Healthy individuals can adjust their movements when changes arise to the body or the environment. Advanced age is associated with central and peripheral changes that may negatively impact one’s ability to adapt motor performance, such us upper-limb (UL) reaching movements. In this thesis, we conducted four studies to address the impact of aging on coordination and adaptation of goal-directed reaches. In the first experiment, we examined compensatory arm–trunk coordination in older adults during trunk-assisted reaching, using two motor tasks : 1) the Stationary Hand Task (SHT) in which older and young participants were asked to maintain a fixed hand position while flexing forward at the trunk, and 2) the Reaching Hand Task (RHT) in which participants were instructed to reach to a within-arm’s reach target while simultaneously flexing forward at the trunk (Raptis et al., 2007; Sibindi et al., 2013). We found that in SHT, young and older participants were able to maintain a stable hand position and compensate for trunk movement by appropriate angular rotations at the elbow and shoulder joints. As well, in the RHT, both groups made similar small overshoot errors. However, older participants performance was significantly more variable compared to young adults. These results suggest that older adult preserve their ability to coordinate arm and trunk movements efficiently during reaching but are not as consistent as young adults. In the second experiment, we sought to determine the ability of older adults to adjust shoulder and elbow coordination in response to changing task demands. Thus, we asked young and older adults to perform the RHT of Raptis et al. (2007) from the first experiment. A detailed comparison of UL kinematics during reaches in the presence and absence of trunk motion (i.e., free- vs. blocked-trunk trials) was performed and compared between young and older adults. We found that participants in both age group were able to coordinate motion at the elbow and shoulder joints in accordance with motion at the trunk. However, the extent of changes at the UL joints was smaller and more variable in older adults compared to young ones, especially when trunk motion was involved. These results imply that older adults can coordinate their UL movements based on task requirements, but with less consistent performance compared to young adults. In the third experiment, we investigated the preservation of intermanual transfer and retention of implicit visuomotor adaptation in older adults. We had young and older participants train to reach with visual feedback rotated 30° counter-clockwise relative to their actual hand motion. Furthermore, we examined whether providing augmented somatosensory feedback regarding movement endpoint would enhance visuomotor adaptation. We found that older adults demonstrated a comparable magnitude of implicit adaptation, transfer, and retention of visuomotor adaptation as observed in young adults, regardless of the presence of augmented somatosensory feedback. These results indicate that intermanual transfer and retention do not differ significantly between young and older adults when adaptation is driven implicitly, regardless the availability of augmented somatosensory feedback. In the fourth experiment, we looked to determine age-related differences in the engagement of offline and online control processes during implicit visuomotor adaptation. A detailed analysis of reaching performance was conducted and between young and older adults, during and after visuomotor adaptation. We found that when rotation was introduced, participants in both age took longer time to complete their movements, reached with a lower peak velocity and spent more time homing in on the target compared to reaches with aligned cursor feedback. Additionally, older adults had more curved paths with rotated cursor feedback compared to their reaches with aligned cursor feedback. Moreover, these changes in reaching performance continued following adaptation for both groups. These results suggest that young and older adults engage more in online control processes during implicit visuomotor adaptation. Together, these studies show that older adults: 1) maintain the ability to use compensatory arm-trunk coordination to maintain reaching accuracy, 2) preserve the ability to adjust the coordination between UL joints to meet task demands, 3) maintain the ability to adjust reaches to meet changes in the reaching environment, as well as transfer and retain the newly acquired movement, and 4) preserve the ability to modify the control processes underlying these adapted movements to meet the demands of the reaching environment. In conclusion, the flexibility to coordinate and adapt upper limb reaching performance to meet changes in task demands is maintained across lifespan.

Les Personnes à Mobilité Réduite (PMR) souffrent souvent de limitation au cours de la locomotion. Plusieurs outils sont à leurs dispositions pour les aider et les assister dans leur déplacement, plus particulièrement le Fauteuil Roulant Manuel (FRM) ou électrique. Ces travaux de thèse se focalisent sur les interactions entre le sujet et le FRM afin de déterminer des éléments prédictifs du déplacement à venir dans l’optique d’une aide à la locomotion. Comprendre l'Interaction Homme-FRM (IHF) est un enjeu actuel afin d'améliorer le déplacement et d’optimiser la performance ainsi que le confort, en minimisant la fatigue et les risques des blessures. Pour contribuer à mieux comprendre voire améliorer cette IHF, un ensemble d'études centrées sur une analyse biomécanique et électromyographique des éléments d'anticipation de l'Initiation du Déplacement en FRM (ID-FRM) a été réalisé au cours de ces travaux de thèse. Cet ensemble d’études porte sur i) les Activités Musculaires Anticipatoires (AMA) en position assise pour interroger la prédictibilité des mouvements au regard de ses paramètres préalables ; ii) l’évolution des patterns musculaires et des pressions sur l'assise lors de l'ID-FRM, pour interroger l’adaptabilité ou la spécificité des paramètres initiaux lors de différents déplacements en FRM chez des sujets valides. ; iii) l’évolutions des patterns musculaires et des pressions sur l'assise lors de l'ID-FRM chez des sujets blessés médullaires et ; iv) l’analyse multi-variée des paramètres de l'ID-FRM. Le SNC présente des capacités d'adaptabilité importantes au mouvement à venir en ajustant les commandes motrices aux conditions physiologiques globales du sujet. Cette adaptabilité a conduit à des pourcentages importants de prédiction et elle peut être enregistrée au travers d'éléments macroscopiques permettant d'envisager des systèmes d'aide au déplacement, notamment pour les PMR. / People with Disabled Mobility (PDM) often suffer from limitation during locomotion. Several tools are at their disposal to help and assist them in their displacement, specifically the electric or the Manual Wheelchair (MW). This thesis works focuses on the Human-MW-Interaction (H-MW-I) subject to determine elements predictors of forthcoming displacement with a view of aid to locomotion. Understanding (H-MW-I) is a current challenge to improve the displacement and optimize the performance as well as comfort, by minimizing fatigue and injury risk. In order to enhance understanding and even improving the (H-MW-I), a set of studies focusing on Biomechanics and electromyographic analysis of anticipation elements of the Manual Wheelchair Initiation Displacement (MW-ID) was performed during the thesis works. This set of studies focuses on i) Anticipatory Muscular Activities (AMA) in the sitting position to question the predictability of the movements in relation to its prior parameters; ii) evolution of muscular patterns and pressures on the seat when MW-ID to question the adaptability or specificity of the initial parameters during various displacements using MW in healthy subjects ; iii) evolution of muscular patterns and pressures on the seat when MW-ID in spinal cord injury subjects and; iv) multivariate analysis of the MW-ID parameters. The CNS presents an important capacity of adaptability to the forthcoming movement by adjusting motor commands to the global physiological condition of the subject. This adaptability has led to significant percentages prediction and it can be saved through macroscopic elements to envisage help systems to displacement for PDM.

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