PROJECT SUMMARY / ABSTRACT Following a stroke, deficits in independent mobility are a primary contributor to decreased quality of life, as many people with chronic stroke (PwCS) are prevented from returning to their prior levels of activity participation or involvement in the community. A major cause of mobility deficits is gait instability, which can increase the risk of falls and limit independent function. While several common rehabilitation methods (e.g. locomotor training, traditional balance training, strengthening) can improve some aspects of function, they have failed to address fall incidence among PwCS. In part, this lack of success is likely due to current interventions not being targeted toward the specific mechanisms causing post-stroke gait instability. A long-term goal of this research is to develop a toolbox of mechanism-based interventions to improve various aspects of post-stroke function. As a step toward this goal, the objective of the present proposal is to determine whether targeted perturbation training can reduce falls in PwCS by improving gait stability. While perturbation training has previously been used to reduce fall risk in other clinical populations, it has thus far been unsuccessful at doing so among PwCS. This lack of success may be due to the nature of the applied perturbations, which are traditionally designed to elicit discrete Reactive responses to avoid a loss of balance. In contrast, the present proposal will apply mechanical perturbations designed to elicit Proactive adjustments in the neuromechanical strategies used to ensure walking stability with every step. Such Proactive perturbations may be better suited to prevent the intrinsic movement errors that are a primary contributor to losses of balance and falls among PwCS. The central hypothesis of this work is that that unlike Reactive methods, Proactive perturbation training will retrain generalized gait stabilization strategies, reducing the risk of falls. This hypothesis will be addressed through three Specific Aims. The first Specific Aim is to determine whether fall rate in PwCS is reduced by Proactive or Reactive training, with immediate clinical implications for the development of interventions that can be applied in the real world. The second Specific Aim is to characterize the neuromechanical mechanisms that underlie gait changes with perturbation training, revealing whether the stabilization strategies normally used to ensure walking balance can be strengthened with appropriately targeted perturbations. Finally, the third Specific Aim is to determine whether Proactive or Reactive training produces generalized gait stabilization, as an ideal intervention would improve resilience even to untrained perturbations that may be experienced in real world walking. The proposed project is based on a neuromechanical framework of bipedal walking control, and thus allows investigation of both clinically relevant outcomes (fall rate) and mechanistic measures o...