PROJECT SUMMARY Impaired mobility is the leading cause of disability among older adults, impacting independence and quality of life. The ability to maintain balance in response to unexpected perturbations such as a bump, trip, or uneven terrain is a critical component of healthy mobility. Older adults, however, have a reduced capacity to do so, increasing their risk of falling. Despite many training methods aimed at improving the ability to adapt to perturbations, outcomes are highly variable. This may be due, in part, to improperly targeted training. The ability to rapidly and appropriately generate a corrective torque to maintain balance in response to a perturbation is mediated by neural and biomechanical factors. The ankle is critical in the response to whole-body perturbations as it generates a substantial portion of the corrective torque. Ankle torques opposing a perturbation arise from the impedance of the ankle, the relationship between imposed displacements and the resultant torques, and neurally-mediated changes in muscle activation. Increased reaction times and altered musculotendon properties in older adults have been associated with an impaired ability to respond. However, the relative importance of these changes to the response to perturbations remains unknown. This proposal will focus on determining the relative contributions of ankle impedance and reaction time to the ability to resist perturbations of posture. Impedance is directly related to the mechanical properties of the structures spanning the joint (i.e. muscle, tendon, ligaments). While age-dependent changes in musculotendon properties should impact ankle impedance, the magnitude of their effect remains unknown. This is a critical barrier to understanding their contributions to altered motor performance. Aim 1 will determine if there are age- dependent changes in ankle impedance and quantify their magnitude. Ankle, muscle, and tendon contributions to impedance will be quantified using a novel technique employing robotic assessments of ankle mechanics and ultrasound imaging of muscle and tendon motions. Impedance of these structures will be quantified using system identification. This simultaneous assessment is vital for determining how the mechanical properties of the muscle and tendon contribute to the mechanics of the ankle. Aim 2 will determine the extent to which ankle impedance and reaction time contribute to age-related differences in the response to postural perturbations. Ankle posture will be unexpectedly perturbed using a robotic device as the subject balances a virtual inverted pendulum simulating the ankle loads generated during standing. Performance will be quantified by the ability to balance the pendulum. Different conditions will be used to differentiate the impact of ankle impedance and reaction time on task performance. Together, the results from this work will provide insight to the mechanism impairing older adults’ ability to respond to unexpected postu...