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While we walk around, busy with our daily routines, we rarely consider how complex our gait needs to be in order to handle all the irregularities of our environment. We walk on uneven curbs, avoid puddles of water, pets, running children etc., almost without thinking about it. Even if we lose balance and trip over something, we are often able to recover. This ability deteriorates with aging, when falls become a prominent problem, but one should keep in mind that, if we were unable to adjust our ongoing steps when faced with sudden changes in our environment, falls would occur at any age. Although adjusting step trajectories is crucial for our ability to navigate the environment, it is not known under which circumstances or how this can be accomplished. Therefore, the ability to adjust leg movements during ongoing gait, either perturbed or unperturbed, is the focus of this thesis.In general, our bodies move by executing motor commands. For example, we decide to ‘walk to the sofa’ and send this command to our executing elements (i.e., our legs). At the same time the motor command is sent to the legs, its copy is sent to a neural controller called the internal model. The internal model predicts what kind of movement is expected and monitors feedback information from our body and new information from the environment. If there is a mismatch between our movement and what is appropriate and expected, our internal model can determine a movement adjustment is needed. For example, if a cat runs in your path while you are walking towards your sofa, your internal model should act to trigger an adjustment of your step so that you do not step on it. Depending on the situation, sometimes a simple online correction of the ongoing movement is sufficient and sometimes the movement needs to be replaced by a whole new motor plan. In case of walking, our leg swing trajectories are planned in advance based on the visual information from the environment, our goals etc. Therefore, if we encounter a change in the environment our pre-planned step needs to be stopped before an alternative foot landing position can be found.The objective of this thesis was to provide insights into the ability of humans to adjust leg movements during ongoing gait, both unperturbed and perturbed. The background and rationale for this thesis are described in Chapter 1. Until now, adjustments of ongoing movements were mostly investigated using arm movements or simple leg movements like step initiation, but gait is more challenging, since it is an ongoing movement that poses considerable balance constraints. Furthermore, it is one of the most common daily life activities requiring leg movement adjustments. Therefore, insights into the ability to adjust ongoing gait, both unperturbed and perturbed, might provide useful information in the context of fall prevention and rehabilitation of various patient populations.In the first part of this thesis we focused on unperturbed gait and developed novel walking tasks in which subjects had to walk on a treadmill by following virtual stepping stones projected onto the treadmill’s surface. These stepping stones could change during the approach, forcing the subjects to adjust their precisely aimed steps. This method is described in Chapter 2.In our first experiment, described in Chapter 3, the stepping stones could change color suddenly, which was an indication they became obstacles to avoid. Therefore, subjects had to adjust their ongoing steps to land outside the obstacle in any way they preferred. Using this task we were able to show that response inhibition (i.e., the ability to stop a movement) plays an important role in obstacle avoidance and we could see that older adults performed worse than young. However, unlike young, older adults also showed learning effects and improved with practice. Furthermore, we paired the walking task with a cognitive task that required inhibition, and found that the performance of both groups deteriorated when the two tasks were performed simultaneously. However, the two groups handled this problem differently and only older adults prioritized their performance on the walking task. Finally, we could see that the difficulties older adults experienced were related to response inhibition, since their performance deteriorated specifically when inhibition was required.In the second experiment (Chapter 4), instead of changing color, the stepping stones could shift position and subjects had to adjust their steps to follow the stepping stone. Unlike the previous task, this forced subjects to adjust their movements in a specific direction and we found that the direction of stepping stone displacement influenced the accuracy of movement adjustments in young adults. Adjustments were most accurate when step lengthening was required and least accurate for step shortening. Furthermore, the difference in accuracy between step lengthening and shortening became smaller with increasing time pressure. This difference in accuracy suggests a higher risk of unsuccessfully executing a leg movement adjustment when a step is being shortened as opposed to lengthened. Since both step shortening and lengthening are viable options for obstacle avoidance, this difference in risk might affect the way obstacle avoidance strategies are chosen.In the second part of the thesis we focused on a condition even more challenging than unperturbed gait by investigating the ability to adjust leg movements during tripping. Tripping occurs frequently in our daily lives and leads to falling, unless we are able to make an appropriate recovery step that recovers balance and lands in a safe area. In the final experiment, as described in Chapter 5, we investigated whether it is possible to adjust such steps, which are already adjustments of ongoing gait in order to recover balance, and how this is accomplished. We tripped young adults and presented them with a forbidden landing zone (FZ) at trip onset. Since this FZ was positioned in the area where they would normally land following tripping, they were forced to adjust the ongoing trip recovery step in order to avoid it. All of our subjects were able to avoid landing in the FZ, but there were individual differences in performance. Some subjects succeeded already in their first trial while others improved over the course of the experiment and succeeded only in the final, fifth trial. Different strategies were used, subjects either shortened their steps or stepped to the side of the FZ. While most subjects used step shortening, shorter subjects tended to step to the size of the FZ, probably because it was positioned too close to the tripping obstacle since their trip recovery steps were shorter. Strikingly, some subjects were even able to switch between strategies. However, irrespective of the strategy used and success of FZ avoidance, balance recovery following tripping was not compromised. Furthermore, we observed strong anticipation effects and subjects adjusted their trip responses even on trials that did not involve a FZ.Finally, to describe the mechanisms driving these adjustments in Chapter 6 we analyzed muscle activity changes occurring during step shortening, the dominant FZ avoidance strategy. Step shortening was driven by muscle activity changes occurring in two functionally different stages. The first stage of muscle activity change occurred around 100 ms following trip onset, which is too early to be considered voluntary, and did not contribute to the observed step shortening. Therefore, we suggest this initial stage might have served as a ‘pause’ until an appropriate movement adjustment was initiated. Second stage of the muscle activity changes occurred at latencies corresponding to voluntary reaction and clearly led to the observed step adjustments (i.e., step shortening and landing on the toes). Interestingly, we found similar muscle activity changes on trials that did not involve a FZ, in line with step adjustments that occurred under the influence of anticipation.In conclusion, and as discussed in Chapter 7, this thesis shows that it is possible, albeit challenging, to investigate movement adjustments during gait using paradigms common in fundamental arm and eye movement research. These paradigms typically investigate online adjustments and response inhibition separately, but our work demonstrates both are involved in adjustments of gait. In general, our data show that unperturbed and perturbed gait can be modified quickly. Apparently, fast movement adjustments are not only possible for eye, arm, and simple leg movements, but even for extremely challenging whole body movements, such as balance recovery following tripping. Finally, learning effects observed in our experiments show that it is possible to improve leg movement adjustment abilities, which is promising for fall prevention, especially in light of our aging society.