Does head extension and flexion increase postural instability in elderly subjects when visual information is kept constant?

The present study determined the effects of flexing and extending the head on the postural stability and mean anterior-posterior (A-P) center of mass (CM) position during upright stance in the elderly. To ensure visual input to stability was not a confounding variable, visual information was kept as constant as possible for all head positions. Twelve healthy elderly subjects (72.3 +/- 4.7 years) were asked to stand stationary on a single force-platform. Postural stability (assessed using the rms A-P excursion of the center of pressure (CP)) was determined for standing with the head erect, and with the head flexed and extended. The vestibular contribution to postural stability becomes increasingly important under challenging conditions, so to highlight the effects of vestibular system input, measurements of postural stability under conditions where visual and somatosensory inputs were disrupted were included. Changes in the mean A-P CM position when tilting the head were assessed by determining changes in the mean A-P location of the CP from standing with the head erect. Compared to standing with the head erect and looking straight ahead, postural stability was reduced when the head was flexed or extended (P < 0.01). Changes in mean A-P CM position were only significant when standing with the head flexed (P < 0.05). This suggests that increases in postural instability with the head tilted from the erect position may be in part due to mechanical perturbation rather than solely vestibular disruption.