Coordination of the head with respect to the trunk and pelvis in the roll and pitch planes during quiet stance.

This study examined the relationship between head and trunk sway during quiet stance and compared this relationship with that of the pelvis to the trunk. Sixteen younger and 14 elderly subjects participated, performing four different sensory tasks: standing quietly on a firm or foam support surface, with eyes open or closed. Roll and pitch angular velocities were recorded with six body-worn gyroscopes; a set of two mounted at the upper trunk, an identical set at the hips, and another set on a head band. Angle correlation analysis was performed in three frequency bands: below 0.7 Hz (LP), above 3 Hz (HP) and in between (BP) using the integrated angle velocity signals. Angular velocities were spectrally analysed. Greater head than trunk motion was observed in angle correlations, power spectral density (PSD) ratios, and transfer functions (TFs). Head on trunk motion could be divided for all sensory conditions into a low-frequency (<0.7 Hz) "head locked to trunk" inverted pendulum mode, a mid-frequency (ca. 3 Hz), resonant mode, and a slightly anti-phasic head motion on stabilised trunk, high-frequency (>3 Hz) mode. There was coherent motion between head and trunk but not between head and pelvis. Trunk and pelvis data were consistent with previously reported in-phase and anti-phase movements between these segments. Significant age differences were not found. These data indicate that during quiet stance body motion increases in the order of pelvis, trunk, head and quiet stance involves control of at least two separate links: trunk on pelvis and head on trunk dominated by head resonance. The head is locked to the trunk for low-frequency motion possibly because motion is just supra-vestibular threshold. The head is not stabilised in space during stance, rather the pelvis is.