Objectives: The objectives are to understand the mechanisms that maintain medial-lateral (ML) balance during stance and walking gait and how age affects these mechanisms. The focus is on ML balance because previous evidence indicates that stability is more problematic in the ML direction than in the anterior-posterior direction and that a sizable proportion of falls in older individuals occur in the ML direction. Based on preliminary results, an additional objective is to begin prototype development of a balance aid that could potentially improve balance during gait and reduce the likelihood of falls in subjects with deficits that affect balance. Plan: The proposa includes 4 Aims, three of which investigate different aspects of the neural control of balance during gait and one Aim that proposes to apply results from the other Aims to explore development of a prosthesis to improve stability during gait. The neural control of balance during gait requires the regulation of two different aspects of movement. The first aspect involves the mechanism that controls body orientation in space and relative to the environment. This aspect requires the integration of information from multiple sensory systems (visual, vestibular, proprioceptive, and somatosensory) and the subsequent generation of appropriate motor actions that maintain a desired body posture. Aim 1 will characterize the multi-sensory influences on orientation control during gait, will compare them to results from stance, and will determine how age influences these orientation control mechanisms. The second aspect of balance control during gait involves mechanisms that maintain dynamic balance. Dynamic balance refers to the oscillating pattern of body motion that occurs with each step cycle. Multiple control mechanisms are potential contributors to dynamic balance, including a mechanism based on the regulation of step timing. This step-timing mechanism is not currently recognized as a major contributor to dynamic balance, but our preliminary results reveal its importance. Aim 2 experiments will characterize the mechanisms contributing to dynamic balance and determine the influence of age on these mechanisms. Aim 3 will primarily determine whether gait measures obtained using our proposed methods correspond to gait measures obtained by conventional methods. Aim 4 will explore development of a balance aid to improve balance during gait by triggering stepping based on measures of body motion. Methods: Participants will be recruited primarily from an existing NCRAR database of volunteers who have expressed willingness to participate in research projects at NCRAR. At least 50% of the subjects in this database are Veterans. Subject groups will include young Veterans (ages 18 to 45 years) and older Veterans (>65 years) with 20 subjects in each group. Subjects will fill out screening questionnaires that provide a self- rating of balance confidence and have been shown to correlate with clinical examinations that predict the likelihood of falls. Aim 1 and 2 experiments will be performed on a custom balance test device that can deliver controlled disturbances by tilting the platform surface or the visual scene viewed by the subject. Participants either stand on the platform surface or perform a stepping-in-place gait that has similar characteristics, in terms of ML balance control, as a normal walking gait. Recorded data include body sway measures, platform forces, step width, timing of steps, and actual surface and visual scene motion. Aims 3 and 4 will use galvanic vestibular stimulation to perturb balance during walking. Data analysis will quantify body sway measures, identify stimulus-response relationships, and identify the contributions of different mechanisms for dynamic balance control. Existing mathematical models of balance during stance and during gait will be combined to account for experimental data such that identified model parameters are physiologically meaningful and also provide outcome measures for comparison across age groups. Statistical comparison between results from younger and older adults will determine the extent to which age influences balance during gait.