Ankle reflex stiffness during unperceived perturbation of standing in elderly subjects.

Postural reflex activity during unperceived perturbation of standing was investigated in 38 elderly subjects (70-96 years old) and 10 younger adults (19-48 years old), and it was related in the elderly group to a history of unexplained falling in the 12 months prior to testing. Ankle torque (T) and ankle angle (A) were recorded during unperceived forward pulls to obtain the ankle stiffness (DeltaT/DeltaA), providing a measure of postural reflex activity at the ankle. Elderly nonfallers and younger adults had similar ranges of normalized ankle reflex stiffness. High ankle stiffness was significantly more common in elderly multiple fallers than in elderly nonfallers (p =.018). Furthermore, a majority of unstable elderly subjects who had reported a single unexplained fall also had unusually high ankle stiffness (p =.004). Multiple fallers and unstable subjects tended to overshoot backward on pull release in comparison with nonfallers (p =.003), which is suggestive of an overactive reflex response that might contribute to postural instability.