<p> A key mechanism to maintain balance during walking is the foot placement strategy,  where the person steps in the direction of an impending fall. On a clear walkway, the foot  placement strategy translates to maintaining a consistent relationship between the center of mass  state and the base of support (a body-centric constraint on foot placement), which is reflected in  a consistent step length. However, to safely navigate in the community, foot placement must  maintain certain spatial relations with environmental features as well (environmental constraints on foot placement). For stepping over obstacles, the environmental constraint takes the form of  targeting. That is, the feet must be placed at precise locations relative to the obstacle to minimize  the likelihood of tripping.  My dissertation focused on proactive adaptations to foot placements while navigating  cluttered environments. I developed the interstep covariation (ISC) index that quantifies the covariation between consecutive foot placements relative to stationary, visible environmental  features (an obstacle and a visual target). The across-step (or group) changes in this index  indicate how the two constraints (body-centric and environmental) on foot placement are  managed during adaptive gait tasks. I quantified how the ISC index changed (1) across steps  while approaching and crossing an obstacle, (2) due to healthy aging and (3) when the proximity  of two environmental features was systematically altered. Specifically, in Study 1, the ISC index  was quantified for the obstacle crossing step for healthy younger and older adults. In Study 2, proactive changes in the ISC index as healthy young adults approached and crossed an obstacle were characterized. In Study 3, the changes in the dynamics of the across-step ISC index due to  an additional visual stepping target in the approach to the obstacle were identified.  I found that there exists a covariance strategy that healthy adults use to navigate the  environment safely and successfully. First, I found that individuals prioritize the environmental  constraint at the expense of the body-centric constraint when the environment poses a larger risk  to balance (the obstacle), or to satisfy a specified constraint (stepping on a visual target). Second,  I found that the shift in prioritization is proactive, i.e., it occurs while approaching an obstacle.  The strategy to shift priorities is influenced by age (Study 1), environmental features (Study 2  and Study 3), and the proximity of two environmental features (Study 3). These studies add to  the current understanding of foot placement control by demonstrating how this well-known and 15 fundamental strategy to maintain balance while walking is systematically influenced by the  environment and task constraints. These findings can be further extended to study proactive and  reactive adaptations during walking in different populations.   </p>

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