The direction of postural instability affects postural reactions to ankle muscle vibration in humans.

Postural instability changes the weighting of different types of sensory information and the state of the equilibrium maintenance system. We have recently found that the effect of Achilles tendon vibration (so-called 'vibration-induced falling') strongly diminishes during unstable posture. However, it is unclear whether the state of postural instability has directionally specific or non-specific effects on the attenuation of proprioceptive influences from ankle muscles. To check this hypothesis, we varied the direction of support mobility and applied shank muscle vibration to subjects standing on a rocking platform movable in the sagittal, frontal or both directions. Postural reactions were present only on those supports that were stable in the sagittal direction. We suggest that the direction of postural instability affects information processing from ankle muscle receptors.