Postural Control during Downward Head Movements in Young Subjects

Abstract

We investigated postural sway in young subjects who were asked to rotate the head in the direction of visual targets. Thirteen subjects wore a helmet and stood on a force plate. They were asked to look at the targets quickly by directing the laser spot attached to the helmet on the target. Every subject showed consistent changes in the ground reaction force (Fx, Fy, Fz) and in the center of pressure (COP) associated with head movements. In 31% of all trials, force changes in Fx, Fy and Fz preceded head movements. During downward head movements, the anterior-posterior component of COP (COPx) exhibited the largest changes and the shortest latency. Ground reaction force in the anterior-posterior direction (Fx) also showed changes before the onset of downward head movements in 85% of the subjects (mean latency=-20 ms). However, the mean latency in other movements lagged behind the head movement onset. Electromyographic activities (EMGs) of the biceps femoris preceded the initiation of downward head movements by 22-54 ms in 2 subjects. These results indicate that goal-directed rotational head movements elicit COP changes. In addition, the COP changes preceding downward head movement suggest preparatory reactions related to anticipatory postural adjustments (APAs).