Preventing mobility impairment is crucial for our rapidly aging population to lead longer, more independent lives. Walking in older adults is commonly characterized, biomechanically, by deficits in “push-off intensity” - a reduction in ankle moment and power output thought to contribute to slower preferred speeds, and ultimately, reduced independence. Unfortunately, conventional assessments of muscle force-generating capacity fail to fully characterize push-off capacity during walking. Therefore, the mechanisms governing limitations in elderly gait remain unknown. In addition, conventional interventions aimed at enhancing push-off intensity in older adults convey benefits only during maximum speed walking without improving habitual walking speeds or push-off intensity. The first purpose of this dissertation was to engineer and implement a novel system to functionally quantify push-off capacity at the individual limb-, joint-, and muscle-levels during walking. The second purpose was to leverage this system in a 6-week training paradigm designed to improve push-off intensity during walking in older adults.

In study 1, we engineered a biomechanical “stress-test” – a motor-driven, impeding force system that functionally assesses push-off intensity by increasing the propulsive demands of walking. Applying this system in older adults in study 2, we found that, unlike all other biomechanical determinants of push-off intensity, older adults appear unable to overcome their deficits in peak ankle moment during walking, alluding to a genuine functionally limiting impairment. In study 3, we used in vivo imaging to investigate the muscles responsible for generating ankle moments during walking (i.e., plantarflexors). We found that shorter plantarflexor fascicle lengths in older adults associate with worse capacity to enhance push-off intensity in walking. Finally, we tested the efficacy of an impeding force intervention aimed at targeting deficits in push-off intensity. We found that, after 6-weeks of impeding force training, older adults increased plantarflexor strength and cross-sectional area, maximum walking speed, and 6-minute walk distance. Unlike conventional interventions, older adults also increased their ankle power by a significant and clinically-meaningful 12%. Combined, these studies contribute to our mechanistic understanding of deficits in push-off intensity in older adults, and also inform rehabilitation programs towards improvements in independent mobility and quality of life for our aging population.