Coordination of the head with respect to the trunk, pelvis, and lower leg during quiet stance after vestibular loss.

This study examined the relationship between head and trunk sway and between pelvis and leg sway during quiet stance in subjects with long-standing bilateral peripheral vestibular loss (BVLs) comparing these relationships to those of age-matched healthy controls (HCs). All subjects performed three different stance tasks: standing quietly on a firm or foam support surface, with eyes closed (ECF or eyes closed on normal) and on foam with eyes open. Data were recorded with four pairs of body-worn gyroscopes to measure roll and pitch angular velocities at the head, upper trunk, pelvis and lower-leg. These velocities were spectrally analysed and integrated for angle correlation analysis in three frequency bands: below 0.7Hz (low pass, LP), above 3 Hz (high pass, HP) and in between (band pass, BP). For both groups head motion was greater than trunk and pelvis motion except for BVL subjects (BVLs) under ECF conditions. BVLs had greater motion than HCs at all measurement locations for ECF conditions. Angle correlation analysis indicated that the head was almost "locked" to the trunk for BVLs over the LP and BP frequency bands. Head movements for both groups were relatively independent of the trunk in the HP band. Power spectral density ratios, and transfer functions showed a similar result - head relative to trunk movements were less up to 3 Hz in all tests for BVLs. The resonant frequency of head-on-trunk motion was shifted to a higher frequency for BVLs: from 3.2 to 4.3 Hz in pitch, 4.6 to 5.4 Hz in roll. Both groups show greater lower-leg than pelvis motion. These data indicate that during quiet stance BVLs change the characteristics of their head on shoulder motion, reducing relative motion of the head below 3 Hz and increasing head resonant frequency. Presumably these changes are accomplished with increased use of proprioceptive neck reflexes.