Thresholds for sensing foot dorsi- and plantarflexion during upright stance: effects of age and velocity.

Background: The objective of this study was to determine in healthy young and old adult females the influence of age, rotation direction, angle, and speed on the threshold for sensing foot dorsi- and plantarflexion when standing and bearing weight on the limb.
Methods: Twelve young (YF, mean age 22 years) and 12 old (OF, 70 years) healthy adult females stood with their dominant foot on a servo-controlled platform and the other foot on a fixed platform. The platform induced either dorsi- or plantarflexion rotations at angular velocities of 0.1, 0.5, or 2.5 degrees/s to angles of 0.05, 0.1, 0.2, 0.4 or 0.8 degree. Subjects performed five trials at each velocity-angle combination and 30 dummy trials in which no platform rotation occurred, for a total of 180 trials. Success rates were determined for detecting both rotation (SRR) and rotation direction (SRD) for each test condition. The angular thresholds required to achieve an SRD of 75% were estimated using logistic regression.
Results: Age, rotation angle, and rotation speed significantly affected SRD (repeated measures ANOVA: p < .001). For the YF, DF thresholds were 0.04, 0.09, and 0.41 degree at the fast, moderate, and slow velocities, respectively. Threshold angles were three to four times larger in the OF than in the YF. A 10-fold reduction in the angular threshold was observed upon increasing the speed of rotation from 0.1 to 2.5 degrees/s.
Conclusions: Both age and speed significantly affected the thresholds for sensing foot dorsiflexion and plantarflexion in women.