Velocity-dependence of Vestibular Information for Postural Control on Tilting Surfaces

Vestibular information is known to be important for postural stability on tilting surfaces, but the relative importance of vestibular information across a wide range of surface tilt velocities is less clear. We compared how tilt velocity influences postural orientation and stability in 9 subjects with bilateral vestibular loss and 9 age-matched, control subjects. Subjects stood on a force platform that tilted 6 degrees, toes-up at eight velocities (0.25-32 deg/s), with and without vision. Results showed that visual information effectively compensated for lack of vestibular information at all tilt velocities. However, with eyes closed, vestibular loss subjects were most unstable within a critical tilt velocity range of 2-8 deg/s. Vestibular-deficient subjects lost their balance in over 90% of trials during the 4 deg/s condition, but never fell during slower tilts (0.25-1 deg/s) and fell only very rarely during faster tilts (16-32 deg/s). At the critical velocity range in which falls occurred, the body CoM stayed aligned with respect to the surface, onset of ankle dorsiflexion was delayed, and there was delayed or absent gastrocnemius inhibition, suggesting that subjects were attempting to actively align their upper bodies with respect to the moving surface instead of to gravity. Vestibular information may be critical for stability at velocities of 2-8 deg/s because the associated postural sway above 8 deg/s may be too fast to elicit stabilizing responses through the graviceptive somatosensory system and postural sway below 2 deg/sec may be too slow for somatosensory-triggered responses or passive stabilization from trunk inertia.

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