Perception of movement at the human ankle: effects of leg position.

1. Recent studies show that subjects perceive smaller ankle movements when they are upright in the standing position than when they are seated. To examine this improvement, the ability to perceive ankle movements was tested in five positions of body, knee and ankle. Subjects reported the direction of slow ramp movements of the ankles. 2. The threshold for perceiving ankle movements was unchanged when only one ankle was moved rather than both together. When seated with the knees bent and ankles slightly plantarflexed, subjects perceived movements of 0.65 deg at 0.05 deg s-1. However, when upright or when seated with their knees and ankles in the standing position, subjects perceived movements that were one-third of this size. 3. These findings show that the knee and ankle positions, rather than being upright, explain the better performance in the standing position. During standing, knee extension and ankle dorsiflexion stretch the calf muscles. Thus, enhanced input from intramuscular stretch receptors appears responsible for the better performance.