The relationship between the movement of the centre of foot pressure in the anterio-posterior plane and the rotation which occurs in the same plane at the level of the ankles, hips and shoulders during postural sway was examined in a group of normal human subjects. Expressed as three frequency-dependent functions this relationship consists of both relative magnitude and phase components. The magnitude component indicates that the pressure centre was most sensitive to ankle rotation, less to hip and least sensitive to rotation at the level of the shoulders. The sensitivity of the pressure centre was also a function of the frequency of the rotations and this is evidenced by the presence of several resonant peaks in the relationship. It is suggested that these could be due to body inertia and transmission delays. The resonant peaks became more prominent when the subjects stood with eyes closed. The growth of resonant peaks is taken as a sign of reduced postural stability and on this basis it is argued that a distinction should probably be made between the concepts of the stability and the steadiness of stance.
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