Mechanisms underlying age-related differences in ability to recover balance with the ankle strategy.

Falls cause substantial death and morbidity in the elderly. An important prerequisite to the development of fall prevention interventions is improved understanding of the biomechanical and neuromuscular variables that govern ability to recover balance. In the present study, we examined the relative importance of strength versus speed of response variables in explaining differences in balance recovery performance with the ankle strategy between young and elderly women. Twenty-five young (19-36 years) and 25 community-dwelling elderly women (66-90 years) participated in balance recovery experiments. Subjects were supported in an inclined standing position by a horizontal tether and instructed to recover an upright vertical standing position by contracting their ankle muscles. The maximum initial lean angle from which 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 from which they could recover balance after the tether was suddenly released (which depends on both strength and speed of response) was 36.1% smaller for elderly than young. 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 dynamic recovery 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 differences in ability to recover balance between young and elderly women associate with variables related to strength and speed of response, and that exercise-based fall prevention programs should include balance and agility training, in addition to strength training.