different risk factors affect the patterns of human movement. In two studies, we compared movement patterns of experienced workers and novices in a repetitive lifting task. In a simple, brief lifting task, novices had more variations in their movement patterns. However, novices did not have the same level of variation in asymmetric lifting tasks, and constrained their movement more than experienced workers. Experienced workers, though, had a similar level of variation in both simple and more difficult lifting conditions. We concluded that whether stable or flexible movement pattern are used depends on task difficulty and the level of experience. In a longer-duration lifting task, both experienced workers and novices increased variations in their movement patterns over time, and we believe that these increases were an adaptation to fatigue. In a third study, we investigated the differences between variations in walking pattern when people use different types of information display (i.e., paper, cellphone, and smart glasses). Using smart glasses had a smaller effect on movement patterns, suggesting that this technology is potentially is safer than other types of display. In summary, these results suggest that studying the variations in human movement patterns can be a useful indicator to evaluate the risk of injury.

While we are witnessing an acceleration of the phenomenon of demographic aging in Canada, a spectacular increase in the participation rate of the elderly has been felt since the end of the 1990s. Several factors may be at the origin of this increase, but few studies really focus on empirically verifying these. It is in this register that this research is focused. In fact, in 2012, the government of Quebec implemented a tax initiative dedicated to the maintenance and return to the labor market of the elderly, namely the “Tax credit for experienced workers”, today. renamed as the “Tax Credit for career extension”. However, to date, no study has been carried out on the effectiveness of this Quebec tax measure in which the government has spent nearly $ 700 million and is preparing to spend more in the coming years. Testing the effect of this incentive policy for experienced workers will therefore be the main objective of this research. In order to meet this objective, we performed multivariate regression analyzes using a statistical model developed from our review of the literature on labor force participation rates of people aged 55 and over. Our sample consists of 360 data or 36 observations (one for each of the years covering the period 1983 to 2018) for each of the 10 provinces of Canada. For the purposes of our analyzes, we selected the participation rate of those aged 55 and over as the dependent variable as well as a realistic selection of 10 independent variables divisible into three categories, i.e., trend and non-trend structural variables (term linear trend, squared trend term and set of 9 provincial dichotomies), economic variables (unemployment rate, interest rate and average weekly earnings) and public policy variables (income tax benefit work, incentive nature within pension schemes and tax credit for experienced workers). Our results show that the Quebec incentive, the tax credit for career extension, did not have the expected effects on workers aged 55 and over, while particularly noticeable changes are observable for this age group. It seems that the good performance of the economic situation is sufficient to explain a good part of the behavior of the participation rates of people aged 55 and over in Quebec. These results are sufficient to encourage further evaluation of the tax credit program, as well as all current and future incentive policies put in place by governments. / Alors que nous assistons à une accélération du phénomène du vieillissement démographique au Canada, une remontée spectaculaire du taux d’activité des personnes âgées se fait sentir depuis la fin des années 1990. Plusieurs facteurs peuvent être à l’origine de cette hausse, mais peu d’études se concentrent réellement sur la vérification empirique de ceux-ci. C’est d’ailleurs dans ce registre que se concentre cette recherche. En effet, en 2012, le gouvernement du Québec s’est vu mettre en place une initiative fiscale explicitement dédiée au maintien et au retour sur le marché du travail des personnes âgées, soit le crédit d’impôt pour les travailleurs expérimentés, aujourd’hui renommée sous le nom de « crédit d’impôt pour le prolongement de carrière ». Or, à ce jour, aucune étude n’a été réalisée sur l’efficacité de cette mesure fiscale québécoise dans laquelle le gouvernement a dépensé près de 700 millions de dollars et s’apprête à en dépenser davantage dans les prochaines années. Tester l’effet de cette politique d’incitation pour les travailleurs expérimentés sera donc l’objectif principal de cette recherche. Afin de répondre à cet objectif, nous avons réalisé des analyses de régression multivariée à l’aide d’un modèle statistique développé à partir de notre recension des écrits sur les taux d’activité des personnes âgées de 55 ans et plus. Notre échantillon se compose de 360 données, soit 36 observations (une pour chacune des années couvrant la période de 1983 à 2018) pour chacune des 10 provinces du Canada. Pour les fins de nos analyses, nous avons choisi le taux d’activité des 55 ans et plus à titre de variable dépendante ainsi qu’une sélection réaliste de 10 variables indépendantes divisibles en trois catégories, soit des variables structurelles tendancielles et non tendancielles (terme de tendance linéaire, terme de tendance au carré et ensemble de 9 dichotomiques provinciales), des variables économiques (taux de chômage, taux d’intérêt et rémunération hebdomadaire moyenne) et des variables de politiques publiques (prime au travail, prestation fiscale pour revenu de travail, caractère incitatif à l’intérieur des régimes de pensions et crédit d’impôt pour les travailleurs expérimentés). Nos résultats démontrent d’ailleurs que l’incitative québécoise, soit le crédit d’impôt pour le prolongement de carrière n’a pas eu les effets escomptés sur les travailleurs âgés de 55 ans et plus, alors que des changements particulièrement marquant sont observables pour ce groupe d’âge. Il semble que la bonne tenue de la conjoncture économique soit suffisante afin d’expliquer une bonne part des comportements des taux d’activité des 55 ans et plus au Québec. Ces résultats sont suffisants afin d’inciter à la poursuite de l’évaluation du programme de crédit d’impôt, de même que l’ensemble des politiques d’incitation actuelles et futures mises en place par les gouvernements.