Correlation between the directions of baby's sway

Abstract

To clarify the correlations among the swaying movements of the human body, the cross-power, the coherence and the phase were calculated by means of a computer. Five normal subjects were examined. The subject stood on a gravicorder with his feet close together. Head movement was picked up by means of a two channel accelerometer attached to a helmet on the head.
Two dimensional sways of the body's center of gravity and head, anteroposterior and lateral movements were registrated on a magnetic tape. The data were transmitted to a computer in the form of digital values after A/D transformation. Auto-powers of arbitrarily chosen two movements were calculated with fast fourier transform of these two time series.
Cross-powers, coherence and phase angle of these two movements could be produced from these two auto-powers. These results were displayed on a CRT. There was no remarkable difference in the cross-powers of any two movements, sways of body's center of gravity and head accelerations, in any direction, antero-posterior and lateral. On the contrary, the coherence and phase showed a remarkable contrast between the different combinations of moving directions. In the same direction, coherence between sways of body's center of gravity and head acceleration showed a high value and the phase difference was slight. Between different directions, coherence between any combination of sway of body's center of gravity and head acceleration showed a low value and the phase difference was large.
Therefore, coherence and phase are much more important than crosspower when discussing the relationships between any two movements. There is only a weak correlation between movements of any portion of the body in different directions, while on the contrary, movements of different portions of the body in the same direction show strong correlations.