Abstract: Previous lower limb weighting studies have placed a load on the legs bilaterally and tested different placement locations. It was previously determined that kinematic changes occur with greater masses and at joints proximal to weight placement [1]. Other studies have determined that these changes exist for a short adaptation period before parameters revert to a steady state [2]. Tasks that require voluntary gait modifications such as obstacle clearance have also been performed with lower leg bilateral weight addition [4]. In cases of normal obstacle clearance increased flexion at all three joints in the lower limb is needed to safely traverse the obstacle [3]. The goal of this study was to investigate joint kinematics and kinetics of unilaterally weighted participants using level ground force platform collection techniques, rather than a treadmill. It was hoped that this would allow for new insight into the adaptation periods and strategic motor pattern changes seen at the ankle, knee and hip. Kinematic and force platform data were collected on two groups of 10 healthy male subjects. Group 1 (mean age = 23years, mean weight = 82.181kg, mean height = 1.798m) was a normal walking group and group 2 (mean age = 24.8years, mean weight = 79.901kg, mean height = 1.773m) was an obstacle clearance group. Both groups participated in 20 trials each of three different conditions; normal, weighted and weight off using a 2.27kg limb mass attached just proximal to the right maleoli markers. A repeated-measures two-way ANOVA was carried out on relevant variables in order to determine statistical significance. Weight addition and removal affected the kinematics and kinetics of the normal walking and obstacle clearance groups. This effect was more prominent in the normal walking group. If changes were seen, trials 1 through 3 were the locations showing a quick adaptation followed by a leveling off back to a new steady state in later trials. Participants in the normal walking group chose to utilize the hip joint in order to control for weight addition and removal. Kinematically, changes in the hip joint angle occurred at all instances analyzed throughout the gait cycle with this effect being more prominent in the weight off condition. In conjunction with this, the hip joint energy generation increased during all phases of the gait cycle while the ankle and knee joints either decreased energy generation or increased energy absorption. In the obstacle group, participants also chose to increase flexion at the hip joint. However, the ankle joint also had either decreased plantarflexion or increased dorsiflexion at all the instances analyzed during the gait cycle. However, joint energy generation increases at these joints were only found during stance and at heel contact. The toe obstacle clearance values also showed a marked increase in trial 1 for the weighted condition which demonstrates a voluntary gait modification made by participants to safely traverse the obstacle that was quickly adapted for. Overall, the results found by previous studies using treadmill collection techniques were still seen in overground force platform data but they were not as robust. References: 1.Martin PE et al. J Biomech. 1990; 23(6):529-536. 2.Noble et al. Exp Brain Res. 2006; 169: 482-495. 3.Patla AE et al. Exp Brain Res. 1995; 196: 499-504. 4.Reid MJ et al. Neurosci Res Comm. 2001; 29(2): 79-87.

Abstract: Previous lower limb weighting studies have placed a load on the legs bilaterally and tested different placement locations. It was previously determined that kinematic changes occur with greater masses and at joints proximal to weight placement [1]. Other studies have determined that these changes exist for a short adaptation period before parameters revert to a steady state [2]. Tasks that require voluntary gait modifications such as obstacle clearance have also been performed with lower leg bilateral weight addition [4]. In cases of normal obstacle clearance increased flexion at all three joints in the lower limb is needed to safely traverse the obstacle [3]. The goal of this study was to investigate joint kinematics and kinetics of unilaterally weighted participants using level ground force platform collection techniques, rather than a treadmill. It was hoped that this would allow for new insight into the adaptation periods and strategic motor pattern changes seen at the ankle, knee and hip. Kinematic and force platform data were collected on two groups of 10 healthy male subjects. Group 1 (mean age = 23years, mean weight = 82.181kg, mean height = 1.798m) was a normal walking group and group 2 (mean age = 24.8years, mean weight = 79.901kg, mean height = 1.773m) was an obstacle clearance group. Both groups participated in 20 trials each of three different conditions; normal, weighted and weight off using a 2.27kg limb mass attached just proximal to the right maleoli markers. A repeated-measures two-way ANOVA was carried out on relevant variables in order to determine statistical significance. Weight addition and removal affected the kinematics and kinetics of the normal walking and obstacle clearance groups. This effect was more prominent in the normal walking group. If changes were seen, trials 1 through 3 were the locations showing a quick adaptation followed by a leveling off back to a new steady state in later trials. Participants in the normal walking group chose to utilize the hip joint in order to control for weight addition and removal. Kinematically, changes in the hip joint angle occurred at all instances analyzed throughout the gait cycle with this effect being more prominent in the weight off condition. In conjunction with this, the hip joint energy generation increased during all phases of the gait cycle while the ankle and knee joints either decreased energy generation or increased energy absorption. In the obstacle group, participants also chose to increase flexion at the hip joint. However, the ankle joint also had either decreased plantarflexion or increased dorsiflexion at all the instances analyzed during the gait cycle. However, joint energy generation increases at these joints were only found during stance and at heel contact. The toe obstacle clearance values also showed a marked increase in trial 1 for the weighted condition which demonstrates a voluntary gait modification made by participants to safely traverse the obstacle that was quickly adapted for. Overall, the results found by previous studies using treadmill collection techniques were still seen in overground force platform data but they were not as robust. References: 1.Martin PE et al. J Biomech. 1990; 23(6):529-536. 2.Noble et al. Exp Brain Res. 2006; 169: 482-495. 3.Patla AE et al. Exp Brain Res. 1995; 196: 499-504. 4.Reid MJ et al. Neurosci Res Comm. 2001; 29(2): 79-87.

It is well established that aging is associated with numerous health concerns, including poor balance. Deteriorations in attention demand also place older adults at a greater risk for falls. Emerging experiments have explored the impact of dual-task training programs and have improved dual-tasking in older adults. However, it is unknown whether these performance-related improvements are a function of the intervention itself or the repeated exposure to the testing protocol. Study 1 explored the implications of repeated administration, once per week for 5 weeks, of a protocol involving standing postural sway while concurrently performing reaction time (RT) tasks in older adults. Results revealed that postural sway was stable across testing sessions whereas the difficult RT task gradually improved over time. Study 2 examined the influence of repeated exposure, once per week for 5 weeks, of a protocol involving negotiating a series of obstacles while performing RT tasks in older adults. Participants walked significantly faster with repeated exposure and gradually improved RT. Study 3 investigated the impact of repeated exposure, once per week for 5 weeks, to three functional mobility measures in older adults. It also examined the influence of a 12-week balance and mobility training (BMT) program as well as a 12-week balance and mobility plus cognitive training (BMT+C) program on functional mobility in older adults. Functional mobility served to be stable over time. Both the BMT and BMT+C groups significantly improved functional mobility and sustained these improvements at the 12-week follow-up, while no changes were observed in the control group. No differences between the BMT and BMT+C groups emerged. Experiment 4 examined the influence of BMT and BMT+C on postural sway and RT in older adults. Participants in both training groups significantly improved RT and sustained these improvements at the follow-up, while no changes were observed in the control group. No changes to postural control were shown in any group. No differences between the BMT and BMT+C groups emerged. Experiment 5 examined the influence of BMT and BMT+C on negotiating a series of obstacles while performing RT tasks in older adults. Both the BMT and BMT+C groups significantly improved RT and sustained these improvements at the follow-up, while no changes were observed in the control group. All groups showed faster time to completion of the obstacle series. No differences between the BMT and BMT+C groups emerged. Collectively, these findings suggest that BMT and BMT+C significantly improve functional mobility and divided attention, and sustain these improvements over time. Although some improvements were observed after repeated exposure over 5 weeks, no changes in the control group were observed. Therefore, the improvements exhibited from BMT and BMT+C are likely not a function of repeated exposure to the testing protocol, as participants may be more susceptible to performance-related improvements when the testing sessions are close in proximity. Altogether, these findings propose that, whether or not cognitive training is included, attention demanding dual-task training not only improves functional mobility and RT, but also sustains these improvements over time in older adults. These results may be used to improve the prescription of exercise in older adults.

L’objectif général de cette thèse était d’investiguer l’organisation posturale de l’initiation de la marche (IM) lors de l’application cumulée de contraintes temporelles et spatiales. L’hypothèse générale était que le système postural, chez le jeune adulte sain, est capable de s’adapter à une variation du degré de contrainte imposé expérimentalement, pour maintenir un niveau de performance motrice et de stabilité invariant. Trois études ont été réalisées pour tester cette hypothèse générale, auxquelles s’est ajoutée une étude de validation de mesure des dimensions de la base de support (BS). L’objectif de la première étude était d’analyser l’effet de la pression temporelle (PT) et de la présence d’un obstacle à franchir sur l’organisation posturale de l’IM. Les résultats ont montré que la durée des ajustements posturaux anticipateurs (APA) était réduite en condition de PT forte comparativement à la condition de PT faible. Cette contrainte n’entrainait cependant pas d’altération de la stabilité et de la performance motrice, vraisemblablement grâce à l’augmentation concomitante de l’amplitude des APA. Par ailleurs, il a été montré que l’enjambement de l’obstacle induisait une augmentation de la durée de la phase oscillante provoquant, de fait, une augmentation potentielle de l’instabilité posturale. Cet effet négatif était cependant contrebalancé par un développement d’APA plus important qu’en condition sans obstacle. Dans cette première étude, la hauteur et la distance à l’obstacle étaient fixes. L’objectif de la deuxième étude était d’analyser l’effet d’une modification des caractéristiques de l’obstacle, combinée à une variation de la contrainte temporelle sur l’organisation posturale de l’IM. Trois hauteurs et distances d’obstacle, et deux niveaux de PT étaient combinés. Pour mettre en évidence le caractère adaptatif de la modulation des caractéristiques des APA en fonction des contraintes spatiotemporelles imposées, un modèle mécanique original du corps humain permettant de formaliser la trajectoire du centre des masses a été élaboré. En accord avec la première étude, les résultats ont montré que la stabilité posturale et la performance motrice demeuraient équivalentes dans les différentes conditions, malgré les larges variations des contraintes imposées. Le modèle mécanique nous a permis de démontrer que cette invariance était liée à la modulation de l’amplitude des APA, témoignant ainsi de l’adaptabilité du système postural aux contraintes imposées. Cette expérimentation a également permis de mettre en évidence de façon fortuite que plus la distance à l’obstacle était importante, plus le pourcentage de pose avant-pied augmentait. L’objectif de la troisième étude était d’analyser l’effet de la pose de pied (avant ou arrière-pied) sur l’organisation posturale de l’IM avec franchissement d’obstacle. Les résultats ont montré que cette organisation posturale dépendait étroitement de la stratégie de pose du pied. Ceux-ci suggéraient l’existence d’une relation d'interdépendance entre les mécanismes de contrôle de l'équilibre de l’IM et la stratégie de pose du pied permettant ainsi, un contrôle optimal de la stabilité. L’objectif de la quatrième étude était de valider la mesure des dimensions de la BS au cours de l’IM à l’aide d’une plateforme de force (méthode dynamique), en prenant pour « gold standard » le système VICON. Les résultats ont montré que la méthode dynamique était suffisamment précise pour pouvoir être comparée au gold standard. En conclusion, l’ensemble de ces résultats suggère que chez le jeune adulte sain, le SNC est capable de moduler de façon adaptative et optimale les mécanismes de contrôle de l’équilibre en fonction des contraintes spatiotemporelles imposées. En termes d’implications cliniques, l’IM avec franchissement d’obstacle serait une méthode de rééducation intéressante aussi bien en évaluation (test-retest) qu’en rééducation où l’obstacle permettrait de proposer une contrainte dosable et reproductible / The main objective of this thesis was to investigate the postural organization of gait initiation (GI) during the application of temporal and spatial constraints. The main hypothesis was that the postural system, in the healthy young adult, is able to adapt to the degree of constraint imposed experimentally, in order to maintain an invariant level of motor performance and stability. Three studies were conducted to test this main hypothesis, with the addition of a validation study measuring the dimensions of the base of support (BOS). The objective of the first study was to analyze the effect of temporal pressure (TP) and the presence (or not) of an obstacle to be cleared on the postural organization of GI. The results showed that the duration of anticipatory postural adjustments (APA) was drastically reduced under the condition of high TP (GI in reaction time) compared to the condition of low TP (GI in self-initiated). This constraint didn’t result in a decrease of stability and motor performance, probably due to the increase in APA amplitude. In addition, it has been shown that clearing an obstacle induces an increase in the duration of the swing phase causing, a potential increase of the postural instability. This negative effect was, however, counterbalanced by the development of more significant APA than without obstacle. In this first study, the height and the distance of the obstacle were fixed. The objective of the second study was to analyze the effect of a modification in the characteristics of the obstacle to be cleared, combined with a variation of temporal constraint on the postural organization of GI. Three heights and three obstacle distances, and two TP levels were combined. To insist on the adaptive character of the characteristics of APA modulation according to the spatiotemporal constraints imposed, an original mechanical model of the human body formalizing the centre of mass trajectory has been elaborated. In agreement with the first study, the results showed that postural stability and motor performance remained equivalent under the different experimental conditions, despite wide variations in the imposed stress level. The mechanical model allowed us to demonstrate that this invariance was related to the amplitude modulation of APA, thereby demonstrating the adaptability of the postural system to imposed constraints. This experiment also highlights "fortuitously" that as the obstacle distance increase, the higher percentage of forefoot strike increase. Also, the objective of the third study was to analyze the effect of the foot strike strategy (front or rear foot) on the postural organization of the GI with an obstacle to be cleared. The results showed that this postural organization was dependent on the swing foot-off strategy. These results suggested the existence of an interdependent relationship between GI's balance control mechanisms and the foot strike strategy, allowing an optimal control of stability. Finally, the objective of the fourth study was to validate the measurement of the BOS dimensions during GI using a force platform (dynamic method), taking the VICON system as gold standard. The results showed that the dynamic method was sufficiently precise to be compared to the gold standard. In conclusion, all of these results suggest that in the healthy young adult, the CNS is able to adaptively and optimally modulate the balance control mechanisms according to the spatiotemporal constraints imposed experimentally. For clinical implications, GI with obstacle to be cleared would be an interesting method of rehabilitation in both test-retest and rehabilitation where the obstacle would provide a measurable and reproducible constraint.