Falls cause substantial death and morbidity in the elderly. Fall risk depends on ability to maintain balance during daily activities and ability to recover balance following a perturbation such as a slip or trip. To guide the design of fall prevention programs, we need an improved understanding of the biomechanical variables that govern ability to recover balance. The aims of this thesis were to determine (1) the relative importance of strength versus speed-of-response variables in explaining age differences in balance recovery performance with the ankle strategy, and (2) the association between variables related to ability to recover balance and variables related to ability to maintain balance. To address Aim 1, young and elderly women were supported in a forward leaning position by a horizontal tether and instructed to recover an upright vertical stance by contracting their ankle muscles. The maximum initial lean angle where they could recover balance without release of the tether (which depends primarily on strength) was 19.6% smaller for elderly than young. The maximum initial lean angle where they could recover balance after the tether was suddenly released (which depends on strength and speed-of-response) was 36.1% smaller for elderly. Moreover, between-group differences in performance were related to both strength and speed-of-response. Peak ankle torque was 7.7% smaller in elderly than young during tether release trials, reaction time was 27% slower in elderly, due to a lengthened muscle response latency, and rate of ankle torque generation was 15.6% slower in elderly. These results suggest that exercise-based fall prevention programs should include balance and agility training, in addition to strength training. To address Aim 2, the same elderly subjects participated in postural steadiness experiments, where the amplitude, velocity, and frequency of their centre-of-pressure displacement were measured during quiet stance. Postural steadiness during quiet stance and ability to recover balance with the ankle strategy were not associated, perhaps because postural steadiness during quiet stance is controlled partly by anticipatory strategies, while balance recovery following a perturbation is governed by reactive strategies. These results support the need to measure both balance recovery and postural steadiness in balance assessments of the elderly.
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