The ability to mentally represent actions is suggested to have a role in the online control of movement in healthy adults. Children’s movement imagery ability and online control have been shown to develop at similar non-linear rates. The current study investigated the relationship between movement imagery and online control in children by comparing implicit and explicit movement imagery measures with the ability to make online trajectory corrections. Imagery ability was a significant predictor of children’s online control of reaching once age and general reaching efficiency were controlled for. These findings extend the proposed relationship between movement imagery and online control.

No / The importance of peripheral visual cues in the control of minimum-foot-clearance during overground locomotion on a clear path was investigated. Eleven subjects walked at their natural speed whilst wearing goggles providing four different visual conditions: upper occlusion, lower occlusion, circumferential–peripheral occlusion and full vision. Results showed that under circumferential– peripheral occlusion, subjects were more cautious and increased minimum-foot-clearance and decreased walking speed and step length. The minimum-foot-clearance increase can be interpreted as a motor control strategy aiming to safely clear the ground when online visual exproprioceptive cues from the body are not available. The lack of minimum-foot-clearance increase in lower occlusion suggests that the view of a clear pathway from beyond two steps combined with visual exproprioception and optic flow in the upper field were adequate to guide gait. A suggested accompanying safety strategy of reducing the amount of variability of minimum-foot-clearance under circumferential–peripheral occlusion conditions was not found, likely due to the lack of online visual exproprioceptive cues provided by the peripheral visual field for fine-tuning foot trajectory.

Human movements are remarkably adaptive, such that we are capable of completing movements in a novel environment with similar accuracy to those performed in a typical environment. Our ability to perform in these environments involves accurate processing of sensory feedback for online and offline control. These processes of control have been widely studied for well learned actions, but not for actions in a novel visuomotor environment. In two experiments, we examined control processes underlying reaches when participants were first introduced to a visuomotor rotation (Experiment 1) and then following visuomotor adaptation (Experiment 2). All participants completed 150 reach training trials when (1) a cursor accurately represented their hand motion (i.e., aligned cursor) and (2) a cursor was rotated 45 degrees clockwise relative to their hand motion (i.e., rotated cursor). In Experiment 1, we sought to determine if the control processes underlying movements in typical and novel visuomotor conditions were comparable. Participants (n = 16) received either continuous visual feedback or terminal visual feedback regarding movement endpoint during reach training. Analyses revealed that participants were able to demonstrate similar outcomes (i.e., movement time and endpoint errors) regardless of visual or cursor feedback, but also demonstrated more offline control (i.e., took more time planning and were less consistent in initiating their movements) when reaching with a rotated cursor compared to an aligned cursor, even at the end of training. Together, the results suggest a greater contribution of offline control processes and less effective online corrective processes when reaching in a novel environment compared to when reaching in a typical environment. In attempt to promote online corrective processes, participants (n = 16) in Experiment 2 first completed the training trials with continuous visual feedback and then completed an additional 45 reaches under (1) slow movement time (i.e., Slow MT: 800-1000 ms) and (2) fast movement time (i.e., Fast MT: 400-500ms) constraints. Results showed a shift to online control (i.e., greater endpoint accuracy) when reaching with an aligned and rotated cursor, when sufficiently more time was provided (i.e., Slow MT). Specifically, participants were able to more effectively utilize visual feedback for online control under the Slow MT constraint compared to when reaching quickly (i.e., Fast MT). Together, these experiments demonstrate a flexibility in control processes underlying reaches with rotated visual feedback of the hand. In that reaches first engage in offline control processes during adaptation to a visuomotor rotation, and then shift to online corrective processes following visuomotor adaptation.

Lors d’une tâche de pointage manuel, la présence de corrections rapides, adaptées, automatiques et même réflexes (Franklin et Wolpert, 2008) suite à une perturbation par saut de curseur a pu être observée dans de nombreuses études. Ici, nous avons souhaité déterminer si ces corrections étaient purement réflexes où si elles étaient amorcées seulement lorsque la perturbation mettait en péril l’atteinte de la cible ; ces corrections ont-elles aussi un aspect fonctionnel ? Dans une première expérience nous avons fait varier la taille des cibles (5 ou 30 mm de diamètre) et des sauts du curseur (5, 15 ou 25 mm) de manière à obtenir certaines combinaisons où la cible pourrait être atteinte sans qu’aucune correction du mouvement pour contrecarrer l’effet du saut du curseur ne soit nécessaire. Des corrections réduisant l’erreur d’environ 65% ont été observées dans toutes les conditions. Dans une seconde expérience, les participants devaient atteindre une très grande cible (arc de 30°) et un saut de curseur de 15 mm était introduit pour certains essais peu de temps après l’amorce du mouvement. Les participants ont modifié leur mouvement dans le sens opposé à celui de la perturbation, et cela même s’ils n’avaient pas détecté consciemment le saut. Cependant, ces corrections étaient moins rapides et plus petites (42% de l’amplitude du saut de curseur) que celles observées lors de la première expérience. Nos résultats supportent le fait que l’amorce des corrections pour des erreurs de trajectoire induites expérimentalement soit de nature réflexe. Un deuxième processus serait alors responsable du déroulement de ces corrections ; ce deuxième processus est basé, entre autres, sur les caractéristiques de la cible. / Cursor-jump experiments have suggested the existence of quick, efficient, automatic and even reflexive (Franklin and Wolpert, 2008) online correction processes in manual aiming movements. In the present study, we wanted to determine whether corrections for a cursor jump are purely automatic/reflexive or whether they are functional in that they occur only when they are required for the target to be reached. In a first experiment, we used different target sizes (5 mm to 30 mm) and cursor-jump amplitudes (5 mm to 25 mm) so that for some target size/cursor-jump combinations, no correction would be needed to reach the target. In all cases, we observed a correction for the cursor-jump. This correction reduced the error induced by the cursor jump by 60-70%, regardless of target size. In a second experiment, we asked participants to point at a large wedge (30° of circular arc). For some trials, a cursor-jump translated the location of the cursor laterally by 15 mm soon after movement initiation. Participants never consciously detected the cursor-jump but clearly modified the trajectory of their movement in the direction opposite to that of the cursor-jump. These corrections were smaller than those observed in the first experiment (42% of the cursor-jump). Our results indicate that the initiation of a correction for a cursor-jump is more reflexive than it is functional. A second correction process would tailor the movement's initial impulse based on the target characteristics.

Visual cues have previously been classified as visual exproprioceptive, when defining the relative position of the body within the environment and are continuously updated while moving (online), and visual exteroceptive when describing static features of the environment which are typically elaborated offline (feedforward). However peripheral visual cues involved in the control of movement have not previously been clearly defined using this classification. Hence the role played by peripheral visual cues in the planning and/or online control of movement remains unclear. The aim of this thesis was to provide a systematic understanding of the importance of peripheral visual cues in several types of movement, namely overground locomotion, adaptive gait, postural stability and reaching and grasping. 3D motion capture techniques were used to collect limb and whole body kinematics during such movements. Visual peripheral cues were manipulated by visual field occlusion conditions or by the employment of point-lights in a dark room. Results showed that the visual cues provided by different parts of the peripheral visual field are mainly used for online fine tuning of limb trajectory towards a target (either a floor-based obstacle or an object to grasp). The absence of peripheral visual cues while moving disrupted the spatio-temporal dynamic relationship between subject and target and resulted in increased margins of safety between body and target and increased time and variability of several dependent measures. These findings argue in favour of the classification of peripheral visual cues as visual exproprioceptive.

Lors d’une tâche de pointage manuel, la présence de corrections rapides, adaptées, automatiques et même réflexes (Franklin et Wolpert, 2008) suite à une perturbation par saut de curseur a pu être observée dans de nombreuses études. Ici, nous avons souhaité déterminer si ces corrections étaient purement réflexes où si elles étaient amorcées seulement lorsque la perturbation mettait en péril l’atteinte de la cible ; ces corrections ont-elles aussi un aspect fonctionnel ? Dans une première expérience nous avons fait varier la taille des cibles (5 ou 30 mm de diamètre) et des sauts du curseur (5, 15 ou 25 mm) de manière à obtenir certaines combinaisons où la cible pourrait être atteinte sans qu’aucune correction du mouvement pour contrecarrer l’effet du saut du curseur ne soit nécessaire. Des corrections réduisant l’erreur d’environ 65% ont été observées dans toutes les conditions. Dans une seconde expérience, les participants devaient atteindre une très grande cible (arc de 30°) et un saut de curseur de 15 mm était introduit pour certains essais peu de temps après l’amorce du mouvement. Les participants ont modifié leur mouvement dans le sens opposé à celui de la perturbation, et cela même s’ils n’avaient pas détecté consciemment le saut. Cependant, ces corrections étaient moins rapides et plus petites (42% de l’amplitude du saut de curseur) que celles observées lors de la première expérience. Nos résultats supportent le fait que l’amorce des corrections pour des erreurs de trajectoire induites expérimentalement soit de nature réflexe. Un deuxième processus serait alors responsable du déroulement de ces corrections ; ce deuxième processus est basé, entre autres, sur les caractéristiques de la cible. / Cursor-jump experiments have suggested the existence of quick, efficient, automatic and even reflexive (Franklin and Wolpert, 2008) online correction processes in manual aiming movements. In the present study, we wanted to determine whether corrections for a cursor jump are purely automatic/reflexive or whether they are functional in that they occur only when they are required for the target to be reached. In a first experiment, we used different target sizes (5 mm to 30 mm) and cursor-jump amplitudes (5 mm to 25 mm) so that for some target size/cursor-jump combinations, no correction would be needed to reach the target. In all cases, we observed a correction for the cursor-jump. This correction reduced the error induced by the cursor jump by 60-70%, regardless of target size. In a second experiment, we asked participants to point at a large wedge (30° of circular arc). For some trials, a cursor-jump translated the location of the cursor laterally by 15 mm soon after movement initiation. Participants never consciously detected the cursor-jump but clearly modified the trajectory of their movement in the direction opposite to that of the cursor-jump. These corrections were smaller than those observed in the first experiment (42% of the cursor-jump). Our results indicate that the initiation of a correction for a cursor-jump is more reflexive than it is functional. A second correction process would tailor the movement's initial impulse based on the target characteristics.

Visual cues have previously been classified as visual exproprioceptive, when defining the relative position of the body within the environment and are continuously updated while moving (online), and visual exteroceptive when describing static features of the environment which are typically elaborated offline (feedforward). However peripheral visual cues involved in the control of movement have not previously been clearly defined using this classification. Hence the role played by peripheral visual cues in the planning and/or online control of movement remains unclear. The aim of this thesis was to provide a systematic understanding of the importance of peripheral visual cues in several types of movement, namely overground locomotion, adaptive gait, postural stability and reaching and grasping. 3D motion capture techniques were used to collect limb and whole body kinematics during such movements. Visual peripheral cues were manipulated by visual field occlusion conditions or by the employment of point-lights in a dark room. Results showed that the visual cues provided by different parts of the peripheral visual field are mainly used for online fine tuning of limb trajectory towards a target (either a floor-based obstacle or an object to grasp). The absence of peripheral visual cues while moving disrupted the spatio-temporal dynamic relationship between subject and target and resulted in increased margins of safety between body and target and increased time and variability of several dependent measures. These findings argue in favour of the classification of peripheral visual cues as visual exproprioceptive.

L’objectif principal de la présente thèse était de déterminer les facteurs susceptibles d’influencer l’efficacité des processus de contrôle en ligne des mouvements d’atteinte manuelle. De nos jours, les mouvements d’atteinte manuelle réalisés dans un environnement virtuel (déplacer une souris d’ordinateur pour contrôler un curseur à l’écran, par exemple) sont devenus chose commune. Par comparaison aux mouvements réalisés en contexte naturel (appuyer sur le bouton de mise en marche de l’ordinateur), ceux réalisés en contexte virtuel imposent au système nerveux central des contraintes importantes parce que l’information visuelle et proprioceptive définissant la position de l’effecteur n’est pas parfaitement congruente. Par conséquent, la présente thèse s’articule autour des effets d’un contexte virtuel sur le contrôle des mouvements d’atteinte manuelle. Dans notre premier article, nous avons tenté de déterminer si des facteurs tels que (a) la quantité de pratique, (b) l’orientation du montage virtuel (aligné vs. non-aligné) ou encore (c) l’alternance d’un essai réalisé avec et sans la vision de l’effecteur pouvaient augmenter l’efficacité des processus de contrôle en ligne de mouvement réalisés en contexte virtuel. Ces facteurs n’ont pas influencé l’efficacité des processus de contrôle de mouvements réalisés en contexte virtuel, suggérant qu’il est difficile d’optimiser le contrôle des mouvements d’atteinte manuelle lorsque ceux-ci sont réalisés dans un contexte virtuel. L’un des résultats les plus surprenants de cette étude est que nous n’avons pas rapporté d’effet concernant l’orientation de l’écran sur la performance des participants, ce qui était en contradiction avec la littérature existante sur ce sujet. L’article 2 avait pour but de pousser plus en avant notre compréhension du contrôle du mouvement réalisé en contexte virtuel et naturel. Dans le deuxième article, nous avons mis en évidence les effets néfastes d’un contexte virtuel sur le contrôle en ligne des mouvements d’atteinte manuelle. Plus précisément, nous avons observé que l’utilisation d’un montage non-aligné (écran vertical/mouvement sur un plan horizontal) pour présenter l’information visuelle résultait en une importante diminution de la performance comparativement à un montage virtuel aligné et un montage naturel. Nous avons aussi observé une diminution de la performance lorsque les mouvements étaient réalisés dans un contexte virtuel aligné comparativement à un contexte naturel. La diminution de la performance notée dans les deux conditions virtuelles s’expliquait largement par une réduction de l’efficacité des processus de contrôle en ligne. Nous avons donc suggéré que l’utilisation d’une représentation virtuelle de la main introduisait de l’incertitude relative à sa position dans l’espace. Dans l’article 3, nous avons donc voulu déterminer l’origine de cette incertitude. Dans ce troisième article, deux hypothèses étaient à l’étude. La première suggérait que l’augmentation de l’incertitude rapportée dans le contexte virtuel de la précédente étude était due à une perte d’information visuelle relative à la configuration du bras. La seconde suggérait plutôt que l’incertitude provenait de l’information visuelle et proprioceptive qui n’est pas parfaitement congruente dans un contexte virtuel comparativement à un contexte naturel (le curseur n’est pas directement aligné avec le bout du doigt, par exemple). Les données n’ont pas supporté notre première hypothèse. Plutôt, il semble que l’incertitude soit causée par la dissociation de l’information visuelle et proprioceptive. Nous avons aussi démontré que l’information relative à la position de la main disponible sur la base de départ influence largement les processus de contrôle en ligne, même lorsque la vision de l’effecteur est disponible durant le mouvement. Ce résultat suggère que des boucles de feedback interne utilisent cette information afin de moduler le mouvement en cours d’exécution. / The main objective of this thesis was to identify factors that may influence the effectiveness of the online control processes of manual reaching movements. Nowadays, producing manual movements made in a virtual environment (moving a computer mouse to control a cursor on the screen, for example) have become commonplace. As compared to movements made in natural settings (pushing the start button on the computer), those made in virtual context pose serious challenges to the central nervous system because the proprioceptive and visual information defining the position of the effector is not perfectly congruent. This thesis focuses on the effects of a virtual environment on the control of manual reaching movements. In our first article, we examined whether such factors as (a) the amount of practice, (b) the orientation of the virtual display (aligned vs. non-aligned) or (c) the availability of the visual feedback on alternated trials could increase the efficiency of online control processes of movements made under virtual context. These factors did not influence the effectiveness of process control movements made in virtual context, suggesting that it is difficult to optimize the control of manual reaching movements when they are made under a virtual context. One of the most surprising results of this study is that we have not reported detrimental effects of screen orientation on the performance of participants, which was inconsistent with the existing literature on this subject. Section 2 was intended to push forward our understanding of online control processes of movements made in virtual and natural contexts. In the second article, we highlighted the adverse effects of a virtual environment on the online control of manual reaching movements. More specifically, we observed that the use of a non-aligned display to present visual information resulted in a significant decrease in performance as compared to an aligned or natural display. We also observed a decrease in performance when the movements were performed in a virtual aligned display as compared to a natural context. The decrease in performance observed in the two virtual conditions was largely due to a decrease of the effectiveness of online control processes. We therefore suggest that the use of a virtual representation of the hand introduced uncertainty as to its position in space, which in turn led to less efficient online control. In our third article, we wanted to determine the origin of this uncertainty and how it impacted on movement control. In the third article, two hypotheses were considered. The first suggested that the increase in uncertainty reported in the virtual context of the previous study was due to loss of visual information relative to the configuration of the arm. The second suggested instead that the uncertainty came from the proprioceptive and visual information that is not perfectly congruent in a virtual context as compared to a natural one (the cursor is not directly aligned with the finger for example). The data have not supported our first hypothesis. It rather appears that the uncertainty is caused by the dissociation of visual and proprioceptive information. We also demonstrated that the information on the starting base on the position of the hand greatly influences the control process online, even when vision is available during the effector movement. This result suggests that internal feedback loops use this information to modulate the ongoing movement.

L’objectif principal de la présente thèse était de déterminer les facteurs susceptibles d’influencer l’efficacité des processus de contrôle en ligne des mouvements d’atteinte manuelle. De nos jours, les mouvements d’atteinte manuelle réalisés dans un environnement virtuel (déplacer une souris d’ordinateur pour contrôler un curseur à l’écran, par exemple) sont devenus chose commune. Par comparaison aux mouvements réalisés en contexte naturel (appuyer sur le bouton de mise en marche de l’ordinateur), ceux réalisés en contexte virtuel imposent au système nerveux central des contraintes importantes parce que l’information visuelle et proprioceptive définissant la position de l’effecteur n’est pas parfaitement congruente. Par conséquent, la présente thèse s’articule autour des effets d’un contexte virtuel sur le contrôle des mouvements d’atteinte manuelle. Dans notre premier article, nous avons tenté de déterminer si des facteurs tels que (a) la quantité de pratique, (b) l’orientation du montage virtuel (aligné vs. non-aligné) ou encore (c) l’alternance d’un essai réalisé avec et sans la vision de l’effecteur pouvaient augmenter l’efficacité des processus de contrôle en ligne de mouvement réalisés en contexte virtuel. Ces facteurs n’ont pas influencé l’efficacité des processus de contrôle de mouvements réalisés en contexte virtuel, suggérant qu’il est difficile d’optimiser le contrôle des mouvements d’atteinte manuelle lorsque ceux-ci sont réalisés dans un contexte virtuel. L’un des résultats les plus surprenants de cette étude est que nous n’avons pas rapporté d’effet concernant l’orientation de l’écran sur la performance des participants, ce qui était en contradiction avec la littérature existante sur ce sujet. L’article 2 avait pour but de pousser plus en avant notre compréhension du contrôle du mouvement réalisé en contexte virtuel et naturel. Dans le deuxième article, nous avons mis en évidence les effets néfastes d’un contexte virtuel sur le contrôle en ligne des mouvements d’atteinte manuelle. Plus précisément, nous avons observé que l’utilisation d’un montage non-aligné (écran vertical/mouvement sur un plan horizontal) pour présenter l’information visuelle résultait en une importante diminution de la performance comparativement à un montage virtuel aligné et un montage naturel. Nous avons aussi observé une diminution de la performance lorsque les mouvements étaient réalisés dans un contexte virtuel aligné comparativement à un contexte naturel. La diminution de la performance notée dans les deux conditions virtuelles s’expliquait largement par une réduction de l’efficacité des processus de contrôle en ligne. Nous avons donc suggéré que l’utilisation d’une représentation virtuelle de la main introduisait de l’incertitude relative à sa position dans l’espace. Dans l’article 3, nous avons donc voulu déterminer l’origine de cette incertitude. Dans ce troisième article, deux hypothèses étaient à l’étude. La première suggérait que l’augmentation de l’incertitude rapportée dans le contexte virtuel de la précédente étude était due à une perte d’information visuelle relative à la configuration du bras. La seconde suggérait plutôt que l’incertitude provenait de l’information visuelle et proprioceptive qui n’est pas parfaitement congruente dans un contexte virtuel comparativement à un contexte naturel (le curseur n’est pas directement aligné avec le bout du doigt, par exemple). Les données n’ont pas supporté notre première hypothèse. Plutôt, il semble que l’incertitude soit causée par la dissociation de l’information visuelle et proprioceptive. Nous avons aussi démontré que l’information relative à la position de la main disponible sur la base de départ influence largement les processus de contrôle en ligne, même lorsque la vision de l’effecteur est disponible durant le mouvement. Ce résultat suggère que des boucles de feedback interne utilisent cette information afin de moduler le mouvement en cours d’exécution. / The main objective of this thesis was to identify factors that may influence the effectiveness of the online control processes of manual reaching movements. Nowadays, producing manual movements made in a virtual environment (moving a computer mouse to control a cursor on the screen, for example) have become commonplace. As compared to movements made in natural settings (pushing the start button on the computer), those made in virtual context pose serious challenges to the central nervous system because the proprioceptive and visual information defining the position of the effector is not perfectly congruent. This thesis focuses on the effects of a virtual environment on the control of manual reaching movements. In our first article, we examined whether such factors as (a) the amount of practice, (b) the orientation of the virtual display (aligned vs. non-aligned) or (c) the availability of the visual feedback on alternated trials could increase the efficiency of online control processes of movements made under virtual context. These factors did not influence the effectiveness of process control movements made in virtual context, suggesting that it is difficult to optimize the control of manual reaching movements when they are made under a virtual context. One of the most surprising results of this study is that we have not reported detrimental effects of screen orientation on the performance of participants, which was inconsistent with the existing literature on this subject. Section 2 was intended to push forward our understanding of online control processes of movements made in virtual and natural contexts. In the second article, we highlighted the adverse effects of a virtual environment on the online control of manual reaching movements. More specifically, we observed that the use of a non-aligned display to present visual information resulted in a significant decrease in performance as compared to an aligned or natural display. We also observed a decrease in performance when the movements were performed in a virtual aligned display as compared to a natural context. The decrease in performance observed in the two virtual conditions was largely due to a decrease of the effectiveness of online control processes. We therefore suggest that the use of a virtual representation of the hand introduced uncertainty as to its position in space, which in turn led to less efficient online control. In our third article, we wanted to determine the origin of this uncertainty and how it impacted on movement control. In the third article, two hypotheses were considered. The first suggested that the increase in uncertainty reported in the virtual context of the previous study was due to loss of visual information relative to the configuration of the arm. The second suggested instead that the uncertainty came from the proprioceptive and visual information that is not perfectly congruent in a virtual context as compared to a natural one (the cursor is not directly aligned with the finger for example). The data have not supported our first hypothesis. It rather appears that the uncertainty is caused by the dissociation of visual and proprioceptive information. We also demonstrated that the information on the starting base on the position of the hand greatly influences the control process online, even when vision is available during the effector movement. This result suggests that internal feedback loops use this information to modulate the ongoing movement.

[pt] Na medida em que as redes de transmissão de energia elétrica ficaram mais malhadas, os limites térmicos de linhas e transformadores passaram a restringir menos a transmissão de potência. Similarmente, o uso de sistemas estáticos de compensação de potência reativa e estabilizadores na excitação dos geradores aumentou a capacidade de transmissão de potência nos sistemas antes limitados por problemas de estabilidade angular. Hoje as linhas de transmissão estão mais carregadas e isto deu origem ao problema da instabilidade de tensão.Neste trabalho, as condições de estabilidade de tensão são avaliadas por condições nodais associadas ao máximo fluxo de potência ativa e reativa que pode ser transmitida dos geradores para as cargas. Estas condições nodais são avaliadas por uma ferramenta analítica com base em modelo matemático, simples mas poderoso, de uma direta interpretação física do fenômeno.Índices abrangentes e significativos são obtidos a partir da matriz Jacobiano do sistema. Eles indicam a região de operação na curva V x P, Q , a margem em MVA para o máximo carregamento, a importância relativa entre as barras, uma medida de dificuldade de transmissão, e o índice de influência que relaciona as margens de potência entre dois pontos de operação, o que caracteriza a eficácia ou não, por exemplo, de uma ação de controle. O método proposto nesta tese para reforçar as condições de estabilidade de tensão consiste de três etapas seqüenciais. Primeiramente, avalia-se as condições de estabilidade de tensão determinando-se a barra crítica da rede através do cálculo da margem de potência. Determinase o caminho de transmissão crítico, conceito novo usado neste trabalho, entre os vários existentes para transportar potência de geradores para aquela barra crítica. Determina-se então o ramo crítico deste caminho, conceito introduzido neste trabalho. Um programa de fluxo de potência ótimo é usado para aliviar o carregamento desse ramo crítico. A seqüência começa novamente com a avaliação das condições no novo ponto de operação. Todas as etapas são repetidas até que as margens resultantes sejam julgadas adequadas.Barras de carga, de geração e de tensão controlada por compensadores de potência reativa em paralelo com a rede podem ser eleitas como a barra crítica. Somente o método de avaliação nodal usado é capaz de lidar com qualquer tipo de barra. Da mesma forma, o procedimento proposto para reforçar as condições de estabilidade de tensão é adequado para qualquer tipo de barra.São mostrados inúmeros testes, tanto ilustrativos como com sistemas reais, em pontos de operação também reais, inclusive na situação de iminente colapso de tensão. Verifica-se que o método proposto realmente produz os resultados desejados. / [en] As the electric power transmission networks became more interconnected, the thermal limits of lines and transformers restrict less the power transmission. Similarly, the use of static systems of reactive power compensation increases the power transmission capacity in systems whereas before they were limited by problems of angular stability. Actually, transmission lines are more loaded and create the voltage stability problem. In this work, voltage stability conditions are assessment by nodal conditions associated to the maximum active and reactive power flow that can be transmitted from generators to loads.These nodal conditions are assessment using an analytical tool, based on a simple but sound mathematical background, modelling a straightforward physical haracterisation of the phenomena. Comprehensive and meaningful indices are obtained from system Jacobean matrix. They indices indicate the operating region in V x P,Q curve, the MVA margin to the maximum load, the relative importance among buses, the buses loading ranking, a measure of difficult for power transmission, and the influence indices that relates power margins between two operating points, which characterises efficiency or not, for example, of a control action.In order to reinforce voltage stability condition, the thesis proposes a method consisting of three sequential stages. Firstly, voltage stability is analysed, deciding network critical bus using the power margin calculation. Next, the critical transmission path is decided, which is a new concept used in this work, in between several existing used to transport generators power for that critical bus. Then, critical branch is obtained through this path, concept introduced in this work. An optimal power flow program is used to alleviate load flow in the critical branch. The sequence starts again with the stability condition assessment in the new operating point. All stages are repeated until resultant margins are judged suitable. Load, generation and voltage-controlled bus by shunt reactive power compensators could be considered critical bus. The nodal method used is the only one capable of handling any bus type.Several cases are shown, illustrative as well as real systems using real operating points,including imminent voltage collapse situations. It is verified that the proposed method really produces the desired results.