Normal subjects stood upright on a platform that moved horizontally, and were examined their visual, vestibular and proprioceptive reflexes.
Postural responses induced by horizontal sinusoidal swaying of the platform were measured in terms of movements of the head, shoulders, hips and knees and the integrated EMG of the gastrocnemius muscle (GM). The transfer function was calculated with platform movements as input and postural response as output. The results were displayed as Bode plots, and the patterns of postural response were deduced from these Bode plots.
1. A basic pattern of postural response was clearly demonstrated. In subjects with their eyes open or closed, at platform movements of 0.3Hz, the various parts of the body moved together with the platform movement. When the platform movement exceeded 1Hz, the lower half of the body and the upper half of the body swayed with the hips as fulcrum. When the eyes were closed, at 0.3 and 1Hz the amplitude of movement of each part of the body was larger than when the eyes were open.
2. The role of visual, vestibular and proprioceptive reflexes in this type of postural response was clear.
1) Visual input reduced the amplitude of body sway at frequencies less than 1Hz.
2) The labyrinthine reflex was presumed to have participated in the delay in phase of head to platform movement at frequencies of 1 to 3Hz.
3) At a platform movement of 3Hz, regardless of whether the eyes were open or closed, GM activity appeared when the muscle was extended, presumably induced by the stretch reflex.