PROJECT SUMMARY Falls are a severe societal issue with vast physical and economic consequences. As we age, there are a variety of factors that are associated with an increased falls risk such as declines in sensor, motor, and cognitive-motor acuity. These changes may affect how we compensate for being in a fatigued state to better navigate dynamic walking challenges due to the additional risks that they put older adults at for falls. The overarching scientific premise of this fellowship proposal is that local muscle fatigability compromises the neuromuscular control of walking balance and importunately precipitates gait instability in a direction-dependent and context-specific ways in older adults. This project will recruit 30 young adults, 30 older adult non-fallers, and 30 older adult fallers to participate in two sessions: one designed to objectively target the local fatigability of muscles that regulate sagittal plane motion and stability and another designed to objectively target the local fatigability of muscles that regulate coronal plane motion and stability. This project strategically combines sophisticated motion capture and electromyographic time-frequency analyses with an innovative suite of walking balance perturbations designed to emulate real-world balance challenges to identify novel mechanistic causes for increased falls risk. This research study will be the first to establish the mechanisms behind fatigues effect on walking stability through the usage of directional local muscle fatigability and a comprehensive and task specific collection of walking balance perturbations integrated with musculoskeletal modeling and simulations. This area of mechanistic and hypothesis-driven research has been severely understudied, but has significant and immediate potential to inform novel advances in diagnostics, rehabilitation, mobile monitoring, and wearable assistive technologies to mitigate falls.