Vestibular and proprioceptive influences on trunk movements during quiet standing.

We characterized upper trunk and pelvis motion in normal subjects and in subjects with vestibular or proprioceptive loss, to document upper body movement modes in the pitch and roll planes during quiet stance. Six bilateral vestibular loss (VL), six bilateral lower-leg proprioceptive loss (PL) and 28 healthy subjects performed four stance tasks: standing on firm or foam surface with eyes open or closed. Motion of the upper body was measured using two pairs of body-worn gyroscopes, one mounted at the pelvis and the other pair at the shoulders. Pitch and roll angular velocities recorded from the gyroscopes were analyzed separately for low-frequency (<0.7 Hz) and high-frequency (>3 Hz) motion. Low-frequency pitch motion was similar for all groups, consisting of in-phase pelvis and shoulder motion. High-frequency pitch motion in controls and VL subjects was dominated by pelvis motion with little shoulder motion, but vice versa in PL subjects. Low-frequency roll motion changed for all groups from mainly shoulder and little pelvis motion to in-phase pelvis and shoulder motion after moving from a firm to foam surface. In contrast, high-frequency roll motion changed from mainly shoulder motion to mainly pelvis motion with the change to a foam surface, except for PL subjects with eyes closed. Coherent low-frequency sway between pelvis and shoulder was only pronounced in VL patients. These results indicate that relative motion between the pelvis and shoulder depends on the support surface, the type of sensory loss, and whether the motion is in roll or pitch plane. Furthermore, relative motion between the pelvis and upper trunk is an integral part of movement modes used to control quiet stance. Vestibular loss patients showed very similar movement modes as controls, with larger amplitudes. Proprioceptive loss patients, however, used more shoulder motion and stabilized the pelvis for the high-frequency mode. We conclude that there is relative motion between the upper trunk and pelvis during quiet stance and suggest that it may contribute to balance control.