The long-term objective of this proposal is to develop screening tools for early identification of future fall risk in the elderly and to evaluate new therapeutic interventions that would counteract the underlying pathological process. Falls are a major health concern and efforts are needed to address the causes of gait unsteadiness in elderly fallers. It is increasingly clear that walking is tightly linked to executive functioning. Investigations using dual-tasking paradigms, where performance on attention-demanding tasks and walking is evaluated when they are performed separately and concurrently, suggest that executive changes play a role in the causal pathway of falls. Furthermore, the subtle but important fluctuations present from one stride to the next during walking seem to be sensitive markers of fall risk. It is possible that cognitive decline is responsible for these changes in stride-to-stride variability, but this has not been investigated. Objectives. Through the use of dual-task paradigms, this proposal will: 1/ investigate the viability of the U-shaped relationship between stride-to-stride variability during gait and difficulty of concurrent cognitive tasks, 2/ disentangle the interrelationships between specific aspects of stride-to-stride variability (e.g., stride width variability vs. step length variability) and components of executive function (e.g., working memory, response inhibition, and cognitive flexibility), and 3/ compare the efficiency of different training strategies to understand the mechanisms underlying training-related changes in stride-to-stride variability control of elderly fallers and develop targeted intervention for preventing falls. K99 phase (cross-sectional study). Forty two healthy young adults, forty two elderly non-fallers, and forty two elderly fallers will be assessed with cognitive tasks involving different levels of difficulty and challenging various executive functioning abilities. Participants will be tested with auditory and linguistic tasks without and with walking on the treadmill. We predict that 1/ each complementary aspect of stride-to-stride variability is related in a U- shaped manner to cognitive load and that the turning point of the function occurs at a lower cognitive load for elderly fallers than for young and elderly non-fallers, and 2/ fallers more "centrally" impaired would show different walking profiles than fallers more "sensory" impaired under both single and dual-task walking conditions. R00 phase (two-arm randomized controlled trial). One hundred more individuals will be recruited for a total of 142 elderly fallers and randomly assigned to a single-task walking training and a dual-task training combining walking and cognitive loading. All subjects will be evaluated before and after a three-month training period, and after a six-month follow-up period under the conditions utilized in Experiment 1 with the addition of novel dual tasks. We predict that dual-task training will be better than single-task training in defining fall-risk profiles and developing therapeutic strategies that are relevant to specific executive function deficits. Methods. Walking performance will be assessed using stride-to-stride variability and traditional biomechanical measures. For the R00 trial, walking performance assessment will be based on two selected measures, one primarily related to central nervous impairments and another one primarily related to sensory impairments that are strongly correlated with the risk of falling, as determined during the K99 study. PUBLIC HEALTH RELEVANCE: Risk of falls may be related to changes in certain aspects of brain function. This research seeks to develop tools to identify older people at risk for falls. Walking under dual-task conditions could be helpful in detecting walking disorders at a preclinical stage and planning targeted therapeutic interventions to prevent falls.